Authors: Susanne Schwinning Benjamin I Starr James R Ehleringer
Publish Date: 2003/04/15
Volume: 136, Issue: 2, Pages: 252-260
Abstract
We conducted experiments to examine the quantitative relationships between rainfall event size and rainwater uptake and use by four common native plant species of the Colorado Plateau including two perennial grasses Hilaria jamesii C4 and Oryzopsis hymenoides C3 and two shrubs Ceratoides lanata C3 and Gutierrezia sarothrae C3 Specifically we tested the hypothesis that grasses use small rainfall events more efficiently than shrubs and lose this advantage when events are large Rainfall events between 2 and 20 mm were simulated in spring and summer by applying pulses of deuteriumlabeled irrigation water Afterwards pulse water fractions in stems and the rates of leaf gas exchange were monitored for 9 days Cumulative pulse water uptake over this interval estimated by integrating the product of pulse fraction in stem water and daytime transpiration rate over time was approximately linearly related to the amount of pulse water added to the ground in all four species Across species consistently more pulse water was taken up in summer than in spring Relative to their leaf areas the two grass species took up more pulse water than the two shrub species across all event sizes and in both seasons thus refuting the initial hypothesis In spring pulse water uptake did not significantly increase photosynthetic rates and in summer pulse water uptake had similar but relatively small effects on the photosynthetic rates of the three C3 plants and a larger effect on the C4 plant H jamesii Based on these data we introduce an alternative hypothesis for the responses of plant functional types to rainfall events of different sizes building on costbenefit considerations for active physiological responses to sudden unpredictable changes in water availabilityThis work was supported by the National Science Foundation IBN 9814510 We would like to thank Danielle Pierce and Kim Davis for their important contributions to field and laboratory work and two anonymous reviewers who greatly helped us find our focus in the presentation of this study
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