SOIL WATER REGIME THROUGH CONTRASTING PASTURE COMMUNITIES IN A SUBMEDITERRANEAN LANDSCAPE

Abstract

Submediterranean landscape around Vic includes, scattered over intensively farmed ground, a number of hills holding diverse mosaics of natural and seminatural communities. We chose two contrasting communities of pastures to investigate the soil water regime, assumed to be the cause of noticeable shifts in the landscape mosaics. Brachypodio-Aphyllanthetum is a Mediterranean, xerophilous, open pasture dominated by the chamaephytic grass Brachypodium retusum, whereas Plantagini-Aphyllanthetum is a dense grassland of Brachypodium phoenicoides, similar to Medioeuropean mesoxerophilous grasslands.We recorded the soil water content in three plots for each community in different hills, during two and a half years. From gravimetric water percentages and soil water retention curves in each plot, we evaluated the soil water potential and the available water during the study period at different depths.Both communities showed roughly similar seasonal soil water dynamics, which included three clearly differentiated hydric periods: (i) humid (October–April), with soils not far from field capacity; (ii) dry (July–August), when soils were drier than wilting point; and (iii) transitional (May–June), defined by a dramatic water loss. The summer values of water content and hydric potential were similar in the two analysed communities, thus rejecting the hypothesised better summer conditions in the mesoxerophilous grassland. The only clear difference was in the way in which the spring transition occurred, namely it was faster and showed sharper ups and downs in the xerophilous Mediterranean pasture. The inverse seasonal transition, from dry to humid, was clearly a sudden event, strongly depending on the timing of late summer rainfall, and it seems not to be a key factor in shaping the functioning of the communities. The lower moisture values at deep soil layers during the humid periods suggest a deficient hydrological functioning, perhaps connected with the pasture-type vegetation.