Climate change in experimental ecology - The impact of warming and drought on key European tree, heath, and grassland species
During climate change rising mean air temperatures will be accompanied by climatic extremes such as drought. The frequency and magnitude of those extreme weather events are expected to increase in the future. Additionally, climatic extremes may have stronger effects on plants than a gradual temperature shift. Therefore, investigations of plant response to extreme weather events are needed to understand ecological and economic implications better. Apart from the consequences of climate change for plants, this thesis also asks the question: Can we simulate global warming reliably with our available warming techniques? In particular, the work in hand is intended to give answers to four research questions in this context (corresponding to four manuscripts). 1. How reliable are our experimental evidences and techniques (passive warming versus warming by transplantation) (manuscript 1)? 2. Is the leaf palatability of Quercus pubescens influenced by warming and drought (manuscript 2)? 3. Does drought response of Fagus sylvatica differ between central and marginal provenances (manuscript 3)? 4. Does stress response of plant communities and species (heath and grassland) differ if drought reoccurs (manuscript 4)? Concerning the first of the above stated research questions, the comparison of the two warming techniques "passive warming" and "warming by transplantation" revealed that the reliability of the experimental results and therefore the resulting predictions for future plant response depend on the chosen technique. Experimental passive warming techniques are complicated in execution and should consider further parameters besides temperature increases such as: evapotranspiration, photosynthetically active radiation, and wind speed in order to improve the knowledge of warming technique effects (manuscript 1). The climatic manipulations "warming" and "drought" were simulated in the EVENT 3 experiment to investigate their impact on the leaf palatability of Quercus pubescens of four European provenances from Germany, Bulgaria, Hungary, and Italy (second research question). This was done in a palatability experiment using the leaf consumption of the forest pest Lymantria dispar (gypsy moth) as response parameter (manuscript 2). Surprisingly, "warming" and "drought" did not effect the leaf palatability. However, leaf palatability was negatively correlated with an increasing density of trichomes (i.e. leaf hairs) and Bulgarian leaves were preferred compared to Hungarian and Italian oak leaves. This study suggests that the attractiveness of the four tested European Q. pubescens provenances might be stable in a changing climate at least as far as L. dispar is concerned. Regarding the third research question, beech seedlings originating from three marginal (Bulgaria, Spain, and Poland) and three central (Germany) Fagus sylvatica provenances out of its geographic range were exposed to drought in two different soil types (sand, loam) (manuscript 3). Drought had a negative impact on all tree growth parameters, especially in sandy substrate. The provenances differed in their response to drought and soil type, but marginal F. sylvatica provenances generally did not withstand drought exposure better than central provenances. The performance of the Bulgarian F. sylvatica provenance might indicate a trade-off between growth and drought tolerance, as the overall total increment was low but stable under drought conditions. Furthermore, local adaptation to summer drought could be detected with respect to mortality. Regarding forest management, focusing only on drought-resistant marginal provenances might not be beneficial, as those provenances could be less adapted to other climatic parameters. Therefore, intermixing Central European forests with drought-resistant tree provenances might be an option to ensure forest resistance and yield in the future. Beside the impact of warming and drought on tree species, the fourth research question assessed the response of temperate grassland and heath communities as well as single species to a very severe drought event (more than half of the growing season) (manuscript 4). The plants experienced previously within their life history different precipitation manipulations ("drought", "heavy rain", "regular watering", and "ambient control" conditions) in addition to natural drought occurrences in the EVENT 1 experiment over a six-year period. Differences in drought stress resistance and the influence of plant community composition on the ecological stress memory of single plant species were tested. Recurrent mild drought stress over several years seems to improve plant resistance and thus lead to less tissue die-back compared to plants with hardly any drought experience in the face of a very severe drought. Additionally, plant community composition might have an influence on the ecological stress memory of single plant species. To sum up, the findings of this dissertation showed that the design of the chosen warming technique has to be thoroughly considered to make reliable predictions of plant response to climate change. Moreover, warming and drought did not impact the leaf palatability of different Q. pubescens provenances. On the other hand, drought influenced the growth of central and marginal F. sylvatica provenances negatively. Beyond that, drought occurrences do not have to be extreme themselves to promote drought resistance of heath and grassland communities and single species.
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