Abstract
In arid and semi-arid regions water availability is the main factor determining vegetation patterns. On gypsum outcrops, vegetation is also conditioned by soil physical (i.e. surface crust) and chemical (i.e. ion unbalance) restrictions, which promote a highly specialist and endemic gypsophile flora. Topographical and climatic variability affect the rigors of gypsum soil environment, and thus, plant community distribution along environmental gradients can differ between gypsum and non-gypsum substrates. The aim of the paper was to evaluate the relative importance of different abiotic factors as drivers of plant communities in gypsum and non-gypsum substrates. While non-gypsophile vegetation is expected to be determined mainly by aridity, gypsophile vegetation could show more complex patterns, following the relaxation of the rigors of gypsum soil under environmental gradients such as steep slopes and mesic conditions. We compared the relationships between presence and cover of three plant communities (open woodland, dwarf-shrubland and garrigue) on gypsum and non-gypsum substrates with environmental factors, after removing the effect of human use. Generalized additive models (GAM) and generalized linear models (GLM) were employed. Results showed that xeric conditions inhibited development and recovery of vegetation, but plant communities on gypsum substrate did not always respond in the same way as non-gypsum vegetation. While the degree of aridity (determined by precipitation and temperature) conditioned non-gypsum vegetation strongly, favouring less developed plant communities, topographical factors (slope aspect and angle) were the main determinants of gypsum plant community patterns. We conclude that gypsum substrate determines strongly the plant community patterns in a semi-arid Mediterranean landscape, as it can be observed by the strong response of gypsophile vegetation to the relaxation of the rigors of gypsum soils with topography (i.e. steep and north oriented slopes).
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