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Abstract. Facilitation of the establishment of certain plant species by nurse plants is a common phenomenon in arid and semiarid ecosystems. The most commonly reported mechanisms of facilitation include cooler temperatures and increased soil nutrients beneath the nurse plant canopy, which favor establishment of other plant species. During conversion of upland grasslands to thorn woodland in southern Texas, Prosopis glandulosa appears to facilitate establishment of other woody plants, including Celtis pallida, whereas Acacia smallii occurs only in habitats between P. glandulosa canopies. We tested the hypothesis that light intensity and soils under P. glandulosa canopies facilitate seedling emergence and growth of C. pallida but inhibit seedling emergence and growth of A. smallii. In the field, C. pallida and A. smallii seeds were planted under P. glandulosa canopies and in adjacent interspaces. Percent emergence of C. pallida seedlings was greater under the canopy of P. glandulosa, whereas percent emergence of A. smallii seedlings was greater in interspaces. In a greenhouse experiment, seeds of each species were planted in pots filled with soil from under P. glandulosa canopies or from adjacent interspaces. Two treatments, shade and sunlight, were imposed and plants harvested seven weeks later. Seedling mass of both species was greater in canopy soil than in interspace soil in sunlight but mass of the two species did not differ between soil sources in shade. Canopy soils contained more total and available nitrogen than interspace soils. These results suggest that light is more limiting than nutrients under shaded conditions and so neither species can take advantage 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. pallida 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 temperatures) operating only during the seed germination stage with successional mechanisms other than facilitation operating during later stages of shrub establishment and growth.

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Spiny hackberry (Celtic pugida Torr.) occurs almost exclusively beneath honey mesquite (Prosopis glandulosa Torr.) canopies. We hypothesized that maximum early seedling growth of spiny hackberry occurs at light intensities similar to those beneath honey mesquite, whereas maximum growth of huisache (Acacia snuzuii Isely), a plant characteristic of noushaded habitats, occurs at light intensities near full sunlight. Photosynthetic photon flux densities (PPFD) under 4 honey mesquite canopies and in adjacent interspaces were recorded during July 1990 to July 1992. Seedling growth indices of spiny hackberry and huisache were determined at PPFDs characteristic of those found under honey mesquite canopies and in herbaceous interspaces. In contrast to our prediction, relative growth rate (Kw), net assimiition rate (NAR), and seedling mass of spiny hackberry were greater in sunlight than in shade characteristic of honey mesquite canopies. Huisache also had greater K,, NAR, and total seedling mass in sunlight. Aggregation of spiny hackberry beneath honey mesquite canopies does not result from an ability to maintain maximum seedling growth at low light intensities.

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