title: Comparative drought resistance of temperate grassland species : mechanisms, prediction and relation to species distribution across moisture gradients
Creators: name: Sun, Shanwen
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: drought resistance
subject: functional traits
subject: species distribution
subject: grasslands
subject: grass
subject: forb
subject: desiccation avoidance
subject: desiccation tolerance
subject: 550

description: Grasslands are important ecosystems worldwide, exhibiting high biodiversity and providing important ecosystem services. In Europe, they occupy more than one-third of the total agricultural land area. However, grasslands experience periodical drought during the growing season, which reduces their productivity, influences species abundance and distribution, and leads to changes in grassland functions. Climate change is expected to increase the intensity and frequency of drought and thus intensify the impacts of drought on grasslands. To improve our ability to predict the consequences of these changes on grasslands, thorough understandings of species responses to drought, of the traits governing drought responses, and of how they affect community assembly and ecosystem function are imperative. However, lacking comparative datasets on whole-plant drought resistance and comprehensive sets of traits across grassland species currently impedes our understanding of the role of drought under past and current climate regimes, and our projections for the future.
To cover the gaps, in this thesis we comparatively assessed whole-plant drought resistance and an extensive suite of traits. To our knowledge, these are the first comparative datasets across multiple grassland species. With them, we aimed to address the following questions: (1) Do species drought responses vary across species? (2) Do performance trade-offs exist at the whole-plant and trait levels? (3) Are whole-plant drought resistance and traits related to species distribution along moisture gradients? (4) What is the mechanism of drought resistance? (5) Which traits can be used to predict drought resistance? 
Whole-plant drought resistance was assessed for 41 common temperate grassland species including 20 forbs and 21 grasses in a common garden drought experiment. The experimental design allowed us to comparatively assess the effects of drought on performances of individual species by only inducing delines in soil water potentials while minimizing effects of other cofounding abiotic and biotic factors that influence plant performances. Additionally, midday leaf water potential under drought was measured to indicate species efficiency of desiccation avoidance. Further, we assessed 38 morphological, anatomical and physiological traits. All traits are hypothesized to be mechanistically related to drought resistance, including ‘hard’ traits that pertain to plant key processes, i.e., water uptake, water storage, water transport, water loss, membrane vulnerability, and carbon gain. 
We found that drought significantly reduced species survival and growth, while its effects varied significantly across species. The ranks of species performances remained consistent across drought periods. Many species showed high drought resistance, and a few may suffer greatly under drought. Forbs and grasses had similar whole-plant drought resistance, while they were divergent along trait axes.
Performance trade-offs were not supported. At whole-plant level, growth under irrigated conditions (optimal growth) was unrelated to whole-plant drought resistance of survival and was positively correlated with growth under drought conditions. At trait level, traits that are known to enable high optimal growth had relatively low importance for whole-plant drought resistance. A ‘fast-slow’ plant economics spectrum did not emerge, suggesting that traits and processes involved in drought resistance and growth under optimal conditions should be independent. Further, whole-plant drought resistance was not related to species distribution along moisture gradients, while drought sensitive species were restricted to wet habitats, suggesting that the relation between drought resistance and species distribution was weak. Consistently, several traits were significantly correlated with species distribution, but they had relatively low importance for whole-plant drought resistance. Together, these results suggest that drought acts as a filter excluding drought sensitive species from dry habitats, but other factors additionally determine species segregation along realized hydrological niches.
The dominance of mechanisms of desiccation avoidance over tolerance was supported. Midday leaf water potential under drought was positively correlated with whole-plant drought resistance. Traits of desiccation avoidance, such as rooting depth, root mass, and stomatal index, had the highest importance for whole-plant drought resistance and species distribution. However, the coordination among traits was complex and no main axes emerged to define a drought resistance spectrum. Many combinations of traits existed to confer grassland species the same or different drought resistance. 
Traits can predict species drought resistance. However, a single trait, such as turgor loss point, had relatively low prediction accuracy. Using more root traits and treating forbs and grasses separately, the predictive models were improved (r2 ≥ 56%). 
In summary, the differential species drought resistance suggests that increasing drought may have a great impact on temperate grasslands. Our models had relatively high predictive power, which will contribute to predicting the consequences of drought on grasslands and may provide applicable models for grassland conservation and management under climate change. The comparative datasets of whole-plant drought resistance and traits can allow to further explore of the direct and indirect effects of moisture on community assembly and grassland dynamics at community levels within the framework of the Biodiversity Exploratories.