PARENT SESSION
Oral Session # 45: Plant Ecology III: Plant - Water Relations I.
Presiding: R Giuliani
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 200.

The importance of precipitation seasonality to the growth dynamics of a desert shrub.

Ogle, Kiona^*,1, Reynolds, James¹, ¹ Department of Biology, Durham, NC

ABSTRACT- Climate models predict shifts in precipitation to winter months in the American Southwest. To elucidate how such shifts may influence desert plants, we are developing a plant growth model to explore the effects of seasonality on the growth of the desert shrub, Larrea tridenata (creosotebush). The model was parameterized for Larrea growing at the Jornada Long-Term Ecological Research site in southern New Mexico. The Jornada is characterized by three distinct seasons: hot, dry springs (Apr-Jun); hot, moist summers (Jul-Oct); and cold, moderately dry winters (Nov-Mar). Average long-term precipitation is around 240 mm/yr, of which approx. 65% occurs in the summer, 25% in the winter and 10% in the spring. We employed the model and Jornada weather data from 1997-2000 (mean precip. = 249 mm/yr) to examine the importance of seasonality to Larrea's growth. In a simulation experiment, we redistributed rainfall across the seasons while maintaining total annual rainfall. The greatest predicted change in peak biomass (26.2% increase over 4 years) occurred when 65% of the rain fell in the winter. Under drought conditions, growth dynamics were more variable and growth was greatest when summer rainfall was high. However, relative growth rate (RGR), peak biomass, growing season length, and timing of growth were more strongly determined by interactions between rainfall in different seasons and were less coupled to precipitation in a single season. For example, a shift in precipitation to favor spring months resulted in increased RGR, especially if the following summer was drier than the spring. Larrea's greatest growth increment is expected during years of average to above average rainfall that is distributed such that the winter-spring period is relatively wet, followed by a relatively dry summer. This suggests that a re-distribution of precipitation favoring winter-spring months would enhance Larrea's performance.

Key words: seasonality, Larrea tridentata, plant growth, simulation