Abstract
Plant establishment in semiarid ecosystems is affected by the limited spatial and temporal availability of resources and adequate microsites provided by nurse plants. There has been little research on plant establishment in these ecosystems that consider both the ecological roles of different plant types and the abiotic properties of their microsites. Such studies could provide important insights about the functioning of semiarid ecosystems. Here, we investigated the links between the patterns of plant establishment and the hydrological and microsite properties of shrubs and grasses in a semiarid ecosystem northeastern Spain. For ecological experiments, we measured the spatial patterns of the establishment of shrubs and grasses in eight 6 x 6 m(2) quadrats over 2 years; we also sowed seeds of Salsola vermiculata (a dominant shrub) and Lygeum spartum (the dominant perennial grass) under adult shrubs (S. vermiculata) and grasses (L. spartum) and in bare soil and then examined seedling germination, survival, and growth over 4 years. For hydrological experiments, we analyzed soil water content under the two codominant shrubs (S. vermiculata and Artemisia herba-alba), the dominant perennial grass (L. spartum), and in bare soil over 18 months; we also measured water infiltration and solar radiation at the same four microsites to identify the hydrological processes responsible for the observed ecohydrological patterns. The three potential nurse plants greatly improved the hydrological and microsite conditions. They increased soil water content after rainfall relative to bare soil. Moreover, S. vermiculata and L. spartum slowed the drying process. However, only S. vermiculata acted as a nurse plant. It improved plant density, diversity, performance, and survival during the whole study period. L. spartum facilitated plant establishment during early stages, but interfered with seedling performance and survival during later stages, probably because of the increasing competition for water with seedlings. A. herba-alba did not facilitate plant establishment at any stage, most likely because of water scarcity during prolonged dry periods and its allelopathic effects. We conclude that the ecological role of a plant cannot be directly inferred from its hydrological or microsite properties. Long-term ecohydrological studies are required to understand the role of nurse plants on seedling establishment.