Water use trade-offs and optimal adaptations to pulse-driven arid ecosystems

1 We introduce a hydraulic soil-plant model with water uptake from two soil layers; one a pulse-dominated shallow soil layer, the other a deeper soil layer with continuous, but generally less than saturated soil moisture. Wat er uptake is linked to photosynthetic carbon assimilation through a photosy nthesis model for C-3 plants. 2 A genetic algorithm is used to identify character suites that maximize ph otosynthetic carbon gain for plants that experience a particular soil moist ure pattern. The character suites include allocation fraction to stem, leav es and shallow root, stem capacitance and stem water storage capacity, maxi mal leaf conductance and sensitivity of leaf conductance to plant water pot ential, and a critical soil water potential at which shallow roots cease to transfer water. 3 We find that if pulse water is a more important water source than deeper soil water in the environment, optimal phenotypes lean towards adaptations that maximize pulse water use (small root : shoot ratio, predominantly shal low root system, high leaf conductance with high stomatal sensitivity to pl ant water status). if deeper soil water is more important, phenotypes lean towards adaptations that maximize deeper soil water use (large root : shoot ratio, predominantly deep root system, lower leaf conductance with low sto matal sensitivity). Stem succulence is adaptive only when deeper soil water is unavailable. 4 From among the continuum of derived phenotypes, four phenotypes are selec ted that resemble the character suites of winter annuals, drought-deciduous perennials, evergreen perennials and stem succulents. Under common conditi ons, these phenotypes reproduce many of the responses to drought and water pulse observed in their respective life-form counterparts. The comparison a lso highlights the differences in plant life-form sensitivity to summer and winter drought conditions. 5 Based on these results, we discuss the possible role of annual precipitat ion patterns in shaping plant adaptations and determining the plant composi tion of arid and semi-arid environments.