The emergence of plant community dynamics and functional traits in response to the hydrological regime – a modelling approach

Abstract

To predict vegetation response to today’s rapid changes in the hydrological regime due to climate change and other anthropogenic alterations calls for a truly dynamic representation of vegetation in a coupled vegetation-hydrology model. However, existing models are deficient to capture the complex, spatiotemporal dynamics of multi-species natural ecosystems because they model few plant functional types or life forms defined a priori by a small set of postulated characteristics. This dissertation aims to surpass this deficiency and refine the prediction of plant community dynamics and functional trait composition in response to the hydrological regime. Therefore, I developed a novel, highly dynamic, spatially-explicit, individual-based model that simulates plant functional trait abundance solely as a function of soil water potentials and individual behavior. An important innovation is that the model is devoid of a priori defined functional trait trade-offs but instead is able to represent continuous variation of functional traits. The model is used for simulation experiments to investigate possible changes in the function and structure of plant communities subjected to gradients of soil water availability. I show that plant functional traits and their combinations (plant functional types) segregate in a predictable manner across the full range of soil water availability confirming accumulating evidence of hydrological niche segregation from field experiments. Interestingly, this segregation was not a consequence of universal functional trait trade-offs and trait correlations. Instead the correlation intensities, including the classical competition-colonization trade-off, were a function of the soil water availability. Furthermore, the entire biomass-density relationship of the plant communities, among this the value of the slope of the classical self-thinning line, was strongly and consistently modified by water stress. Specifically, the growth reduction in water stressed communities decreased the potential for density-dependent mortality which consequently steepened the thinning slope in communities with high density and levelled the thinning slope in communities with low density. Remarkably, temporally-varying soil water promoted diversity even in humid climates. This suggests that the temporal-storage effect can act globally as a coexistence mechanism driving hydrological niche segregation. Functional diversity of plant communities boosted coexistence under temporally-varying soil water through a change in the functional traits which buffered the population growth rate against increasing temporal variability of soil water from seed dispersal distance to water stress tolerance and life form. Interestingly, many of the predicted patterns of the plant community dynamics and functional traits changed similarly at both ends of the soil water range, i.e. excessive and insufficient water availability selected for a comparable vegetation response. This implies that species’ specific water stress intensity and the corresponding spatiotemporal variation of this water stress intensity, rather than climatic means, might determine vegetation response and thus, species distribution and abundance. In general, the model design allowed for the addition of few key characteristics of natural plant communities such as variation within functional traits, local interactions as well as complete lifecycle including recruitment, and thus allowed for a better understanding of the mechanisms underlying the response of these communities to changes in the hydrological regime.