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Functional diversity (FD) has the potential to address many ecological questions, from impacts of global change on biodiversity to ecological restoration. There are several methods estimating the different components of FD. However, most of these methods can only be computed at limited spatial scales and cannot account for intraspecific trait variability (ITV), despite its significant contribution to FD. Trait probability density (TPD) functions (which explicitly account for ITV) reflect the probabilistic nature of niches. By doing so, the TPD approach reconciles existing methods for estimating FD within a unifying framework, allowing FD to be partitioned seamlessly across multiple scales (from individuals to species, and from local to global scales), and accounting for ITV. We present methods to estimate TPD functions at different spatial scales and probabilistic implementations of several FD concepts, including the primary components of FD (functional richness, evenness, and divergence), functional redundancy, functional rarity, and solutions to decompose beta FD into nested and unique components. The TPD framework has the potential to unify and expand analyses of functional ecology across scales, capturing the probabilistic and multidimensional nature of FD. The R package TPD (https://CRAN.R-project.org/package=TPD) will allow users to achieve more comparative results across regions and case studies. ; CPC was supported by the Estonian Research Council (project PSG293), and by the European Union through the European Regional Development Fund (Centre of Excellence EcolChange)

This is the peer reviewed version of the following article: Journal of Vegetation Science 26.3 (2015): 538-551, which has been published in final form at http://dx.doi.org/10.1111/jvs.12260. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. ; Questions: How do inter-annual fluctuations in water availability affect the functional trait patterns along spatial gradients of resource availability and disturbance? Location: Mediterranean grasslands in central Spain, near Madrid. Methods: We surveyed plant communities from 66 sites under different grazing regimes (from heavy grazing to grazing abandonment) in productive and unproductive habitats (corresponding to upper and lower topographic zones) in 2 yr with contrasting rainfall conditions. Community weighted mean (CWM) and Rao quadratic entropy for three key plant ecological strategy traits (specific leaf area, height and seed mass) were calculated for each community. We used null models to estimate functional richness (FR) and functional divergence (FD), the two components of functional diversity with the highest power to detect changes in community assembly processes across environmental gradients. Results: The patterns of CWM remained rather constant across years, with the only exception being seed mass, which experienced considerable temporal changes that suggested that heavy-seeded species are favoured under stressful conditions. Patterns for FR were consistent across years. They revealed both trait convergence and divergence depending on the niche axis and context. Convergence was observed for vegetative traits in unproductive habitats and seed mass in productive ones, and divergence for seed mass in unproductive habitats and vegetative traits in productive ones. In contrast, the patterns of FD of the vegetative traits changed considerably between years, as shown by increased divergence during the wet year in unproductive habitats and decreased divergence in grazing-abandoned productive habitats. Conclusions: Temporal changes in Mediterranean grassland composition depend on complex interactions between species traits, resource availability and disturbance. Increased rainfall appeared to have contrasting effects on assembly processes in stressful and productive habitats. In stressful habitats we found evidence that increased rainfall promoted niche complementarity, while in productive habitats, especially in the absence of disturbance, it increased trait convergence ; Financial support was received from the Spanish Ministry of Science (Projects CGL2007-63382 and CGL2011-24871), the Madrid Regional Government (Projects REMEDINAL- S0505/AMB-0335 and REMEDINAL2-S2009/AMB-1783), and the Spanish Ministry of Education (FPI for Carlos Pérez Carmona)