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The 'dehesas' are savannah-like semi-arid grasslands typical of western Spain, which are subject to strong inter-annual variations in biomass production. Over a 20-year period, from 1986 to 2005, above-ground biomass and the nutritional quality of these grasslands in the province of Salamanca ( western Spain) were evaluated to determine the relationships between interannual variations in grassland parameters and climate variables ( precipitation and temperature). Herbage samples were collected from several sites, along a topographic gradient that differentiated two types of herbaceous communities on the upper and lower part of the slope. Nutritional quality was assessed on the basis of protein, acid detergent fibre, neutral detergent fibre, lignin and digestibility.

On both the upper and lower zones, total biomass and biomass of grasses were correlated with annual precipitation calculated from the previous October to the current June. Biomass of legumes and forbs, on the upper zones, was correlated with spring precipitation. Step-wise multiple regression analysis provided different models for grasses, legumes, forbs, and total biomass for the upper and lower zones. Protein concentration was negatively correlated with annual precipitation in both zones of the slope. The number of days in spring with precipitation (>= 1 mm or >= 10mm) was a good predictor of the lignin content and digestibility in both zones of the slope, and of the acid detergent fibre content on the upper zones and the neutral detergent fibre content on the lower zones.