|DE SOYZA, A|
Submitted to: Ecological Monographs
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/1998
Publication Date: N/A
Interpretive Summary: Large tracts of southwestern desert grasslands have been replaced by shrubs over the past century and we are studying what caused this and its continuing consequences in order to understand its implications for the future. Shrubs tend to gather resources around themselves ("resource islands") at the expense of surrounding areas leading to infertile resource-poor areas between shrubs which causes further reductions in grasses. Over a 3-year period, we studied the effect of depriving large and small shrubs of mesquite and creosotebush of water during summer or winter. Both shrub species normally photosynthesized and grew most during late spring. Mesquite did not change this pattern when deprived of water, but creosotebush changed its maximal growth rate to summer and continued some growth even during severe drought. Both species also changed the soil depth from which they drew water when droughted. This was true for large and small shrubs. Droughted shrubs often compensated for reduced growth by growing faster than undroughted shrubs during the next season. Overall, both shrub species exhibited characteristics which adapt them to surviving through periods of drought with little reduction in growth.
Technical Abstract: Large areas of semiarid grasslands in southwestern United States have been virtually replaced by shrubs during the past century. Understanding the causes and consequences of such vegetation dynamics requires that we elucidate the interplay between external forces of change (e.g., climate, human impacts) and the internal forces within these ecosystems that foster resilience and/or stability. We conducted a 3-yr field study in the Jornad Basin of southern New Mexico to explore the relationships between seasonal manipulations of soil water and its impact on soil nutrient dynamics of resource islands and shrub growth and physiology. We simulated seasonal drought in summer (June 1-September 30) and winter/spring (October 1-May 31) by constructing large rainfall-exclusion shelters over shrub resource islands at different stages of development. Our experiment tests two principal hypotheses: 1) creosotebush and mesquite have different growth phenologies, rooting patterns and physiological responses to resource availability (primarily water), 2) different size classes of shrubs (small and large) represent distinct stages of resource island development, hence, different stabilities. Shrubs of both species appeared well-adapted to withstand season-long droughts. The stage of maturity of a resource island complex did not seem to be a significant factor to the growth and physiological activity of the shrub. Both small and large shrubs within each species had relatively similar patterns of growth and activity and both ages of shrub islands had similar responses to drought. Young shrub islands of creosotebush were negatively impacted by drought through lowered water potentials and stem growth, relative to mature islands.