Phenotypic trait variation in forest herbs along environmental gradients in the context of climate change

Abstract

Temporal and spatial environmental variation affects every organism in manifold ways. Especially for sessile life forms like plants it poses a big challenge to deal with fluctuations in their environment, since they are hardly able to quickly escape unfavourable conditions. Recently, it has been shown that the combination of several human-induced, rapid changes in the environment (known as global environmental change) represents large novel selective pressures to which plants need to respond and possibly adapt. Amongst others, the rapid world-wide increase in temperature is assumed to have major ecological and evolutionary consequences being a major threat to species diversity. Hence, it is of great importance to get deeper knowledge about the ability of plant species to react to environmental changes, especially in climate. One way of plant populations to avoid extinction is tracking the shift in climate either in time or space (e.g. shifts in phenology or to higher latitudes / altitudes). Such a response, however, may be impeded by low migration rates of many species and / or current human-induced habitat fragmentation (possibly combined with reduced genetic variation in fragmented populations). Thus, the ability of plant species to adapt themselves to new climatic conditions, via a combination of adaptive and non-adaptive phenotypic plasticity, genetic diversity or local adaptation, may provide an alternative vital strategy for their survival and long-term persistence. To examine the response of plant species to different environmental conditions, fitness-related plant functional traits are commonly used. So far, the variability of these traits within a species (i.e. intraspecific variation), however, has been mostly neglected. Still, there is recent awareness that i) intraspecific variation may differ substantially across different scales (among regions, between populations, within populations or even within individuals) and ii) that it may reflect the general potential of a species' ability to adapt to a new environment via phenotypic plasticity, genetic diversity or local adaptation. Therefore, the major objective of this study was to enlighten patterns and underlying causes of intraspecific variation within and among populations of temperate forest under-storey herbs with regard to different environmental conditions and climatic variation in particular. The forest understorey comprises mainly specialist species, which may be notably threatened by global change due to their low migration rates and colonization success. These species may thus especially rely on their ability to adjust themselves to a changing environment. Overall, phenotypic responses differed between species and plant functional types used in this study. Therefore, impacts of global change on different species are likely diverse and will potentially strongly affect forest community structures. Further research integrating experimental studies and examination of numerous species in natural plant communities and especially the consideration of several (human-induced) environmental changes is required to allow generalization and more explicit prospects for specific species. In summary, this thesis emphasizes the importance of taking intraspecific variation into account when using plant functional traits as proxies for species performance related to environmental variation.