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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.
To understand the resilience of African savannas to global change, quantitative information on the long-term dynamics of vegetation is required. Past dynamics can be reconstructed with the REVEALS model, which requires pollen productivity estimates (PPE) that are calibrated using surface pollen and vegetation data. Here we calculated PPE values for five savanna taxa using the extended R-value (ERV) model and two pollen dispersal options: the Gaussian plume model (GPM) and the Lagrangian stochastic model (LSM). The ERV calculations failed to produce a reliable PPE for Poaceae. We therefore used Combretaceae as the reference taxon – although values obtained with Poaceae as the reference taxon are presented in the supplement. Our results indicate that Combretaceae is the taxon with the highest pollen productivity and Grewia the taxon with the lowest productivity. Acacia and Dichrostachys are intermediate pollen producers. We find no clear indication of whether the GPM PPEs or the LSM PPEs are more realistic, but the differences between these values confirmed that the pollen fall speed has a greater effect in the modelling of GPM than in the LSM. We also applied REVEALS to the pollen record of Lake Otjikoto (northern Namibia) and obtained the first quantitative reconstruction of the last 130 years of vegetation history in the region. Cover estimates for Poaceae indicate the predominance of a semi-open landscape throughout the 20th century, while cover values below 50% since the 21st century correspond to a thick savanna. This change in grass cover is associated with the spread of Vachellia, Senegalia and Grewia reflecting an encroached state.
Duckweeds (Lemnaceae) are the smallest and fastest-growing angiosperms. This feature, together with high starch production and good nutritional properties, makes them suitable for several applications, including wastewater treatment, bioenergy production, or feed and food supplement. Due to their reduced morphology and great similarity between diverse species, taxonomic identification of duckweeds is a challenging issue even for experts. Among molecular genotyping methods, DNA barcoding is the most useful tool for species identification without a need for cluster analysis. The combination of two plastid barcoding loci is now considered the gold standard for duckweed classification. However, not all species can be defined with confidence by these markers, and a fast identification method able to solve doubtful cases is missing. Here we show the potential of tubulin-based polymorphism (TBP), a molecular marker based on the intron length polymorphisms of β-tubulin loci, in the genomic profiling of the genera Spirodela, Landoltia, and Lemna. Ninety-four clones were analyzed, including at least two representatives of each species of the three genera, with a special focus on the very heterogeneous species Lemna minor. We showed that a single PCR amplification with universal primers, followed by agarose gel analysis, was able to provide distinctive fingerprinting profiles for 10 out of 15 species. Cluster analysis of capillary electrophoresis–TBP data provided good separation for the remaining species, although the relationship between L. minor and Lemna japonica was not fully resolved. However, an accurate comparison of TBP profiles provided evidence for the unexpected existence of intraspecific hybrids between Lemna turionifera and L. minor, as further confirmed by amplified fragment length polymorphism and sequence analysis of a specific β-tubulin locus. Such hybrids could possibly correspond to L. japonica, as originally suggested by E. Landolt. The discovery of interspecific hybrids opens a new perspective to understand the speciation mechanisms in the family of duckweeds.
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.
Due to climate change, economically important crop plants will encounter flooding periods causing hypoxic stress more frequently. this may lead to reduced yields and endanger food security. As roots are the first organ to be affected by hypoxia, the ability to sense and respond to hypoxic stress is crucial. At the molecular level, therefore, fine-tuning the regulation of gene expression in the root is essential for hypoxia tolerance. Using an RnA-Seq approach, we investigated transcriptome modulation in tomato roots of the cultivar ‘Moneymaker’, in response to short- (6 h) and long-term (48 h) hypoxia. Hypoxia duration appeared to have a significant impact on gene expression such that the roots of five weeks old tomato plants showed a distinct time-dependent transcriptome response. We observed expression changes in 267 and 1421 genes under short- and long-term hypoxia, respectively. Among these, 243 genes experienced changed expression at both time points. We identified tomato genes with a potential role in aerenchyma formation which facilitates oxygen transport and may act as an escape mechanism enabling hypoxia tolerance. Moreover, we identified differentially regulated genes related to carbon and amino acid metabolism and redox homeostasis. Of particular interest were the differentially regulated transcription factors, which act as master regulators of downstream target genes involved in responses to short and/or long-term hypoxia. Our data suggest a temporal metabolic and anatomic adjustment to hypoxia in tomato root which requires further investigation. We propose that the regulated genes identified in this study are good candidates for further studies regarding hypoxia tolerance in tomato or other crops
Abstract
Individuals of the marine chelicerate lineage Pycnogonida (sea spiders) show considerable regenerative capabilities after appendage injury or loss. In their natural habitats, especially the long legs of sea spiders are commonly lost and regenerated, as is evidenced by the frequent encounter of specimens with missing or miniature legs. In contrast to this, the collection of individuals with abnormally developed appendages or trunk regions is comparably rare. Here, we studied a remarkable malformation in a postlarval instar of the species Phoxichilidium femoratum (Rathke, 1799) and describe the external morphology and internal organization of the specimen using a combination of fluorescent histochemistry and scanning electron microscopy. The individual completely lacks the last trunk segment with leg pair 4 and the normally penultimate trunk segment bears only a single aberrant appendage resembling an extension of the anteroposterior body axis. Externally, the proximal units of the articulated appendage are unpaired, but further distally a bifurcation into two equally developed leg‐like branches is found. Three‐dimensional reconstruction of the musculature reveals components of two regular leg muscle sets in several of the proximal articles. This confirms interpretation of the entire appendage as a malformed leg and reveals an externally hidden paired organization along its entire proximodistal axis. To explain the origin of this unique malformation, early pioneering studies on the regenerative potential of pycnogonids are evaluated and (a) an injury‐induced partial fusion of the developing limb buds of leg pair 3, as well as (b) irregular leg regeneration following near complete loss of trunk segments 3 and 4 are discussed. Which of the two hypotheses is more realistic remains to be tested by dedicated experimental approaches. These will have to rely on pycnogonid species with established laboratory husbandry in order to overcome the limitations of the few short‐term regeneration studies performed to date.