In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a "pulse" of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.

The significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil-root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes.

The influence of local precipitation and temperature on long-term growth dynamics in two species of seasonally dry tropical forest trees were investigated. Growth records were extracted from tree rings in Guanacaste province, Costa Rica. These chronologies provide a long-term (c. 85-y) record of tree growth for two species with contrasting phenologies. Annual growth, in both species, was dependent on annual and/or monthly variation in local precipitation but less so on temperature. For each species, however, patterns of growth reflected unique degrees of sensitivity to monthly rainfall and rainfall during previous years. It is hypothesized that such differences were due to the rooting depth of these species. A review of the literature also indicated similar diverse cambial growth responses by tropical trees to variation in annual and monthly climate. Lastly, it was shown that variation in longer term fluctuations in the Pacific and Atlantic oceans, as measured by the El Niño Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO), significantly influenced local precipitation in Guanacaste only during the wettest portion of the wet season. Such temporal sensitivity may have differentially influenced the longer-term growth of some tropical tree species but not others. Together, these results support the hypothesis that tropical tree species respond individualistically to variation in local and regional climate and that some tropical assemblages may in fact be structured by species-specific differences in soil water-use.

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The temporal pa erns of evapotranspira on were monitored for 2 yr for four species of differing life form that currently form near monoculture communi es in the Great Basin, USA, a region with a growing season spanning early spring to autumn and predictable overwinter water accumula on in the vadose zone. Species included an annual grass (Bromus tectorum L.), a perennial grass [Agropyron desertorum (Fisch. ex Link) Schult.], a shrub (Artemisia tridentata Nu . ssp. wyomingensis Beetle and Young), and a tree [Juniperus osteosperma (Torr.) Li le]. The two grasses and shrub were growing on the same soil type with uniform texture and subject to near surface percola on of the vadose zone only, while J. osteosperma was growing on soils with a petrocalcic layer below which water was near fi eld capacity. These monotypic stands were found to diff er in quan ty and ming of vadose zone water use, in use pa ern of shallow and deeper water resource pools, and in depth and quan ty of rainwater hydraulically redistributed. All species rapidly u lized shallow vadose zone water in the spring when growth was observed, but use of deep vadose zone water varied by life form and was not linked with the period of growth for any species. Water in the vadose zone of the grass species increased between years, with evapotranspira on less than precipita on inputs and contrasted to water use in A. tridentata where water use approximately equaled precipita on inputs. Juniperus osteosperma used water below the petrocalcic zone, par cularly in late summer. Water use by all species was consistent with the concept of a shallow vadose zone "growth pool" of water and a deeper vadose zone "maintenance pool" used during the summer drought period. The pa erns of water use suggest that water per se is not a limited resource for survival, but infl uences the availability of nutrients necessary for plant growth that are associated with shallow vadose zone water. We postulate that cold-adapted plants in the Great Basin have converged on a general pa ern of rapidly u lizing soil moisture in shallow depths, in part, to infl uence nutrient availability. Our results strongly suggest describing the pool dynamics of vadose zone water will be necessary to further our understanding of plant fi tness, interac ons among species for resources, and species coexistence in arid and semiarid ecosystems.Abbrevia ons: ET, evapotranspira on; LAI, leaf area index.The emerging science of ecohydrology explicitly recognizes the fundamental role that vegetation plays in vadose zone hydrology at scales from the community to the watershed. Th ese eff ects modify, and in part control, water inputs, movement, and losses from the vadose zone. For example, plants modify water input to soils and ultimately watersheds through processes including precipitation interception (Bosch and Hewlett, 1982;Brown et al., 2005;LaMalfa and Ryel, 2008), fog condensation (Azevedo, 1974;Ewing et al., 2009), alteration of surface movement by slowing fl ow paths (Schlesinger et al., 1990; Wilcox et al....