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Institute
- Institut für Botanik und Landschaftsökologie & Botanischer Garten (56) (remove)
Ecological Impacts and Phenotypic Plasticity of a Global Invasive Cactus, Opuntia ficus-indica
(2023)
Biological invasions by non-native species pose one of the major threats to biodiversity, the way ecosystems function, and the well-being of humans. These invasions can occur through various means, including accidental or intentional introductions by humans, natural dispersal, and climate change. Non-native species can harm the native species and ecosystems, by homogenizing plant communities, competing for resources, changing how the ecosystem operates, and eventually causing native species to go extinct. Even though not all non-native species become invasive, changes in climate and ecosystems can cause the successful establishment and spread of non-native species. Despite the advancements in our understanding of biological invasions in recent years, research has been biased towards temperate regions, whereas arid and semi-arid regions that are highly impacted by climate change are underrepresented. Thus, particularly focusing on the impacts of biological invasions in subtropical arid and semi-arid regions, the goal of this PhD project was to explore the effects of an invasive cactus on the local native communities and investigate the mechanisms of its successful invasion. Certain species are found to take advantage of the ever-drying climates in the arid/ semi-arid regions of the world. Opuntia ficus-indica, native to Mexico, is an exceptionally successful drought-tolerant invasive cactus that successfully grows in these regions. O. ficus-indica, a most widespread invasive cactus, is considered an ecosystem engineer as it modifies the habitats of indigenous plant species and dependent animals. This project aimed to identify the ecological impacts of O. ficus-indica in the highlands of Eritrea, the competitive potential of O. ficus-indica and the plastic changes that enabled its spread and invasion (Chapters I-III). For this purpose, field observations and common garden experiments were carried out throughout the project.
We investigated the effects of Opuntia ficus-indica on the spatial diversity of native plant communities (Chapter I), its competitive ability against native species (Chapter II) and the phenotypic plasticity of O. ficus-indica (Chapter III). To investigate the main ecological effects of O. ficus-indica on the native community, field data was collected from the highlands of Eritrea and comparisons were made between O. ficus-indica invaded and noninvaded areas (Chapter I). The study aimed to understand the effects of O. ficus-indica by examining species composition, richness, and diversity across vegetation layers and revealed that O. ficus-indica homogenises the species composition of the native ecosystem. This provides evidence that the presence of O. ficus-indica reduces landscape-level heterogeneity or spatial diversity. However, O. ficus-indica did not influence the species richness and diversity of the local communities. The mechanisms of the successful homogenisation of the local communities by O. ficus-indica were attributed to the potential competitive abilities of O. ficus-indica against the native species, and the plastic and adaptive traits it developed in the non-native ranges. The first assumption was tested by setting up a common garden competition experiment between two native Eritrean species, Ricinus communis and Solanum marginatum (Chapter II). The experiment used two water availability treatments, wet and dry, and categorized plants into intraspecific (native or invasive only) and interspecific (native and invasive) competition. The study evaluated the impacts by comparing the growth of O. ficus-indica alone to the growth alongside native species which revealed the weak competitive potential of O. ficus-indica. However, O. ficus-indica was observed to outgrow the native species in several folds which can be attributed to its successful invasion. The second assumption of the successful spread of O. ficus-indica was attributed to the phenotypic plastic traits adapted by O. ficus-indica in the non-native ranges (Chapter III). The phenotypic plasticity of O. ficus-indica was assessed by exposing it to water stress across dry and wet environments. The species were cultivated from a diverse set of 12 populations, encompassing its native range in Mexico with three cultivars and nonnative ranges in Africa (Algeria, Eritrea, Ethiopia), the island of Madeira off the coast of Africa, and in Europe, Italy with two cultivars and in Portugal from three sites. In Mexico and Italy, we collected various cultivars to ensure a wide representation of genotypes. We found that the species originating from the native range of O. ficus-indica exhibited lower plasticity to conditions of elevated water availability. Furthermore, a trial gradient experiment on O. ficus-indica was conducted to determine the appropriate watering levels for the species and the experiment revealed not only the species' capacity to endure a lack of water for nine months but also its ability to withstand prolonged waterlogged conditions.
This thesis illustrates the fact that invasive species are a major threat to biodiversity and ecosystem functioning worldwide, especially in rarely studied regions with dry climates and limited resources. How can invasive plants spread and cause negative impacts on native ecosystems (Chapter I), despite their weak competitive abilities (Chapter II)? This thesis explored these questions by examining the case of O. ficus-indica, an invasive species in arid/ semi-arid climates (Chapter I). It showed that O. ficus-indica has a high growth potential that allows it to overcome resource limitations, that its growth is not affected by competition from native species (Chapter II), and that it exhibits adaptive plasticity that enhances its invasion success in different environments (Chapter III). This thesis revealed the complex mechanisms and consequences of biological invasions in dry climates and contributes to the understanding of invasive species. It also suggests that more research is needed in understudied regions to assess the impacts of O. ficus-indica or invasive species in general on native biodiversity and ecosystem services and to identify the factors that influence the competitive and adaptive potentials.
Forests influence the climate of our Earth and provide habitat and food for many species and resources for human use. These valuable ecosystems are threatened by fast environmental changes caused by human-induced climte change. Negative growth responses and higher tree mortality rates were associated with increasing physiological stress induced by global warming. Especially boreal forests at high latitudes in the arctic region are threatened, a region predicted to undergo the highest increase in temperature during the next decades. Therefore, it is important to assess the adaptation potential in trees. For this purpose, I studied natural populations of white spruce (Picea glauca (Moench) Voss) in Alaska. In this thesis, I present three scientific papers in which my co-authors and I studied the phenotypic plasticity and genetic basis of tree growth, wood anatomy and drought tolerance as well as the genetic structure of white spruce populations in contrasting environments. We established three sites representing two cold-limited treelines and one drought-limited treeline with a paired plot design including one plot located at the treeline and one plot located in a closed-canopy forest, respectively. Additionally, the study design included one forest plot as reference. Within the entire project, in total 3,000 trees were measured, genotyped and dendrochronological data was obtained. I used several approaches to estimate the neutral and adaptive genetic diversity and phenotypic plasticity of white spruce as a model organism to explore the adaptation potential of trees to climate change.
In the first chapter, I combined neutral genetic markers with dendrochronological and climatic data to investigate population structure and individual growth of white spruce. Several individual-based dendrochronological approaches were applied to test the influence of genetic similarity and microenvironment on growth performance. The white spruce populations of the different sites showed high gene flow and high genetic diversity within and low genetic differentiation among populations, rather explained by geographic distance. The individual growth performances showed a high plasticity rather influenced by microenvironment than genetic similarity.
In the second chapter, I investigated the populations of the drought and cold-limited treeline sites to decipher the underlying genetic structure of drought tolerance using different genotype-phenotype association analyses. Based on tree-ring series and climatic data, growth declines caused by drought stress were identified and the individual reaction to the drought stress event was determined. A subset of 458 trees was genotyped, using SNPs in candidate genes and associated with the individual drought response. Most of the associations were revealed by an approach which took into account small-effect size SNPs and their interactions. Populations of the contrasting treelines responded differently to drought stress events. Populations further showed divergent genetic structures associated with drought responsive traits, most of them in the drought-limited site, indicating divergent selection pressure.
In the third chapter, my co-authors and I studied xylem anatomical traits at one of the cold-limited treeline sites to investigate whether genetic or spatial grouping affected the anatomy and growth of white spruce. Annual growth and xylem anatomy were compared between spatial groups and between genetic groups and individuals. Overall, wood traits were rather influenced by spatial than genetic grouping. Genetic effects were only found in earlywood hydraulic diameter and latewood density. Environmental conditions indirectly influenced traits related to water transport.
In conclusion, white spruce showed a high genetic diversity within and a low genetic differentiation among populations influenced by high gene flow rates. Genetic differences among populations are rather caused by geographical distance and therefore genetic drift. Differing selection pressure at the treeline ecotones presumably lead to divergent genetic structures underlying drought-tolerant phenotypes among the populations. Thus, adaptation to drought most likely acts on a local scale and involves small frequency shifts in several interacting genes. The identified genes with adaptive growth traits can be used to further exlore local adaptation in white spruce. Tree growth and wood anatomical traits are rather influenced by the environment than genetics and showed a high phentoypic plasticity. The high genetic diverstiy and phenotypic plasticity of white spruce may help the species to cope with rapid environmental changes. Still, additional work is needed to further explore adaptation processes to estimate how tree species reacted to rapid climate change. The presented thesis shed some light on the adaptation potential of trees by the example of white spruce using several approaches.
Myxomycetes or Myxogastria (supergroup Amoebozoa) are one of several Protistean groups dispersing via airborne spores. The model organism for the group, so far exclusively studied in a laboratory environment, is Physarum polycephalum. Here, molecular evolution, distribution and the ecology of spores dispersal was investigated for the non-model species Physarum albescens. This nivicolous myxomycete fruits with snow melt in most mountain ranges of the northern hemisphere and disperses via spherical, dark-colored and melanin-rich spores. Fruit body development and subsequent spore dispersal occurs within a short time window of a few days. At this time, the fruiting plasmodium is fully exposed to the harsh environment if the protecting snow melts away. The spores, with a diameter of 10–13 µm of the typical size for myxomycetes, can potentially reach all suitable habitats worldwide, which led to the assumption that not only Ph. albescens but most myxomycete species should be ubiquitously distributed over the world.
In the first part of this study (article 1), the question was, if spore dispersal can realize a gene flow sufficient to meet the above-mentioned assumption. A total of 324 accessions of Ph. albescens, collected all over the northern hemisphere, was sequenced for 1-3 genetic markers (SSU, EF1A, COI), and 98 specimens were further analyzed using the genotyping by sequencing technique. As a result, at least 18 reproductively isolated units, which can be seen as cryptic biological species, emerged as phylogroups in a three-gene phylogeny, but as well in a SNP-based phylogeny and were confirmed by a recombination analysis between the three markers. However, this evolutive radiation is not simply caused by geographic fragmentation due to low dispersal capability: within a certain region, multiple phylogroups coexisted next to each other, although some appeared to be regional endemics. Most likely, mutations in mating-type genes, as shown to exist for the cultivable Ph. polycephalum, are the main drivers of speciation. This challenges the hypothesis of ubiquitous distribution of Ph. albescens and corroborates the results of the few available studies for other myxomycete species. In addition, groups of clonal specimens, mostly but not always restricted to a certain slope or valley indicated that sexual and asexual reproduction coexists in the natural populations of Ph. albescens.
In the second part (articles 2), the fundamental niche for Ph. albescens was described using all available records for the species. The resulting set of 537 unique occurrence points was subjected to a correlative spatial approach using the software MaxEnt. In dependence on the predictor variables three species distribution models emerged which differed only in details. The first consisted of only 19 bioclimatic variables and an elevation map from the WorldClim dataset. The second was corrected for pseudo-absences resulting from missing survey activities, and the third was expanded with an additional categorical environment variable on snow cover. High mean AUC (area under the curve) values above 0.97 could be reached with all three models. Variables for snow cover, precipitation of the coldest quarter (of the year), and elevation correlated highly to predict the distribution of Ph. albescens. Only in mid-northern latitudes, elevation alone was a good predictor, but it would cause false-positive predictions in arid mountain ranges and failed to explain occurrence in lowland sites at higher latitudes. Mountains in humid climates showed the highest incidences, confirming recent studies that long-lasting snow covers combined with mild summers are crucial for the ecological guild of nivicolous myxomycetes, with Ph. albescens as a typical species.
Spore size is crucial for dispersal ability and should thus be a character under strong selection. In addition, spores carrying two nuclei with opposite mating types should have a colonization advantage. This was the hypothesis for the last part of this study (articles 3 and 4), which investigated this trait in a quantitative manner. This required a method to analyze thousands of spores automatically (article 3) and with high precision for size and the number of nuclei enclosed. Human errors should be excluded, to reveal even subtle differences in the resulting spore size distributions. Two challenges had to be met for this approach. First, a preparation technique was developed to reduce false segmentations due to overlaying spores by aligning spores on one common plane with a high-frequency vibration device. Second, the segmentation process was automated to allow separating spores that are densely packed in the respective images. A machine learning algorithm was set up and trained to reliable identify and measure dark-colored spores. The technique produced consistent results with high accuracy, and the large number of spores allowed to compile spore size distributions, to check for the constancy of this character, which is impossible with manual measurements limited to low numbers.
The resulting spore size distributions, obtained from over 80 specimens (article 4), were mostly narrow, which is in accordance with our hypothesis. Spore size was as well fairly constant within fructifications from one colony. However, mean spore size within different accessions of Ph. albescens showed large variation (ca. 10%, a range often indicated to key out different morphospecies of myxomycetes), and this was explained only by a minor part with differences between biospecies. Not much smaller (8%) was the variation within a group of clonal specimens collected within 25 m distance. This points to a strong influence of environmental factors even at a micro spatial scale, perhaps caused by microclimatic differences and high phenotypic plasticity for spore size. The influence of large-scale covariates like altitude or latitude was negligible. However, spore size correlated with the variance in this trait, indicating that oversized spores may be caused by detrimental environmental conditions. Two aberrations in spore development were found: First, a few specimens showed a multimodal distribution for spore size with two or even three discernible spore populations. The estimated volumes of those populations correspond to a multiple of the first and most abundant conspicuous spore size population. Second, not all spores were uninucleate as to be expected for meiotic products. This was revealed by fluorescence signals from staining the same spores with DAPI, with a second machine learning algorithm trained to identify the nuclei in a spore. A few specimens showed a significant proportion of binucleated spores in the size range of normal-sized ones, and these specimens were not the ones with multimodal spore size distributions. This indicates that the negative impacts (inbreeding) of multinucleate spores should outweigh a possible colonization advantage and is in accordance with the high genetic diversity found in the worldwide population of Ph. albescens, indicating predominantly sexual reproduction in wild populations of myxomycetes.
Drainage has commonly been a pre-requisite for the productive use of peatlands. The biased focus on agriculture, forestry and peat extraction has long ignored the destructive effects of drainage and the successive degradation of ecosystem functions of wet peatlands. Accelerated by the climate crisis, the finite nature of drainage-based peatland use is increasingly recognised. Consequently, productive land use options for wet or rewetted peatlands (paludiculture) are required as sustainable alternatives. A wide range of paludiculture plants and options of biomass utilisation are identified as suitable and promising. Despite the growing interest, experiences with and research on the economic viability of paludiculture are still rare.
This thesis addresses the lack of knowledge on paludiculture in terms of practical feasibility, costs and benefits at the farm level, market prospects and framework conditions. I selected the two currently most advanced paludicultural practices in Europe: a) Harvesting natural reed beds as a traditional ‘low-input’ paludiculture, i. e. the utilisation of existing ‘wild’ vegetation stands; b) ‘Sphagnum farming’ as a novel ‘high-input’ paludiculture including stand establishment and water management required for the active transformation from drainage-based peatland use to paludiculture. In both cases, I investigate three different biomass utilisation avenues. This thesis adds to the fields of problem-driven sustainability and land-use science. Procedures and costs of paludiculture were studied in transdisciplinary research projects in close cooperation with practitioners. Due to the novelty of the topic, I put special emphasis on the triangulation of methods and data sources: pilot trials, field measurements, semi-structured expert interviews, structured questionnaires, secondary data from trade statistics and literature. To account for uncertainty related to costs and revenues, I conduct stochastic scenario analysis (Monte Carlo simulation) for the extended contribution margin accounting of harvesting reeds and sensitivity analysis for the investment appraisal of Sphagnum farming.
Paludiculture on fens: harvesting reeds
Paper I investigates harvesting procedures for reed-dominated (Phragmites australis) vegetation stands. In many European countries special-purpose tracked machinery is applied for large-scale conservation management and the commercial harvest of thatching reed. Stochastic scenario analysis reveals a wide range of possible economic outcomes (ca. € -1000 to € 1500 ha-1 a-1) and identifies material use of reed superior to its use as a source of energy. Winter harvest of high-quality thatching reed in bundles is the most profitable option. Winter harvest of bales for direct combustion is suitable for low-quality stands and has a limited risk of loss. In the case of summer harvest, revenues for green chaff for biogas production cannot cover harvesting costs but non-market income via subsidies and agri-environmental payments may ensure profitability. While biomass for energy generation is limited to a local market, thatching reed is traded as an international commodity. The market situation for thatching reed is investigated for Europe (Paper II) and Germany (Paper III). The major reed consuming countries in Western Europe (Netherlands, Germany, UK, Denmark) rely on imports of up to 85 % of the national consumption, with reed being imported from Eastern and Southern Europe and since 2005 also from China. The total market volume for reed for thatching in Northern Germany is estimated with 3 ± 0.8 million bundles of reed with a monetary value at sales prices of € 11.6 ± 2.8 million. Most of the thatchers (70 %) did not promote reed of regional origin to their customers due to insufficient availability in the first place and a lack in quality as second reason. The cultivation of reed in paludiculture may improve quantity and quality of domestic thatching reed. An area of 6000 ± 1600 ha with an average yield of 500 bundles per hectare would allow covering the current total demand of 3 million bundles of the German thatching reed market (Paper III).
Paludiculture on bogs: Sphagnum farming
Sphagnum farming provides an alternative to peatland degradation in two ways: Firstly, Sphagnum mosses can be cultivated as new agricultural crops on rewetted peatlands. Secondly, the produced Sphagnum biomass is a high-quality raw material suitable to replace peat in horticultural growing media (Paper V). Pilot trials have demonstrated the practical feasibility of establishing Sphagnum cultures on former bog grassland, cut-over bogs and mats floating on acidic waters bodies; Paper IV compares for the three types of production sites the specific procedures, costs and area potential in Germany. Water-based Sphagnum farming is not recommended for large-scale implementation due to highest establishment costs, major cultivation risks and limited area potential. For soil-based Sphagnum farming, the most important cost positions were Sphagnum shoots to set up pilots, investment for water management and regular weed management. Bog grassland has the highest area potential, i. e. 90,000 ha in NW Germany. Paper V assesses the profitability of Sphagnum farming on former bog grassland based on extrapolating five years of field experience data (establishment ņ management ņ harvest) to a total cultivation time of twenty years. Cultivating Sphagnum biomass as founder material for Sphagnum farming or restoration was profitable even in pessimistic scenarios with high costs, high bulk density and low yields. Selling Sphagnum for orchid production was economically viable in the case of medium to high yields with a low bulk density. Cost-covering prices for Sphagnum biomass substituting peat seem achievable if end consumers pay a surcharge of 10 % on the peat-free cultivated horticultural end-product. An area of 35,000 ha of Sphagnum farming suffices to meet the annual demand of the German growing media industry for slightly decomposed Sphagnum peat.
Framework conditions affecting feasibility of paludiculture
The relation of revenues from selling biomass to its production costs is an important piece of the paludiculture feasibility puzzle. Further aspects effecting the economic viability and competitiveness of paludiculture encompass the market demand, the availability of mature technology, legal restrictions, the eligibility for agricultural subsidies, a remuneration of external benefits and the opportunity costs of present farming activities (Paper I, V). Legal and policy regulations are of major importance for land use decisions on peatlands – both for keeping up drainage and for shifting to paludiculture.
Conclusion and Outlook
This thesis provides a first assessment of the costs and profitability of large-scale harvesting of reeds and Sphagnum farming based on real-life data. The paludicultural practices investigated may be a solution for a minor share of the more than 1 million ha of peatlands drained for agriculture in Germany. Future research should also address other biomass utilisation options and other crops. Large-scale pilots are required to improve technical maturity of procedures and machinery, gather reliable data to replace assumptions on costs and revenues and study long-term effects on economics and ecosystem services. The micro-economic perspective needs to be complemented by the societal perspective quantifying and monetising external effects of peatland restoration, paludiculture and drainage-based peatland use. There is a high need for intensified research, large-scale implementation and accelerated adaption of the policy and legal framework to develop paludiculture as an economically viable option for degraded peatlands.
Forests are ecologically important ecosystems, for example, they absorb CO2 from the
atmosphere, mitigate climate change, and constitute habitats for the majority of terrestrial
flora and fauna. Currently, due to increasing human pressure, forest ecosystems are
increasingly subjected to changing environmental conditions, which may alter forest growth
to varying degrees. However, how exactly different tree species will respond to climate
change remains uncertain and requires further comprehensive studies performed at different
spatial scales and using various tree-ring parameters.
This dissertation aims to advance the knowledge about tree-ring densitometry and
tree responses to climate variability and extremes at different spatial scales, using various
tree species. More specifically, the following aims are pursued: (i) to obtain and compare
wood density data using different techniques, and to assess variability among laboratories
(Chapter I). (ii) To investigate microsite effects on local and regional Scots pine (Pinus
sylvestris L.) responses to climate variability (Chapter II) and extremes (Chapter III),
using ring width (RW) and latewood blue intensity (LBI) parameters. (iii) To give a general
site- and regional-scales overview of Scots pine, pedunculate oak (Quercus robur L.), and
European beach (Fagus sylvatica L.) RW responses to climate variability (Chapter IV). (iv)
To discuss the challenges which may result from compiling tree ring records from different
(micro)sites into large-scale networks. The study area comprises nine coastal dune sites, each
represented by two contrasting microsites: dune ridge and bottom (Chapters II and III), and
310 different sites within the south Baltic Sea lowlands (Chapter IV).
The dissertation confirms that sample processing and wood density measuring are
very important steps, which, if not performed carefully, may result in biases in growth trends,
climate-growth responses, and climate reconstructions. The performed experiment proved
that the mean levels of different wood density-related parameters are never comparable due
to different measurement resolutions between various techniques and laboratories. Further,
the study revealed substantial biases using data measured from rings of varying width due
to resolution issues, where resolution itself and wood density are lowered for narrow rings
compared to wide rings (Chapter I).
The (micro)site-specific investigation showed that, depending on the species,
different climate variables (temperature, precipitation, or drought) constitute important
factors driving tree growth across investigated locations (Chapters II and IV). However,
there is evidence that the strength and/or direction of climate-growth responses differ(s)
between microsite types (Chapter II) and across sites (Chapter IV). Moreover, climategrowth
responses are non-stationary over time regardless of the tree species and tree-ring
parameter used in the analysis (Chapters II and IV). There are also differences in RW and
LBI responses to extreme events at dune ridge and bottom microsites (Chapter III).
The regional-scale investigations revealed that climate-growth responses (strength
and non-stationarity) are quite similar to those observed at the local scale. However,
compiling RW or LBI measurements into regional networks to study tree responses to
extreme events led to weakened signals (Chapter III).
The findings presented in Chapters II and IV suggest that the strength, direction,
and non-stationary responses are very likely caused by several climatic and non-climatic
factors. The mild climate in the south Baltic Sea region presumably does not constitute a
leading limiting growth factor, especially for Scots pine, whose distribution extends from
southern to northern Europe. Thus, the observed climate-growth responses are usually of
weak to moderate strength. In contrast, for other species reaching their distribution limit at
the Baltic coast, the climatic signal can be very strong. However, the observed findings also
result from the effects of microsite conditions, and potentially other factors (e.g.,
management, stand dynamic), which all together alter the physiological response of the tree
at a local scale. Although climate at the south Baltic Sea coast is mild, extreme climate events
may occur and affect tree growth. As demonstrated (Chapter III), extreme climate events
affected tree growth across dune sites, however, to varying degrees. The prominent
differences in tree responses to extreme climate events were significant at the local scale but
averaged out at the regional scale. This is very likely associated with observed microsite
differences, where each microsite experiences different drivers and dynamics of extreme
growth reductions.
This dissertation helped to demonstrate that integrating local tree-ring records into
regional networks involves a series of challenges, which arise at different stages of research.
In fact, not all possible challenges have been discussed in this dissertation. However, it can
be summarized that several steps performed first at the local scale are very important for the
quality and certainty of climate-growth responses, tracking tree recovery after extreme
events, and potential climate reconstructions at the larger scale. Among them, identification
of microsite conditions, sample preparation, and measurement, examination of growth
patterns and trends, and identification of a common limiting growth factor are very
important. Otherwise, the compilation of various tree-ring data into a single dataset could
lead to over- or underestimation of the results and biased interpretations.
Forest ecosystems around the world and especially boreal forests, are facing
drastically changing climatic conditions. It is known that these changes could
challenge their functionality and vitality. Still, the exact impact is not fully
understood, as tree growth is a complex process and depends on countless
environmental and genetic factors. To estimate the effects of climate change
on tree growth and forest development precisely, we must learn more about
tree growth itself. A comprehensive approach is needed where trees and
forests are investigated on different scales and levels of detail, ranging from
global studies to studies on single individuals.
In this dissertation, I follow such a comprehensive approach, using the
North American conifer white spruce as an example. I present three papers
in the form of three chapters in which my co-authors and I studied the
growth and anatomy of white spruce (Picea glauca [Moench] Voss) and how
it is influenced by environmental, climatic, and genetic factors.
We used diverse approaches and methods on different spatial scales, ranging from
investigations on the landscape to the local scale. We established three paired
plots with forest and treeline sites (two cold-limited and one drought-limited).
as well as one additional forest site. In the first chapter, we concentrated
on the genetic diversity of white spruce within and between populations at
all study sites throughout Alaska. The genetic investigations were combined
with analyses on the individual growth response of trees to climatic conditions
to find whether genetic similarities or spatial proximity caused similarities
in growth and climatic sensitivity. In the second chapter, we studied the
direct and indirect effects of environmental conditions on the xylem tissue
of white spruce. We analyzed the impact of precipitation, temperature, and
tree height on four xylem anatomical traits in trees growing at the three
treelines. The investigated traits represented the main functions of xylem
tissue (i.e., water transport and structural support). In the third chapter,
we investigated similar xylem anatomical traits at one cold-limited treeline.
We compared xylem anatomy and annual increment between genetic groups
and individuals and between spatial groups to investigate whether spatial or
genetic grouping influenced the anatomy and growth of white spruce.
We found an overall high gene flow and high genetic diversity in white
spruce. However, the sensitivity of the growth and anatomical traits of white
spruce was driven mainly by spatial rather than genetic effects and differed
between study sites. Trees from the drought-limited site were more sensitive
towards precipitation and a moisture index, while trees from the cold-limited
sites were more sensitive towards temperature. A strong direct effect of tem-
perature was primarily found in latewood traits related to the structural sup-
port of the tree. Earlywood traits related to water transport, however, were
influenced mainly by tree height. Tree height itself was potentially affected
by diverse abiotic and biotic factors (e.g., (micro)climate, soil conditions,
and competition). Thus, traits related to water transport were indirectly
influenced by environmental conditions. Genetic effects in xylem anatomical
traits were found in the earlywood hydraulic diameter and latewood den-
sity, whereas in general, primarily spatial rather than genetic grouping was
influencing the anatomy of white spruce.
Overall, white spruce showed to be a genetically diverse species with a
high gene flow. The effects of spatial proximity and spatial grouping on the
sensitivity and anatomy of white spruce indicate high phenotypic plastic-
ity. This high phenotypic plasticity combined with the vast genetic diversity
translates into an immense potential for the species to adjust (phenotypically)
and possibly adapt (genetically) to changing conditions. Thus, in terms of
climate change, white spruce may be a rather persistent species that manages
to cope with the drastic changes. Though additional work might be needed to
draw a more solid conclusion, the presented work shows how a comprehensive
study approach can help to interpret and understand the growth and ecology
of a tree species. It may be an inspiration for future studies to broaden their
approaches and to use comprehensive methods on different levels of detail to
not only observe trees but to explore and understand them.
In Mitteleuropa kommen innerhalb der Gattung Diphasiastrum neben drei Ausgangsarten (D. alpinum, D. complanatum, D. tristachyum) drei Taxa hybridogenen Ursprungs vor (D. x issleri = D. alpinum x D. complanatum; D. x oellgaardii = D. alpinum x D. tristachyum; D. x zeilleri = D. complanatum x D. tristachyum). Alle sechs Taxa sind diploid. Die homoploiden Hybriden unterscheiden sich sowohl morphologisch als auch hinsichtlich ihres Kern-DNA-Gehaltes deutlich voneinander und nehmen eine intermediäre Stellung zwischen ihren Elternarten ein. Daher ist zu vermuten, dass es genetische Schranken für Rückkreuzungen gibt. Außer den regelmäßig auftretenden diploiden Hybriden konnten drei sehr seltene triploide Diphasiastrum-Hybriden nachgewiesen werden. Auf Grund ihres Kern-DNA-Gehaltes und der Morphologie kann auf folgende Kombinationen geschlossen werden:
Diphasiastrum alpinum x D. x issleri (Genomformel AAC),
Diphasiastrum alpinum x D. x oellgaardii (Genomformel AAT),
Diphasiastrum complanatum ssp. complanatum x D. x issleri (Genomformel ACC).
Es kann vermutet werden, dass diese triploiden Hybriden durch eine Kreuzbefruchtung zwischen einem diploiden Gametophyten, entstanden aus einer Diplospore, und einem haploiden Gametophyten hervorgegangen sind. Diplosporen könnten auch zur Vermehrung der diploiden Hybriden mittels Sporen beitragen; allerdings sind sie bei Flachbärlappen noch nicht experimentell eindeutig nachgewiesen. Bisherige Untersuchungen dreier genetischer Marker (cp, RPB, LFY) sowie die Ergebnisse einer AFLP-Analyse legen jedoch eine überwiegende de-novo-Entstehung durch primäre Kreuzungsereignisse nahe.
Die drei Elternarten unterscheiden sich hinsichtlich ihrer genetischen Diversität erheblich. Während von D. alpinum mindestens zwei genetische Linien existieren, ist D. tristachyum offensichtlich wenig variabel. Die größte genetische Vielfalt weist D. complanatum auf, für das eine sexuelle Reproduktion durch flowzytometrische Untersuchungen der gametophytischen Generation nachgewiesen werden konnte. Auch die Hybriden sind genetisch nicht einheitlich, was für unabhängige Entstehungsereignisse spricht.
Die Vertreter der Gattung Diphasiastrum weisen einen ausgeprägten Pioniercharakter auf und können Lebens-räume mit frühen Sukzessionsstadien erfolgreich besiedeln. Hier bilden sie durch ihr klonales Wachstum flächig ausgedehnte Bestände (Klone) aus. Diese können, längerfristig geeignete Standortbedingungen vorausgesetzt, ein Alter von vielen Jahrzehnten bis zu mehreren Hundert Jahren erreichen. Mit ihren staubfeinen Sporen sind Flachbärlappe auch zur Besiedlung von Gebieten, die von bestehenden Vorkommen weiter entfernt sind, mittels Langstreckentransport durch die Luft befähigt.
Flachbärlappe sind obligate Dunkelkeimer mit sich über mehrere Jahre erstreckenden Entwicklungszyklen. Die heterotrophen unterirdisch lebenden Gametophyten benötigen für Ihre Entwicklung Mykorrhizapilze. Funde von Gametophyten des Alpen-Flachbärlapps boten die Möglichkeit, den assoziierten Mykorrhizapilz morphologisch und genetisch zu untersuchen. Dieser wurde als zur Sebacinales-Gruppe B (Agariomycota) zugehörig identifiziert. Diese Pilzgruppe ist auch als Mykorrhizapartner von Ericaceen (Heidekrautgewächse) bekannt. Da keine Hinweise auf eine Mykorrhizierung des sporophytischen Bärlapp-Gewebes gefunden wurden, ist die Beziehung zwischen Pilz und Bärlapp möglicherweise nicht symbiotischer sondern parasitischer Natur. Der mykoheterotrophe Bärlapp-Gametophyt würde in diesem Fall epiparasitisch auf Vertretern der Ericaceen leben. Dies würde die regelmäßige Vergesellschaftung von Flachbärlappen mit verschiedenen Heidekrautgewächsen erklären. Eine ericoide Mykorrhiza bei Bärlappen, bestehend aus einem Netzwerk zwischen Ericaceen, Mykorrhizapilzen und Bärlapp-Gametophyten, wurde zuvor nicht beobachtet.
Die aktuelle Verbreitung der Flachbärlappe ist in den meisten Landesteilen Deutschlands und auch in einigen anderen Regionen Mitteleuropas weitgehend bekannt. Ihre früheren Arealbilder sind hingegen erst für Teilgebiete geklärt, was auf ihre schwierige Bestimmbarkeit und der über Jahrzehnte in der botanischen Literatur bestehenden taxonomischen Verwirrung zurückzuführen ist. Die frühere Verbreitung konnte auf der Basis kritischer Herbarrevisionen bislang für Niedersachsen und Bremen, Nordrhein-Westfalen, Thüringen und Teilgebiete Hessens und Bayerns rekonstruiert werden.
Die standortökologischen Ansprüche der Flachbärlapp-Sippen sind für Deutschland und einige Regionen angrenzender Länder hingegen gut untersucht. Es werden Sandböden mit unterschiedlich hohen Lehm- und Tonanteilen besiedelt, die relativ humusreich sind und größere Skelettanteile aufweisen können. Die Böden sind trocken bis frisch, reagieren sehr stark bis stark sauer (pH-Werte zwischen 2,9 und 4,5) und sind nährstoffarm (Stickstoffgehalte im Mittel zwischen 0,12 % und 0,25 %). Hinsichtlich ihrer Lichtansprüche unterscheiden sich die Flachbärlapp-Taxa erheblich. D. complanatum, D. tristachyum und ihre Hybride D. x zeilleri besiedeln recht heterogene Wuchsorte und sind sowohl an halbschattigen als auch lichtreichen Standorten zu finden (relativer Lichtgenuss meist zwischen 20 % und 80 %). D. alpinum und seine Hybriden D. x issleri und D. x oellgaardii bevorzugen dagegen offene Wuchsorte mit einem relativen Lichtgenuss zwischen 80 % und 100 %.
Die allermeisten Vorkommen von Flachbärlappen sind in Mitteleuropa heute an Sekundärstandorten anthropogenen Ursprungs zu finden. Primärstandorte stellen außerhalb des Alpenraumes die große Ausnahme dar. Die Vergesellschaftung der Flachbärlappe ist gut dokumentiert. Neben verschiedenen von Nadelhölzern dominierten Wald- und Forstgesellschaften (Leucobryo-Pinetum, Cladonio-Pinetum, Vaccinio myrtilli-Piceetum) treten sie in verschiedenen Vegetationstypen des Offenlandes mit lückiger und kurzrasiger Struktur auf (Vaccinio-Callunetum, Genisto anglicae-Callunetum, Violion- und Nardion-Gesellschaften, Festuca nigrescens-Agrostis capillaris-Bestände).
Die Flachbärlappe sind seit Jahrzehnten von einem dramatischen Bestandsrückgang betroffen und werden daher in den meisten nationalen Roten Listen Mitteleuropas als stark gefährdet oder sogar als vom Aussterben bedroht geführt. Hauptgrund ist das fast vollständige Verschwinden ihrer ehemaligen Lebensräume durch Aufgabe traditioneller Nutzungsformen und Änderungen in der forstlichen Bewirtschaftung. Die zunehmende Eutrophierung durch die ständig intensiver werdende Landwirtschaft stellt einen sukzessionsbeschleunigenden Faktor dar und bedingt, dass die Verweildauer eines Bestandes an einem Sekundärstandort ohne pflegende Eingriffe mittlerweile auf maximal 10 bis 15 Jahre gesunken sein dürfte. Allerdings lassen sich die Bestände durch das regelmäßige manuelle Entfernen bzw. Eindämmen pflanzlicher Konkurrenten stützen und ihre Überlebensdauer damit deutlich erhöhen, wie Erfahrungen im Rahmen diverser Artenhilfsprogramme in verschiedenen Teilen Deutschlands gezeigt haben. Auch die Flachbärlapp-Hybriden bilden langlebige und flächig ausgedehnte Klone aus und können fernab einer oder sogar beider Elternarten auftreten. Unabhängig von ihrer noch ungeklärten generativen Reproduktionsfähigkeit verhalten sie sich wie unabhängige Arten und sollten daher naturschutzfachlich auch als solche bewertet werden.
Der starke Rückgang sowie eine hohe internationale Verantwortlichkeit Deutschlands für einige Diphasiastrum-Taxa, speziell für D. x issleri und D. x oellgaardii, zeigen die dringende Notwendigkeit für gezielte Artenhilfsprogramme für diese faszinierende Pflanzengruppe.
Summary
Raised bogs are raised above the regional ground water level and only fed by rain. To be able to be ‘high yet wet’, they have developed intricate self-regulation mechanisms. The most important of these mechanisms in Sphagnum raised bogs is the acrotelm. This upper layer of peat and vegetation shows a distinct gradient from large pores at the top to small ones at the bottom. When the water table drops, water can only flow through small pores and run-off is effectively reduced. Still, the acrotelm has high storativity, which restricts water table fluctuations to this layer. The acrotelm presents a compromise between small pore space to minimise run-off and large pore space to maximise storativity.
These two ‘tasks’ of the acrotelm can also be split in horizontal space. The dry hummocks on the surface of a raised bog have much lower transmissivity and storativity than the wet hollows. These two surface elements can be organised in strikingly regular patterns of elongated strings of hummocks and so-called flarks of hollows arranged perpendicular to the slope. The origin of regular string-flark patterns was studied in chapter 2.
In a simple, heuristic, spatially explicit simulation model, each cell in a square grid is randomly declared either a hummock or a hollow. The grid is on a slope and water is allowed to flow from each cell to its four neighbouring cells until water tables stabilise. Then, every cells changes state based on its water table: if the water table is low, the cell will more likely be a hummock, if it is high a hollow. If the parameter settings are right, this procedure will result in regular striping patters. Chapter 2 was the first study to search the parameter space for settings that result in patterning. Systematic analysis showed that the parameter space in which patterns develop is sharply delineated, indicating positive feedback mechanisms. Once a pattern develops, water tables in the model diverge: the flarks become wetter and the strings become drier. The hummock and hollow cells have combined into higher order units, the strings and flarks, that emerge as more effective in regulating water flow.
Applying the same model for the first time to the dome shape of a raised bog (capther 3), pattern formation appeared to occur on three organisational levels. On the lowest nanotope level, we find strings and flarks, again combined in a string-flark complex, but this complex occurs alongside an all hummock rand and a wet, featureless central plateau. These three mire sites constitute the second, microtope level. On the third, mesotope level we can distinguish different types of bog domes that are defined by different combinations of mire sites. Classical literature on peatland classification used the same approach to classify bog domes, but also other and larger peatland areas. Our modelling shows that the mire sites actually exist as functional units in a self-organising bog and that they are not mere human classification constructs.
To test our ideas on self-regulation and -organisation as well as the modelling results, we studied a patterned raised bog in Tierra del Fuego in terms of its plant cover, its water and its peat (chapter 4). The studied bog is almost completely covered by Sphagnum magellanicum. In northern peatlands the different niches from high and dry hummock to low and wet hollow are filled by different species of Sphagnum, each with their specific growth form. In the studied bog, all niches from dry to wet are occupied by Spagnum magellanicum, showing a wide range in growth form. Yet, we found it has only limited genetic diversity that is not linked to these niches and growth forms.
Detailed measurements were made along a 498 m long transect crossing the bog, including water table measurements (every metre), vegetation relevés (2 × 2 m), hydraulic conductivity just below the water table (n = 246) and hydraulic conductivity in 11 depth profiles to a depth of 2 m (n = 291); the degree of humification of the corresponding peat was assessed in conjunction with the hydraulic conductivity measurements (n = 537). Sphagnum magellanicum moss samples were collected every 2 m along this transect and genotyped (n = 242). In addition, along short, 26 m long transects crossing strings and flarks water table and hydraulic conductivity just below the water table were measured every metre. Sphagnum growth forms were assessed and the vegetation of the entire bog was mapped in 10 × 10 m relevés (n = 3322). The simulation model was applied to a generalised form of the bog.
There was an almost perfect match between plant cover and water tables. As expected, hydraulic conductivity just below the water table was about 7 times lower in the dry than in the wet measurement spots. These observations are valid on the low level of the nanotope: hummocks and hollows or dry and wet spots in general. Other observations only made sense on higher organisational levels like the microtope. For example, the hydraulic conductivity profiles of the string-flark complex show a gentler gradient than those of the plateau and the rand. The peat in the string-flark complex originates on this level of organisation and combines characteristic of both its dry and wet constituents. On the mesotope level, the simulation model produced a good match with the patterns on the actual dome. We analysed the abundant data on different organisational levels ranging from small single plants to the large mire system of fens and domes of which the studied dome is part. We looked for commonalities and discrepancies to help us better understand how the close link between plants, water and peat functions in reality.
The results of all measurements were integrated with information from literature and discussed in the framework of a self-regulating and -organising raised bog. The field measurements considerably sharpened existing theoretical considerations. We identified nineteen hydrological feedback mechanisms. We found that the various mechanisms overlap both in space and time, which means there is redundancy in the self-regulation capacity of the system. Raised bogs, when in a natural state, are among the most resilient ecosystems known; resilience that is provided by feedbacks and back-up systems to these feedbacks.
We used our ideas and insights on self-regulation in Sphagnum raised bogs to look for similar patterns and responses in tropical domed peat swamps (chapter 5). We know that in Sphagnum raised bogs the tasks of the acrotelm can be split in horizontal space. When we looked at undisturbed tropical peat swamps with this new search image, we recognised how hummocks of root material and litter and particularly buttress roots regulate run-off and storage of water. We could identify several additional hydrological feedback loops that mirror the mechanisms found in Sphagnum raised bogs.
This thesis considerably advances our understanding of raised bogs as self-organising systems. The patterns and processes they display on multiple levels can be seen as a form of ecosystem diversity that exists independent of species and genetic diversity.
Peatlands are the most space-efficient terrestrial carbon sink on earth, storing more carbon than all other vegetation types in the world combined. The amount of carbon input into peatlands is determined by the primary production and decomposition of plants. The fragile relationship between these two processes is massively disturbed by intensive land use and the associated drainage of large peatland areas, releasing as much carbon dioxide annually as global air travel. Aiming for the substantial reduction of greenhouse gas emissions, rewetting measures have been initiated worldwide to protect and sustainably manage peatlands by restoring the waterlogged conditions required for peat formation. However, the increase in droughts across Europe adds another threat for peatlands by lowering water tables and affecting plant productivity, litter decomposition and phenology, which can reduce their potential for carbon storage.
Fens are minerotrophic peatlands that make up over a third of the peatland area in Europe. The growth and turnover of root biomass is particularly important for the formation and degradation of peat in fens; thus, a special focus should lie on root dynamics research. However, despite their pivotal role for peat formation, we still lack knowledge about root responses to environmental changes caused by rewetting or drought in fens. This thesis aims to advance our knowledge about root processes as well as their abiotic drivers in drained and rewetted fen peatlands of NE Germany, and how they may be affected by an extreme drought. For this purpose, destructive (i.e. in-growth cores, litter bags, soil coring) along with non-destructive measurements (i.e. minirhizotrons, NDVI) were used in situ in forested (alder forests) and graminoid-dominated (sedges and grasses) plant communities representative of the prevailing fen peatlands of Central Europe.
In this thesis, I investigate the environmental drivers of root growth (Chapters I-III), the annual production and decomposition (Chapter II), phenology and temporal dynamics of root growth (Chapters I and III), and the response of root biomass distribution and their functional traits to environmental changes linked to rewetting (Chapter IV). To understand the fundamental differences in productivity of plant communities on mineral and organic soils, above-and belowground phenology and their environmental drivers were compared among different temperate ecosystems (i.e. a beech forest, a forested peatland and two graminoid-dominated fen peatlands) in Central Europe (Chapter I). The study provides evidence that generalizations of aboveground to belowground production are not likely to reflect seasonal dynamics in temperate fen peatlands. Furthermore, the study shows that fine root production can be up to 10 times higher for peatland plant communities than for a beech forest on mineral soil, highlighting the importance of roots for contributing substantially to the formation of organic soils. By comparing annual productivity and decomposition between drained and rewetted fens, it is shown that rewetted fens maintained their productivity under the drought conditions experienced in Central Europe in the year 2018, leading to a higher carbon storage potential despite similar decomposition rates (Chapter II). A deeper understanding on the drivers of this high productivity in the rewetted sites is provided by the analysis of temporal dynamics of root growth and their potential abiotic drivers (Chapter III). Here, the important role of root phenology in the maintenance of productivity of rewetted fens under drought conditions is revealed, since higher root productivity in response to rewetting was driven by an extension of the growing season rather than through a higher growth rate (Chapter III). This thesis shows that rewetting can be beneficial for plant production under drought conditions, which is central to the maintenance of the carbon sink function of peatlands (Chapters II and III). Rewetting maintained high water tables, favouring a plant community adapted to water saturation and also to fluctuating environmental conditions, and thus a community able to cope with periodic water table drawdowns that might increase in the future. Contrarily, drainage caused water tables to constantly drop below rooting depth of plants that might be adapted to drier conditions, but not drought. To gain a deeper understanding of the changes that roots undergo with rewetting and their potential effects on soil carbon storage, a fourth study focuses on the changes in biomass distribution and functional traits of roots along the soil profile (Chapter IV). Together with root age determination the study indicates higher rates of carbon turnover in shallow soil layers and higher belowground carbon investments with rewetting compared to drainage in a forested peatland.
This thesis demonstrates that generalizations of phenological events from plant communities of mineral to organic soils, even though they face the same macroclimatic conditions, are misleading, as they are not subject of the same environmental controls (Chapter I). Rewetting of forest and graminoid-dominated fen peatlands supports their function as carbon sink by enhancing renewed carbon sequestration in form of root biomass (Chapters II-IV). Knowledge about root phenology is crucial to understand plant productivity of peatlands, one of the main drivers of organic matter accumulation (Chapter III). Even though roots are pivotal for mediating the input of carbon into the soil, their dynamics remain one of the least understood aspects of plant function. This thesis contributes to fill this knowledge gap by shedding light on root processes that contribute to the formation of peat and the complexity of the underlying abiotic drivers in rewetted and drained fens in face of a warmer and drier climate.
Forests are key biomes linked to biogeochemical cycles, important species reservoirs and major ecosystem services providers. The observed global climate change in the 20th century has the potential to deeply affect the conservation, functioning and structure of these ecosystems. Expressed as rising average temperatures due to the increase in atmospheric concentration of greenhouse gases such as carbon dioxide, nitrate oxide and methane, pollutants which are mostly product of burning fuel for industrial activities. These long-term changes will be heterogeneous in time and space throughout the globe. For northeastern Germany, predictions indicate that summer temperature and winter precipitation will be at a constant rise, whereas summer precipitation is expected to decrease, conditions will increase the risk of drought conditions. The changes in long-term means will be accompanied by increased frequency of weather extremes. The overall effect of climate change, both its long- and short-term components and their interaction with forest growth is uncertain. Tree
species in the temperate forest are highly adapted to seasonal growth, active in late-spring and summer when temperature thresholds activate primary and secondary growth as well as leaf development, given sufficient water availability. During winter, they become dormant as an strategy to decrease damage by freezing temperatures. These adaptations ultimately determine species distributions as they occur along climate gradients within their ecological
optima. Thus climate change can have a significant effect on species distribution ranges and more locally it can change species abundances. Trees being sessile organisms, possess limited dispersal capacities and rely on their adaptation potential, both genetically through selection over generations and through phenotypic plasticity (e.g. the capacity of adapting to changing conditions within a lifetime).
Tree growth can be explored by dendrochronological methods, that is, by analyzing traits of annual xylem bands as produced by the vascular cambium. These traits are width, wood anatomical properties (e.g. cell wall thickness, lumen diameter), and isotopic composition.
Tree-rings are integrators of environmental conditions and indicators of vitality and productivity of trees and forests. Studying these traits allows to understand the effect of climate on growth and physiological function over decadal to centennial scales in the past and by it inform about future growth performance. However, environmental information is not trivially extracted from tree-rings. Environmental signals in tree-rings are often the result of
complex interactions of lagged meteorological conditions and tree-scale characteristics such as size, canopy status (i.e. social status), competition and stand density, among other factors. For this reason the monitoring of secondary growth as it unfolds, for example through dendrometer monitoring (i.e. record of the stem-radial variations at intra-annual temporal scales) and repeated sampling for the study of xylogenesis, is of major importance to understand climate-growth relationships and bridge the gap between dendroecological analysis atdifferent ecological scales (from single trees to stands to populations). Therefore this thesis contains contributions a) to the understanding of long-term climate shifts and its effect on tree growth for species in the Central European temperate forests through dendrochronological assessments and contributions b) to understanding intra-annual growth dynamics and
its relationship to meteorological conditions through the analysis of monitoring records. In the retrospective analysis chapters (I-III), first an assessment was performed of the climate-growth relationships of important species of these region which indicated that deciduous species’ growth (Fagus sylvatica, Quercus robur and Q. petreae) was influenced mostly by summer water availability. For Pinus sylvestris was late spring temperature. Negative correlations between winter temperatures and growth indices of deciduous species increased over the last decades, possibly linked to less snow cover of the soil leading to root damage causing growth reductions. Scots pine presented the opposite, as positive correlations with winter temperatures became more frequent, indicating that this species’ growth rates might
benefit from an elongation of the vegetation period. Afterwards the effect of stand characteristics in the climate response was explored. The climate signal of solitary oak trees growing in northeastern Germany was compared to oaks in closed stands. Solitary trees
expressed higher growth rates and drought signals, which endanger its conservation as dry conditions are expected to increase in the region. As in the temperate forest crowding effects are variable throughout a tree’s lifetime, as well as other limiting factors (e.g. climate), we subsequently developed a methodology based on analysis of individual tree-ring series rather than chronologies (site means) to disentangle these effects on heterogeneous samples and quantify them. By sampling all present crown classes in a site near Rostock (Germany), we found beech was mostly affected by water availability in the previous summer
and this effect was well represented throughout the population. For oak the main climatic driver of growth was previous October temperature with a low representation throughout the obtained sample. For beech, the main trait governing the variability around the response to the main climate driver of growth was cambial age, and for oak was crown-projection/size. On the prospective analysis chapters (IV-VI), monitoring datasets from the years 2013-2019 were used for the analysis of meteorological forcing of dendrometer series, the effect of a multi-year drought event and for the development of a method to combine continuous dendrometer records with discrete histological observations from xylogenesis analysis. The analysis of meteorological forcing on stem-radial variations indicated all observed species (beech, oak, hornbeam in this case) respond similarly to atmospheric water content whereas
the growth phenology displayed contrasting species differences. These findings indicate high-frequency variations in stem dynamics are similar between species as it reflects transpiration and water transport in the stem, whereas the timing of growth reflects life strategies and
wood anatomical adaptations. Next we evaluated the effect of the consecutive drought years 2018-2019 using dendrometer data (beech, oak, hornbeam and sycamore maple). The increment levels after the onset of drought in 2018 were not reduced for the observed individuals, whereas in 2019 all species showed decreased growth levels, particularly beech. Most likely the water moisture reservoirs were adequately filled in winter and spring before summer 2018, which lead to increased buffer capacity to withstand the harsh conditions for radial growth. However in winter, and the spring before the summer of 2019, there was not sufficient precipitation which lead to less resistance to the second bought of the drought event.
This illustrates the complex lagged meteorological effect on radial growth, which is easily obscured in retrospective dendroecological analysis and emphasizes the pivotal role of soil moisture and soil water storage in tree-growth analysis. As a final contribution, while recognizing the importance of prospective growth monitoring, we developed a software tool to visualize and combine dendrometer stem-radial variations with images of histological events, such as those obtained by microcores for xylogenesis analysis. Growth signals in dendrometers are often of smaller magnitude than variations related to stem-water dynamics. By comparing them with histological images of wood-formation it is possible to accurately assign growth phases to dendrometer series and optimize their assessment. The advancement in methodological approaches to study intra-annual tree growth data is of major importance in the context of permanent ecological monitoring plots and its role in the assessment of the consequences of climate change on forest growth and conservation.
Overall the findings of this thesis indicate that climate change impacts in the temperate forest of Central Europe will be and have been varied depending on the species considered with stand, site and tree-level conditions strongly modulating its consequences and even direction. Deciduous species, particularly beech, will be at risk due to decreased water availability during summer for which beech shows a high sensitivity. While oak seems to
be less prone to drought related growth reductions and it is plausible to consider changes in dominance towards drier sites, it is still at risk if vulnerability thresholds are crossed. Scots pine appears to be favored by the increased temperatures during late winter, although these are naturally found on poor sites or sites either too dry or too wet for other dominant deciduous species to establish. Nevertheless, Scots pine has been planted on a variety of site conditions and especially in northeastern Germany is among the most widespread and economically important forest trees. Furthermore, the individual variability we have found in climate responses indicates that heterogeneous stands contain resilient sub-populations that
could guarantee survivorship of the species after stark changes in climate means. However, it appears that strong enough stressors such as hotter droughts can trigger wide ecosystem changes with more efficiency than shifts in climate means. Due to this intra-annual growth
monitoring is particularly relevant to foretell ecosystem changes and to understand the complex relationships found in climate-growth analysis performed in dendroecological studies, as it permits to mechanistically understand how conditions outside the tree-ring formation
period affects wood formation.