Nonlinear response curves are often used to model the physiological responses of plants. These models are preferable to polynomials because the coefficients fit to the curves have biological meaning. The response curves are often generated by repeated measurements on one subject, over a range of values for the environmental variable of interest. However, the typical analysis of differences in coefficients between experimental groups does not include a repeated measures approach. This may lead to inappropriate estimation of error terms. Here, we show how to combine mixed model analysis, available in SAS, that allows for repeated observations on the same experimental unit, with nonlinear response curves. We illustrate the use of this nonlinear mixed model with a study in which two plant species were grown under contrasting light environments. We recorded light levels and net photosynthetic response on anywhere from 8 to 10 points per plant and fit a Mitscherlich model in which each plant has its own coefficients. The coefficients for the photosynthetic light-response curve for each plant were assumed to follow a multivariate normal distribution in which the mean was determined by the treatment. The approach yielded biologically relevant coefficients and unbiased standard error estimates for multiple treatment comparisons.

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....

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Hydraulic redistribution, the movement of water from soil layers of higher water potential to layers of lower water potential through the root systems of plants, has been documented in many taxa worldwide. Hydraulic redistribution is influenced principally by physical properties of roots and soils, and it should occur whenever root systems span soil layers of different water potential. Therefore, hydraulic redistribution should occur through the root systems of plants with aboveground tissue removed or through the root systems of fully senesced plants as long as roots remain intact and hydrated. We examined our hypothesis in field and greenhouse studies with the annual grass Bromus tectorum. We used soil psychrometry to measure soil water potential and performed 2 H-labeling experiments. In the field, following senescence of B. tectorum, we show substantial changes in soil water potential consistent with both upward and downward movement of water through roots. The amount of water redistributed represents a significant proportion of that which can be stored in the rooted zone. We also experimentally demonstrated upward movement of a 2 H label by roots of senesced plants and by roots of plants without aboveground tissues. In the greenhouse, we further demonstrated redistribution by senesced individuals using a 2 H label. Hydraulic redistribution through the roots of senesced plants should receive further attention because it may have important ecological consequences for soil water recharge, survival of plants through drought, and agricultural practices.