Spatial and Temporal Patterns of the Vadose Zone Hydrological Niche Gradient in a Semi-Arid Mediterranean Oak Savannah

Despite common resource demands amongst co-existing plant species inhabiting semi-arid climates, many ecosystems in arid or Mediterranean areas exhibit high plant diversity. Past studies have hypothesized that this diversity is sustained by the co-existence of a variety of different hydrological niches within the ecosystem. Different plant species exhibit different degrees of 'fit' within the boundaries of different niches leading to sustained co-existence and minimal competition for resources among different species. To date, evaluation of the existence of hydrological niche partitioning has focused on detailed and widespread species analysis, with hydrological evaluation surrogated to broad analysis of topographic features. We have taken an alternate approach that provided new perspective by generating a spatially and temporally resolved analysis of water availability as a function of topographic position, local spatial variability and seasonal fluctuations. We measure the fine-scale spatial and temporal patterns of the vadose zone hydrological niche gradient following two topographic transects across a semi-arid Mediterranean oak savannah. The pattern of changes of soil water content in the surface soils (surface to -15 cm) and the entire vadose zone are evaluated during the transitional drying phase between the months of April and June. Preliminary results demonstrate a trend towards homogenization of the surface soil hydrological niche gradient throughout this drying period. Changes in the spatial pattern of moisture availability in the vadose zone are much less pronounced, suggesting that local niches in water availability are preserved during early summer for those species which can exploit the whole vadose zone: findings which are broadly supported by measurements of plant water potential and plant water isotope composition. The co-existence of multiple hydrological niches across topographic gradients, fine-scale spatial heterogeneity and seasonal fluctuations suggests likely differential responses of Mediterranean ecosystems to climatic variability.