EFFECTS OF NURSE-PLANT CANOPY LIGHT-INTENSITY ON SHRUB SEEDLING GROWTH

Spiny hackberry (Celtis pallida Torr.) occurs almost exclusively benea th honey mesquite (Prosopis glandulosa Torr,) canopies, We hypothesize d that maximum early seedling growth of spiny hackberry occurs at ligh t intensities similar to those beneath honey mesquite, whereas maximum growth of huisache (Acacia smallii Isely), a plant characteristic of nonshaded habitats, occurs at light intensities near full sunlight, Ph otosynthetic photon nux densities (PPFD) under 4 honey mesquite canopi es and in adjacent interspaces were recorded during July 1990 to July 1992, Seedling growth indices of spiny hackberry and huisache were det ermined at PPFDs characteristic of those found under honey mesquite ca nopies and in herbaceous interspaces. In contrast to our prediction, r elative growth rate (K-w), net assimilation rate (NAR), and seedling m ass of spiny hackberry were greater in sunlight than in shade characte ristic of honey mesquite canopies. Huisache also had greater K-w, NAR, and total seedling mass in sunlight, Aggregation of spiny hackberry b eneath honey mesquite canopies does not result from an ability to main tain maximum seedling growth at low light intensities.