PARENT SESSION
Oral Session #17: Plant Ecology: Water Relations.
Presiding: W. Pockman
Monday, August 5. 1:00 PM to 3:45 PM. Coconino Meeting Room, TCC.

Reconstructing plant water uptake and root area profiles.

Ogle, Kiona^*,1, Reynolds, James¹, ¹ Duke University, Durham, NC

ABSTRACT- Stable isotopes of hydrogen (D) and oxygen (¹⁸O) bound in plant and soil waters are useful indicators of plant-soil water interactions. Plants obtain water from various depths in the soil and the isotopic signature of stem plant water reflects the average soil isotopic value, weighted by the proportion of water acquired from each soil layer. Simple linear mixing models—which partition soil water into two sources (e.g., surface vs. deep) that have distinct D or ¹⁸O signatures—have been employed to estimate the proportion of plant water uptake from each source. However, no models are available to estimate the proportion of water acquired from multiple (> 3) sources. An algorithm will be presented that reconstructs root area and plant water uptake profiles for multiple soil layers. The algorithm couples a statistical simulation routine, a simple biophysical model for root water uptake, and measurements of D and ¹⁸O. The statistical routine places biologically realistic constraints on possible root area profiles, thereby minimizing the non-uniqueness problem. The model was successful at reproducing a variety of hypothetical water uptake profiles (observed vs. predicted R² = 0.88) and its accuracy increases as the number of potential soil water sources increases. The algorithm is employed to reconstruct root area and water uptake profiles for the desert shrub, Larrea tridentata. These reconstructed profiles are used to determine how rapidly Larrea uses rain water and the plasticity of water sources available to this important desert shrub. These questions are important for elucidating how arid plants such as Larrea respond to rainfall variability and how sources of soil water may change due to projected shifts in rainfall patterns in arid zones.

KEY WORDS: D, ¹⁸O, plant water relations, stable isotopes