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

Authors: DE SOYZA, AMRITA - NEW MEXICO STATE UNIV; FRANCO, AUGUSTO - UNIVERSIDADE DE BRASILIA; VIRGINIA, ROSS - DARTMOUTH COLLEGE; REYNOLDS, JAMES - DUKE UNIVERSITY; Whitford, Walter

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Shrubs such as mesquite (Prosopis glandulosa) have invaded and now dominate areas that once were mainly grasslands. In order to understand and perhaps predict the future course of these invasions we must understand what makes these shrubs so successfull in desert ecosystems. When shrubs first invade an area they are small and must first compete against similar sized grass plants. Later, when the shrubs become relatively large they gather nutrients and water and change the distribution of these resources in the landscape. This research was done to investigate the different physiological responses of plants in these two stages. Small shrubs had greater rates of photosynthesis and water loss through transpiration when water was plentiful, and maintained high plant water content even when water was scarce. This is advantageous when small shrubs have to compete with grasses and other plants during the establishment stage. Large shrubs shad greater photosynthetic rates and low rates of water loss through transpiration when water was scarce and plant water content was closely tied to soil water content. By the time shrubs are large they have little competition from other plants and water saved is not available to a competitor. Overall the study provides evidence of differences in physiological responses of different-sized shrubs. Small shrubs may actually be less coupled to soil water content than are large shrubs, allowing them to avoid conditions when continued water loss could not be matched by equivalent water uptake from the soil.

Technical Abstract: The Jornada del Muerto basin of southern New Mexico has undergone a marked transition of plant communities. Shrubs now dominate in areas that were previously dominated by perennial grasses. The replacement of grasses by shrubs requires an establishment phase where small shrubs must compete with similar-sized grass plants. This is followed by a phase in which large, established shrubs sequester nutrients and water within their biomass and alter soil resources under their canopy, creating "islands" of fertility. We studies gas exchange and water relations of small and large shrubs growing in the Jornada del Muerto as a function of varying soil moisture during the season. The small shrubs had greater net assimilation, stomatal conductance, transpiration, and xylem water potential than large shrubs following high summer rainfall in July and highest seasonal soil moisture. 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 was lowest. Low xylem water potentials and high water-use efficiency would allow large shrubs to continue acquiring and conserving water as soil moisture is depleted. Although the study provides evidence of differences 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.