PhD Thesis

Effects of climate change on plants and ecosystem functioning: Implications for managed temperate grasslands

Mohammed Arfin Khan (10/2011-09/2015)

Abstract:

Global climate change poses challenges to plants and ecosystem functioning. Grasslands have become a major study object in experimental biodiversity and climate impact studies. The great majority of the existing studies investigated the effects of climate change on productivity. However, studies on how climate change (such as 1000-year drought, high precipitation variability, seasonal warming, late frost in spring etc.) affects flowering phenology, plant physiology, community composition, legume facilitation, plant nitrogen (N) and soil N status in managed temperate grasslands are lacking. It is known that land management can improve performances of plants and ecosystem functions. Yet, the relative importance and potential of land management in buffering the negative impacts of climate change are largely unknown. In addition, the rain-out shelters used to study the ecological responses to climate change (mainly drought) are often criticized for creating micro-climatological artifacts, which may influence plant responses. Thus, the main objectives of this thesis were (a) to investigate how selected plants and ecosystems respond to different aspects of climate change (e.g. seasonal warming, precipitation variability, winter rain addition, late frost, heavy rainfall and drought), (b) to investigate three potential land management options to buffer the negative impacts of climate change, and (c) to contribute to the advancing of climate change research by examining whether there are any methodological artifacts in ongoing climate manipulations experiments. To meet these three objectives, responses (mainly related to phenology, productivity, physiology, seedling emergence and N status) of selected plant species, their populations, artificial plant communities as well as a semi-natural managed temperate grassland ecosystem were investigated. Seasonal (winter/summer) warming advanced flowering phenology and altered biomass production of early vs. late flowering species (manuscript 1). Onset of early flowering temperate grassland species was advanced by winter warming (4.9 days) more than by summer warming (2.3 days), while late flowering species were generally less sensitive to warming in either season. Flowering phenology was largely unaffected by experimental changes in precipitation regimes (manuscript 1). However, high precipitation variability during the growing season altered plant cover of early vs. late flowering species. Ecosystem productivity and legume facilitation increased under heavy rainfall compared to control (manuscript 2). Drought reduced plant physiological activities e.g. lower stomatal conductance, lower effective quantum yield, and lower leaf water potential (manuscript 6). Drought effects on plants were altered by the presence of legume species (manuscript 2). Under drought, the presence of a legume species enhanced overall biomass production of three neighboring grassland species by 36% compared to the absence of legume. Species-specific legume facilitation effects were also detected: Arrhenatherum elatius was facilitated by legume presence under drought and heavy rainfall, Plantago lanceolata was facilitated only under heavy rainfall, and Holcus lanatus was facilitated only under control conditions. Positive effects of legume presence found under control also persisted under drought for plant and soil N. European populations/provenances of grass species differed in plant N status under drought. Yet, populations from the wetter sites did not perform worse than presumably drought-adapted populations, indicating no evidence of local adaptation (manuscript 3). Variation in within-species responses was as high as variation in among-species responses under drought and late frost (manuscript 5). Within-species variation during the early life stages of Verbascum thapsus populations (a global plant invader) was detected as different germination and seedling emergence rates under the representative climates of seven biomes (manuscript 4). Furthermore, plant N status was altered by rewetting and harvest delay after drought (manuscript 3). Harvest delay after rewetting could not compensate the negative effects of drought on biomass production, but increased plant N concentration and N content. A detailed quantification of micro-climatological artifacts showed that the strength of drought manipulation using the rain-out shelter technique was dependent on ambient weather conditions (manuscript 6). Plant responses were highly correlated to ambient micro-climate conditions. Therefore, relating drought responses to ambient micro-climatological parameters such as air temperature and vapor pressure deficit can facilitate meaningful interpretation and comparison of studies and of different responses of experimental droughts between years within single studies. Furthermore, rain-out shelters altered temperature and reduced radiation inside the shelter. However, these micro-climatological artifacts had no significant effects on growth responses of grassland plants. Thus, fixed rainout shelters remain a useful tool for ecological drought manipulation experiments. In summary, the present thesis provides evidence on how climate change affects selected plant species and ecosystem functions in managed temperate grasslands. The findings of this thesis have practical implications for grassland ecosystem management in the face of climate change. For instance, negative drought effects can be minimized by legume presence and by rewetting combined with harvest delay. Results show strong differences in population-specific responses to extreme climatic conditions. However, climatic origin of populations cannot predict these response variations. Therefore, increasing within-species diversity (or population mixtures) may help maintain plant productivity and N nutrition in the face of climate change.