EFFECTS OF PLANT SIZE ON PHOTOSYNTHESIS AND WATER RELATIONS IN THE DESERT SHRUB PROSOPIS-GLANDULOSA (FABACEAE)

The Jornada del Muerto basin of the Chihuahuan Desert of southern New Mexico, USA, has undergone a marked transition of plant communities. S hrubs such as mesquite (Prosopis glandulosa) have greatly increased or now dominate in areas that were previously dominated by perennial gra sses. The replacement of grasses by shrubs requires an establishment p hase where small shrubs must compete directly with similar-sized grass plants. This is followed by a phase in which large, established shrub s sequester nutrients and water within their biomass and alter soil re sources directly under their canopy, creating ''islands'' of fertility . We hypothesized that these two phases were associated with shrubs ha ving different physiological response capacities related to their age or size and the resource structure of the environment. As a corollary, we hypothesized that responses of small shrubs would be more tightly coupled to variation in soil moisture availability compared to large s hrubs. To test these hypotheses, we studied gas exchange and water rel ations of small (establishing) and large (established) shrubs growing in the Jornada del Muerto as a function of varying soil moisture durin g the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrub s following high summer rainfall in July, and highest seasonal soil mo isture at 0.3 m. High rates of carbon assimilation and water use would be an advantage for small shrubs competing with grasses when shallow soil moisture was plentiful. Large shrubs had greater net assimilation and water-use efficiency, and lower xylem water potential than small shrubs following a dry period in September, when soil moisture at 0.3 m was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water a s soil moisture is depleted. Although the study provides evidence of d ifferences in physiological responses of different-sized shrubs, there was not support for the hypothesis that small shrubs are more closely coupled to variation in soil moisture availability than large shrubs. Small shrubs may actually be less coupled to soil moisture than large shrubs, and thus avoid conditions when continued transpiration could not be matched by equivalent water uptake.