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Abstract
In the 21st century, most of the world’s glaciers are expected to retreat due to further global warming. The range of this predicted retreat varies widely as a result of uncertainties in climate and glacier models. To calibrate and validate glacier models, past records of glacier mass balance are necessary, which often only span several decades. Long-term reconstructions of glacier mass balance could increase the precision of glacier models by providing the required calibration data. Here we show the possibility of applying shrub growth increments as an on-site proxy for glacier summer mass balance, exemplified by Salix shrubs in Finse, Norway. We further discuss the challenges which this method needs to meet and address the high potential of shrub growth increments for reconstructing glacier summer mass balance in remote areas.
Many of the world’s most biodiverse regions are found in the poorest and second most populous continent of Africa; a continent facing exceptional challenges. Africa is projected to quadruple its population by 2100 and experience increasingly severe climate change and environmental conflict—all of which will ravage biodiversity. Here we assess conservation threats facing Africa and consider how these threats will be affected by human population growth, economic expansion, and climate change. We then evaluate the current capacity and infrastructure available to conserve the continent’s biodiversity. We consider four key questions essential for the future of African conservation: (1) how to build societal support for conservation efforts within Africa; (2) how to build Africa’s education, research, and management capacity; (3) how to finance conservation efforts; and (4) is conservation through development the appropriate approach for Africa? While the challenges are great, ways forward are clear, and we present ideas on how progress can be made. Given Africa’s current modest capacity to address its biodiversity crisis, additional international funding is required, but estimates of the cost of conserving Africa’s biodiversity are within reach. The will to act must build on the sympathy for conservation that is evident in Africa, but this will require building the education capacity within the continent. Considering Africa’s rapidly growing population and the associated huge economic needs, options other than conservation through development need to be more effectively explored. Despite the gravity of the situation, we believe that concerted effort in the coming decades can successfully curb the loss of biodiversity in Africa.
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.
Changing climate can strongly affect tree growth and forest productivity. The dendrochronological approach to assessing the impact of climate change on tree growth is possible through climate–growth correlation analysis. This study uses an individual tree-based approach to model Pinus wallichiana (P. wallichiana) radial growth response to climate across the physiographic gradients in the lower distributional range of Nepal. This study sampled six sites across the Makwanpur district of central Nepal that varied in elevation and aspect, obtaining 180 tree-ring series. Climate data series were obtained from Climate Research Unit (CRU 4.0). The pair correlation approach was used to assess P. wallichiana growth response to climate and site-level physiographic variables such as site-level environmental stress. The study also determined long-term growth trends across the elevation and aspect gradients. Trees at sites with higher elevation and northeast aspect (NEA) were more responsive to winter and spring precipitation, whereas trees with lower elevation and northwest aspect (NWA) were more responsive to winter and spring precipitation. Basal area increment (BAI) analysis showed the variation of growth at site-level environmental stress, suggesting that the sensitivity of forest ecosystems to changing climate will vary across the lower growth limit of P. wallichiana due to differences in local physiographic conditions.
Abstract: The Arctic has experienced a pronounced increase in air temperature over the last four decades, with an average increase of 0.4 °C per decade and thus an increase of almost the double rate than that of temperate regions. Remote sensing studies and repeat photography of historical images have shown large-scale increases of plant productivity in tundra ecosystems over the same time period. A pronounced size, abundance and biomass increase of shrubs has been observed. This so called shrub expansion has important repercussions for the vegetation, the animals, the soil, the energy and the carbon balance of the Arctic tundra and on regional and global climate. As the comparison of historical photographs with recent photographs has shown, this shrub expansion occurs on different temporal and spatial scales with areas of strong increase in shrub cover (expanding patches) and areas without noticeable changes in shrub vegetation (stable patches). While remote sensing approaches for the detection of changes in vegetation are limited in their temporal coverage and so far also in their resolution, historical photographs with high resolution are often not available. Experimental studies have shown that an increase in nutrients or temperature often resulted in increased shrub biomass, but findings were partly contradictory, referred to short term observations and usually confined to small areas. To bridge the gap between spatially limited plot-scale experiments and global large-scale assessment of plant productivity by satellite derived pictures, dendrochronology was used in this thesis to analyze the drivers for and the rate of shrub growth of different widespread evergreen and deciduous shrub species in alpine and arctic tundra and to reconstruct historic environmental conditions. In detail, this doctoral thesis was conducted to study shrub growth and to assess the applicability of traditional dendrochronological methods on shrubs that had been so far mainly applied to trees and to test whether shrubs differed morphologically from trees. Further, I was determined to look for evidence for a possible Scandinavian shrub range expansion and to assess which climatic factors – temperature, precipitation or snow – influenced shrub growth significantly. Moreover, we aimed to find the reason for the observed heterogeneity of the shrub expansion on the landscape and its relevance for the three most common shrubs on the Alaskan tundra. The methods applied followed the routines usually applied for dendrochronological analyses of treerings, with the exception that usually several stem discs of the main stem were analyzed and frequently had to be prepared with help of a microtome as thin-sections, that were stained and sealed on a coverglass before annual shrubrings were measured. The averaged shrubring widths were then compared with environmental factors through correlation and regression methods. This thesis gives first a general introduction to climate change in the Arctic, shrub expansion on the tundra, the scientific discipline of dendrochronology or -ecology on shrubs and its development, the main research questions and the thesis outline. Then seven research papers are presented and the main results and conclusions are synthesized and discussed and finally possible venues of future research are outlined. The most important insights gained from this thesis are the following: I) Dendroecological methods can be applied to shrubs. Insights into shrub morphology have been gained by detecting an interesting mechanism for coping with adverse environmental conditions of both, trees and shrubs that can save resources by confining the production of wood to the upper parts of the stem. II) Further, I found evidence for a shrub expansion in Scandinavia. III) I could establish the causal link between the current climate warming and increased radial and vertical shrub growth by identifying summer temperature as main driver for shrub growth. IV) Results from the Alaskan tundra indicate a strongly adverse role of snow for shrub growth in stable patches, refuting the popular snow-shrub-microbe hypothesis for this extensive area across species. The differing influence of snow is likely linked to the presence of permafrost and shallow active layers and the snow’s contribution to moist or even anoxic conditions in Alaska. V) Furthermore, we found that the different rates and the spatial heterogeneity of shrub expansion are accompanied by strong differences in the surrounding vegetation composition and the soil parameters of expanding (accustomed to more favorable conditions) and stable shrub patches. VI) These differences are predisposed by shrub patch position within the landscape, comprising different levels and rates of disturbance. VII) Additionally, shrub ring records were successfully used as natural archives to model past temperature dynamics respectively summer glacier mass balance with high accuracy. VIII) Finally, a synthesis of the climate-growth relationships of shrubs of more than 25 sites around the Arctic as joined effort together with other leading shrub researchers supports the presence of a circumpolar shrub expansion, gives recommendations for methods used in shrub dendroecology and lays out future research directions. The findings of my dissertation research show that the analysis of shrubs by dendroecological methods yields highly interesting results, and they greatly improved our understanding of factors that influence individual shrub growth, the reconstruction of earlier environmental conditions as well as the reconstruction and assessment of plant population dynamics.
Abstract
The role of future forests in global biogeochemical cycles will depend on how different tree species respond to climate. Interpreting the response of forest growth to climate change requires an understanding of the temporal and spatial patterns of seasonal climatic influences on the growth of common tree species. We constructed a new network of 310 tree‐ring width chronologies from three common tree species (Quercus robur, Pinus sylvestris and Fagus sylvatica) collected for different ecological, management and climate purposes in the south Baltic Sea region at the border of three bioclimatic zones (temperate continental, oceanic, southern boreal). The major climate factors (temperature, precipitation, drought) affecting tree growth at monthly and seasonal scales were identified. Our analysis documents that 20th century Scots pine and deciduous species growth is generally controlled by different climate parameters, and that summer moisture availability is increasingly important for the growth of deciduous species examined. We report changes in the influence of winter climate variables over the last decades, where a decreasing influence of late winter temperature on deciduous tree growth and an increasing influence of winter temperature on Scots pine growth was found. By comparing climate–growth responses for the 1943–1972 and 1973–2002 periods and characterizing site‐level growth response stability, a descriptive application of spatial segregation analysis distinguished sites with stable responses to dominant climate parameters (northeast of the study region), and sites that collectively showed unstable responses to winter climate (southeast of the study region). The findings presented here highlight the temporally unstable and nonuniform responses of tree growth to climate variability, and that there are geographical coherent regions where these changes are similar. Considering continued climate change in the future, our results provide important regional perspectives on recent broad‐scale climate–growth relationships for trees across the temperate to boreal forest transition around the south Baltic Sea.
Summary
Sphagnum farming can substitute peat with renewable biomass and thus help mitigate climate change. Large volumes of the required founder material can only be supplied sustainably by axenic cultivation in bioreactors.
We established axenic in vitro cultures from sporophytes of 19 Sphagnum species collected in Austria, Germany, Latvia, the Netherlands, Russia, and Sweden: S. angustifolium, S. balticum, S. capillifolium, S. centrale, S. compactum, S. cuspidatum, S. fallax, S. fimbriatum, S. fuscum, S. lindbergii, S. medium/divinum, S. palustre, S. papillosum, S. rubellum, S. russowii, S. squarrosum, S. subnitens, S. subfulvum and S. warnstorfii. These species cover five of the six European Sphagnum subgenera; namely, Acutifolia, Cuspidata, Rigida, Sphagnum and Squarrosa.
Their growth was measured in suspension cultures, whereas their ploidy was determined by flow cytometry and compared with the genome size of Physcomitrella patens. We identified haploid and diploid Sphagnum species, found that their cells are predominantly arrested in the G1 phase of the cell cycle, and did not find a correlation between plant productivity and ploidy. DNA barcoding was achieved by sequencing introns of the BRK1 genes.
With this collection, high‐quality founder material for diverse large‐scale applications, but also for basic Sphagnum research, is available from the International Moss Stock Center.
Coastal sand dunes near the Baltic Sea are a dynamic environment marking the boundary between land and sea and oftentimes covered by Scots pine (Pinus sylvestris L.) forests. Complex climate-environmental interactions characterize these ecosystems and largely determine the productivity and state of these coastal forests. In the face of future climate change, understanding interactions between coastal tree growth and climate variability is important to promote sustainable coastal forests. In this study, we assessed the effect of microsite conditions on tree growth and the temporal and spatial variability of the relationship between climate and Scots pine growth at nine coastal sand dune sites located around the south Baltic Sea. At each site, we studied the growth of Scots pine growing at microsites located at the ridge and bottom of a dune and built a network of 18 ring-width and 18 latewood blue intensity chronologies. Across this network, we found that microsite has a minor influence on ring-width variability, basal area increment, latewood blue intensity, and climate sensitivity. However, at the local scale, microsite effects turned out to be important for growth and climate sensitivity at some sites. Correlation analysis indicated that the strength and direction of climate-growth responses for the ring-width and blue intensity chronologies were similar for climate variables over the 1903–2016 period. A strong and positive relationship between ring-width and latewood blue intensity chronologies with winter-spring temperature was detected at local and regional scales. We identified a relatively strong, positive influence of winter-spring/summer moisture availability on both tree-ring proxies. When climate-growth responses between two intervals (1903–1959, 1960–2016) were compared, the strength of growth responses to temperature and moisture availability for both proxies varied. More specifically, for the ring-width network, we identified decreasing temperature-growth responses, which is in contrast to the latewood blue intensity network, where we documented decreasing and increasing temperature-growth relationships in the north and south respectively. We conclude that coastal Scots pine forests are primarily limited by winter-spring temperature and winter-spring/summer drought despite differing microsite conditions. We detected some spatial and temporal variability in climate-growth relationships that warrant further investigation.
1. Anthropogenic climate change is a substantial threat to global biodiversity. It may affect insect herbivores directly and indirectly. Indirect effects are, among others, mediated by climate‐change induced variation in host‐plant quality. Although being potentially important, little is known on the significance of such indirect effects and on interactions among environmental stressors in plant–herbivore interactions.
2. To simulate the potential impact of climate change, we investigated effects of host‐plant temperature and soil moisture on herbivore performance in the tropical butterfly Bicyclus anynana under laboratory conditions.
3. Maize grown at high temperatures or under wet conditions reduced herbivore performance, indicated by decreased body mass, storage reserves, phenoloxidase activity, and increased development time. Temperature and soil moisture acted largely independent of one another. Detrimental effects of the high plant temperature were restricted to males, indicating a higher vulnerability of this sex to environmental stress.
4. In nature, B. anynana might be threatened by increasing temperatures during the wet season negatively affecting host‐plant quality. Our study shows that herbivore performance can be substantially affected by indirect effects mediated through changes in host‐plant quality, which deserves more attention in the current era of global climate change.
Tree growth in northern and upper treeline ecotones of the circumpolar boreal forest is
generally limited by temperature, i.e., trees grow generally more under warm, and less under
cold climatic conditions. Based on the assumption that this relationship between tree growth
and climate is linear and stable through time, dendroclimatologists use tree rings as natural
archives to reconstruct past temperature conditions. Such tree-ring based reconstructions,
together with other natural archives (e.g., ice cores and pollen), constitute our understanding of
past climatic conditions that reach beyond modern instrumental records.
However, a steadily increasing amount of studies reports a recent reduction or loss of the
summer temperature signal for several species and sites of the boreal forest. Such a reduction
of temperature sensitivity results in temporally unstable climate-tree growth relationships,
which challenges the work of dendroclimatologists by potentially leading to miscalibrations of
past climatic conditions. On the upside, this shift in the trees’ climate sensitivity might point to
a shift in tree growth-limiting factors and thus serve as an early indicator of climate change
impacts. There is evidence that this recent reduction in temperature sensitivity might be caused
by the observed strong temperature increase at high latitudes, and thus temperature-induced
drought stress. Other potential drivers and amplifiers of this phenomenon are differing microsite
conditions (dry vs. wet soils) and factors inherent to trees, like genetic properties or age
effects.
In this PhD thesis, I systematically assessed the effects of frequently discussed drivers of
unstable climate-tree growth relationships (climate change, micro-site effects, genetical
predisposition) on two representative species of the boreal forest, white spruce in North
America and Scots pine in Eurasia, across various temporal and spatial scales. I used classical
(tree-ring width) and more novel (wood density, quantitative wood anatomy)
dendrochronological proxies to unravel the effects from annual to sub-monthly resolution.
More precisely, in chapter I, white spruce clones were compared to non-clones at two treeline
sites in Alaska to test whether their growth patterns differ, and whether white spruce clones are
generally suitable for dendroclimatic assessments. Clonal reproduction is frequent at treeline
due to harsh conditions, but might lead to competition among individuals due to the close
proximity among each other, which in turn might obscure their climatic signal. Second, I tested
the effect of warmer and drier climatic conditions on the summer temperature signal of Scots
pine in Eurasia (chapter II) and on the growing season moisture signal of white spruce in North
America (chapter III), respectively. Temperature-induced drought stress is expected to be the
most important driver of unstable climate-growth relationships in the boreal forest. I included
several sites across latitudinal (50-150 km) and longitudinal (1,000-2,200 km) gradients to
cover large parts of the species’ distribution ranges. Since Scots pine covers a wide range of
ecological habitats, I additionally tested the effect of dry and wet micro-site conditions on the
summer temperature signal of Scots pine in chapter II. Finally, in chapter IV, a systematic
literature review was carried out in order to investigate the distribution of unstable climategrowth
relationships in global tree-ring studies, and the usage of such series in climate
reconstructions. Furthermore, the scientific impact of these potentially inaccurate climate
reconstructions was assessed.
In this PhD project, warmer and drier climatic conditions led to temporally unstable climate
signals in both Scots pine (chapter II) and white spruce (chapter III), as expected. Unstable
climate-growth relationships were found for all tested tree-ring proxies and at all sites in North
America, and at most sites in Eurasia. Micro-site effects (chapter II) and clonal growth
(chapter I) had no significant effect on the climate sensitivity and high-frequency variability
of the tested species, but affected absolute growth. The review (chapter IV) revealed that the
phenomenon of unstable climate-growth relationships is globally widespread, and occurs
independent of tree species, geographic location, and tree-ring and climate proxies. While
reconstructions inferred from these unstable relationships are frequent and respective papers
have a high impact, the tree-ring community seems to increasingly recognize the challenge of
unstable climate-growth relationships.
With these findings, this PhD project helped to shed more light on the frequency, underlying
drivers, and the impact of unstable climate-growth relationships in boreal forest trees, as well
as underlying reaction processes in trees. Above all, this PhD project suggests that the loss of
climate sensitivity is caused by a change of growth limiting factors: temperature limitation
seems to be suspended in warmer and drier years for Scots pine in Eurasia, and moisture
limitation first arises under warm/dry conditions for white spruce in North America. Due to
plastic growth responses in trees, the general assumption in dendroclimatology – that climategrowth
relationships are stable through time – seems to be incompatible with the principle of
limiting factors (one factors is always most growth limiting).
To improve the validity of future climate reconstructions, statistical approaches considering
synchronously or changing climatic limiting factors need to be promoted, along with attempts
to select the best responding trees from a dataset. Furthermore, a better understanding of nonclimatic
factors potentially affecting tree growth (e.g., age, disturbance, soil parameters) is
needed. A growth reduction of mature and dominant white spruce trees sampled in this PhD
project seems likely under future warming conditions, with series of wood cells being valuable
early indicators of climate change effects in white spruce. However, inferences cannot be
extended to the entire stand due to the applied sample design. Projected climate warming will
probably lead to a further reduction of the summer temperature signal in trees of the northern
boreal forest, while wider consequences for forest growth and productivity are unclear.