Functional reorganization and productivity of a water-limited annual plant community

Changes in plant community structure in response to environmental change can be defined as community reorganization. In water-limited regions, the size of annual plants is related to soil resource availability; therefore, inter-annual reorganization of annual plant communities is expressed through shifts in the diversity and weighted mean values of plant sizes. Nevertheless, the relative contribution of plastic changes at the individual plant level vs. community changes (species presence and abundance) as reorganization mechanisms on the one hand, and as shapers of ecosystem function on the other hand, is not known. We investigated reorganization through inter-annual changes in species diversity and functional structure (diversity and identity). Species richness, composition, and biomass were monitored over four years at Lehavim site, Israel, among adjacent topographic sites. All of the sampled plant individuals were divided into categories according to plant size. We calculated the inter-annual changes in productivity and examined the relationships between these changes and various reorganization indices. Reorganization indices based on both functional identity (value of size traits) and functional diversity of the community were strongly related to, and demonstrated similar predictive ability for, inter-annual changes in plant productivity. In contrast, reorganization indices based on community taxonomic structure demonstrated weaker relationships with changes in productivity. Different components of functional structure complemented each other during the reorganization process and their relative contributions to changes in productivity varied significantly among years. The level of expression of different reorganization mechanisms changes markedly among years and expresses the obstacles that limit the ability of the community to reorganize.