Response of understorey forage to multiple tree effects in Iberian dehesas

The relative importance of tree effects (shade and changes in soil water and nutrients) on understorey forage production is analysed in three open oak woodlands of Central-Western Spain with different rainfall regimes: 452, 547 and 651 L m−2 of mean annual rainfall. Pasture production was measured in three different zones: under canopy shade, under artificial shade and on the open grassland in six replicate trees per site. In each site, tree and zone, the effect of fertilizer and water application was analysed following a factorial design in four 1-m2 plots (control, watered, fertilised, and watered + fertilised). Results showed a net positive effect of tree canopy on pasture yield of 19% higher beneath the canopy than on open grassland. Neither shade, nor watering, nor fertilisation alone explained this positive effect of trees. It must be explained by a combination of shade with increased soil nutrients and/or water content. The combination of watering and fertilisation produced a significantly higher pasture yield under artificial shade than under the tree canopy, indicating that trees can also have a detrimental effect on pasture yield. This can be better explained in terms of competition for underground resources than by light reduction, especially for soil water in more arid sites.