Effects of nurse-plant canopy light intensity on shrub seedling growth

Abstract

Spiny hackberry (Celtis pallida 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 smallii Isely), a plant characteristic of nonshaded 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 assimilation rate (NAR), and seedling mass of spiny hackberry were greater in sunlight than in shade characteristic of honey mesquite canopies. Huisache also had greater Kw, 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.