Document: DAN-3-32-39

Effects of soil moisture substrate characteristics on mesquite water relations and leaf carbon isotope ratios in semi-desert grasslands.

KOEPKE, D.F.*, D.G.WILLIAMS and A.FRAVOLINI

University of Arizona, Tucson, AZ 85721 USA ¹

Abstract:
The distribution, abundance, and function of woody vegetation in semi-desert grasslands are highly dependent on characteristics of soil substrate. Velvet mesquite occurs throughout semi-desert grassland and savanna in southeastern Arizona, but its abundance varies across soils of differing clay content, affecting surface hydrology and soil moisture availability. Mesquite density (trees/area) is high on young Holocene geomorphic surfaces with coarse-textured soils and is relatively low on older Pleistocene surfaces with a clay-rich argillic horizon. We measured seasonal patterns of plant predawn water potential, water sources (²H and ¹⁸O of stem and soil water), and leaf carbon isotope ratios of small (<1 m height), medium (1 2 m), and large (>2 m) mesquite plants at three sites that differed in the development of an argillic horizon in shallow soil layers. Predawn water potential before the onset of the summer 'monsoon' (July to mid September ) was lowest (4.3 MPa) for plants on the coarse-textured Holocene surface, but increased to 0.8 MPa after summer rains had wet the upper soil layers. Predawn water potential was lowest during pre-monsoon in the smallest plants (4.8 MPa), suggesting that access to deep soil moisture was restricted by a shallow root system. Predawn water potentials were high (overall average of 0.9 MPa) and did not differ among populations at the three sites during the rainy monsoon. In October, predawn water potential values dropped after monsoon rains ceased, but did not differ among the three populations or size classes. Moisture stored in deeper soil layers from prior winter precipitation may have allowed plants on the clay-rich soil to maintain favorable water balance during the pre-monsoon drought. Furthermore, the use of summer rains did not differ among plants at the three sites. Stable isotopes in stem and soil water will be measured to verify our conclusions.

Keywords: Holocene geomorphic surface, Pleistocene geomorphic surface, leaf carbon isotope

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS