PARENT SESSION
Poster Session #24: Modeling.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Soil moisture promotes grass growth, but does this growth have a measurable effect on soil-water content?

Fernandez, Roberto^*,1, Reynolds, James², ¹ IFEVA-Universidad de Buenos Aires, Buenos Aires, Argentina² Duke University, Durham, NC

ABSTRACT- Plants respond to soil water and, at the same time, affect it via root uptake. In arid and semiarid zones, the Noy-Meir "pulse/response" paradigm suggests that responses are generally larger than effects, i.e., abiotic influences on resource availability are more important than biotic ones. We present a simple (5-parameter), coupled plant-soil simulation model of a grass patch devised to distinguish between responses and effects, and apply it to investigate those conditions and temporal scales where each is important. Using a fixed, 1-day time step and observation windows of varying lengths (X=1-512 d), we assess plant responses to episodic precipitation events by computing correlations between net primary production (NPP) and cumulative rainfall during the previous X days; these are positive correlations. Effects, on the other hand, are evaluated by the existence of negative correlations between soil-water content and cumulative NPP during the previous X days. Results suggest that arid-zone vegetation may exert more control on soil-water availability than suggested by the pulse/response paradigm. These biotic influences are more likely to be found: (i) in systems with low total rainfall inputs, (ii) in communities dominated by potentially fast-growing species with rapid turnover of biomass, and (iii) if observation periods span more than a few weeks. The average size of rainfall events has an influence on NPP-rainfall relationships but does not alter the relative importance of responses vs. effects.

KEY WORDS: grassland, arid zones, NPP, desertification