Species-specific effects of the herbaceous layer on recruitment of woody species under different shading and precipitation conditions

Herbs may affect the recruitment of woody species. However, most studies have evaluated the impact of one particular herb species or the herbaceous layer as a whole. It is less known if different herb species have different effects on recruitment of woody species, and whether these effects depend on environmental conditions. In this paper, we evaluated the effects of different native and exotic herb species on recruitment of nine woody species in central Chile, under different conditions of shading and precipitation. The experiment was carried out in an open ruderal area in Santiago, Chile. We conducted a factorial experiment with 360 1 × 1 m pots with monospecific crops of eight herb species and a control without herbs, two levels of irrigation (245 and 534 mm) simulating two precipitation regimes, and the presence or absence of artificial shade simulating a woody canopy. Nine woody species common to the region were sown in each pot. Luminosity (PAR), soil moisture, herb volume, richness of woody species and seedling density of woody species recruited were measured. The effects of herb species on woody species richness varied from neutral in the open condition to mainly negative in the shaded condition. Differences in irrigation did not modify the effects of herbs on species richness. The effects of herb species were more variable when woody species were considered individually. Negative and neutral effects predominated, although some positive effects were observed under shaded and wetter conditions. Our results suggest that different herb species have distinct effects on recruitment of specific woody species and species richness, which mainly depend on shade conditions and secondarily on the precipitation regime, and that herb-woody species interactions are highly specific. Thus, management focused in herb controlling should be species-specific, in terms of herb and woody species, and dependent on climate and woody cover.