SHRUB EMERGENCE AND SEEDLING GROWTH IN MICROENVIRONMENTS CREATED BY PROSOPIS-GLANDULOSA

Facilitation of the establishment of certain plant species by nurse pl ants is a common phenomenon in arid and semiarid ecosystems. The most commonly reported mechanisms of facilitation include cooler temperatur es and increased soil nutrients beneath the nurse plant canopy, which favor establishment of other plant species. During conversion of uplan d grasslands to them woodland in southern Texas, Prosopis glandulosa a ppears to facilitate establishment of other woody plants, including Ce ltis pallida, whereas Acacia smallii occurs only in habitats between P . glandulosa canopies. We tested the hypothesis that light intensity a nd soils under P. glandulosa canopies facilitate seedling emergence an d growth of C. pallida but inhibit seedling emergence and growth of A. smallii. In the field, C. pallida and A. smallii seeds were planted u nder P. glandulosa canopies and in adjacent interspaces. Percent emerg ence of C. pallida seedlings was greater under the canopy of P. glandu losa, whereas percent emergence of A. smallii seedlings was greater in interspaces. In a greenhouse experiment, seeds of each species were p lanted in pots filled with soil from under P. glandulosa canopies or f rom adjacent interspaces. Two treatments, shade and sunlight, were imp osed and plants harvested seven weeks later. Seedling mass of both spe cies was greater in canopy soil than in interspace soil in sunlight bu t mass of the two species did not differ between soil sources in shade . Canopy soils contained more total and available nitrogen than inters pace soils. These results suggest that light is more limiting than nut rients under shaded conditions and so neither species can take advanta ge of the high nutrients beneath P. glandulosa. Shade and greater soil nutrients beneath P. glandulosa do not appear to be the major factors that facilitate C. pallida or inhibit A. smallii. Aggregation of C. p allida beneath P. glandulosa canopies appears to be a complex process that involves both passive facilitation (seed dispersal by birds) and active facilitation (reduction of seed dormancy by under-canopy temper atures) operating only during the seed germination stage with successi onal mechanisms other than facilitation operating during later stages of shrub establishment and growth.