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Changes in the environment will alter the growth rate of trees and forests. Different disciplines assess such growth rates differently, for example, with tree-ring width data, forest inventories or with carbon-flux data from eddy covariance towers. Such data is used to quantify forests biomass increment, forest’s carbon sequestration or to reconstruct environmental variables before instrumental records. However, raw measurement data is typically not considered to be representative for the average growth rate of trees or forests. Depending on the research question, the effects of certain environmental variables or effects of tree and forest structure have to be removed first. It can be challenging to define and quantify a growth trend that can answer a specific research question because trees and forests grow and respond to environmental change in multiple ways simultaneously, for example, with altered radial increment, height growth, and stand density. Further challenges pose time-lagged feedback loops, for example, between height and radial increment or between stand density and radial increment. Generally, different environments will lead to different tree and forest structures, but because of tree’s longevity this adaptation to the new environment will take decades or even centuries. Consequently, there can be an offset between the present forest structure and what we term the potential natural forest (PNF): Similar to the potential natural vegetation (PNV), the PNF represents that forest that would develop under the current environmental conditions in the absence of human intervention. Because growth rates are affected by the tree and forest structure, growth-trend estimates will differ between the present and the potential forest. Consequently, if the legacy effects of the past are not of interest, the PNF is the theoretical baseline to correct and estimate growth trends.
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.
Diversified livelihoods combining farming, livestock keeping and non-farm income arecharacteristic of many rural households worldwide. For the Central Asian and Caucasian region,livestock keeping is especially important in terms of land use and socio-cultural heritage. We contributeto the literature with data from the under-researched Caucasus region and investigate: (i) the extent ofdiversification in smallholder households; (ii) the role of livestock keeping in diversification; (iii) theinfluence of household-specific and location-specific variables and diversification on householdincome. Based on a dataset of 303 households, we calculate contribution margins for the mainagricultural activities, household income, and diversification indices and analyze the influence ofdiversification, asset and location variables on household income with a regression model. Householdincome is generally diversified and a combination of four income sources (crops, livestock, poultry/beesand social benefits) was the most frequent. The econometric analysis shows that higher householdincomes are positively correlated with higher household land and livestock assets, the presence ofnon-farm work and social benefit income sources and with an increasing specialization as measuredby the diversification index. For enhancing rural household incomes and slowing down rural-urbanmigration, the development of non-farm job opportunities is recommended.
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.