Document: KIO-3-30-20

Physiological responses of the desert shrub Larrea tridentata to short-term variation in summer rainfall.

OGLE, K.*, R.J.FERNANDEZ, S.L.BAUER and J.F.REYNOLDS

Duke University, Durham, NC 27708 USA ¹

Abstract:
In the semiarid region of southern New Mexico, plant activity is tightly coupled to water availability. Thus, these ecosystems are highly vulnerable to potential shifts in the variability of seasonal rainfall. To develop a better predictive understanding of this vulnerability, we examined the effects of short-term variation in summer rainfall on growth and diurnal patterns of gas exchange in the evergreen shrub, Larrea tridentata. During June-Aug. 1998, leaf area (LA) production, transpiration (E), and photosynthesis (A) were monitored for two groups of 8 shrubs; one group was irrigated twice during the summer. Soil moisture probes (TDR) were installed under each shrub (0-10, 0-30, 30-60 cm depths) and 4 shrubs per group were instrumented with automated sap-flow sensors. The first irrigation (30 mm; July 2) followed an extended dry period and, in turn, was followed by a series of rains (34 mm; July 2-5). Irrigation did not significantly affect shrub growth and physiology, and neither treatment group showed an immediate Ed (daily transpiration) response to increased soil moisture. All shrubs were characterized by substantial LA production (~13%/d) from July 4 to 9, which may have masked their physiological responses. However, Ed and Ad (daily carbon gain) gradually increased in all shrubs during the subsequent 10-d dry period. In contrast to the first irrigation, a second irrigation (40 mm; July 26), which followed a series of rains (114 mm total), had a significant impact on LA production (p = 0.04). Moreover, for several days after the event, irrigated shrubs exhibited greater Ed and Ad than controls (e.g., Edmax= 938 vs. 750 mol m^-2d^-1; Admax= 662 vs. 518 mmol m^-2d^-1). These data support preliminary modeling results of a strong interaction between mean daytime vapor pressure deficit (VPD) and soil moisture (i.e., the correlation between Ed and VPD decreases as the correlation between Ed and soil water content increases). Both our field and modeling observations illustrate the importance of the timing of rainfall to Larrea growth and physiology.

Keywords: sap-flow sensors, Larrea tridenata, rainfall variability, plant growth, transpiration, photosynthesis, plant-soil-atmosphere interactions

This abstract is being presented at: 9:15 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.