Ecohydrological implications of canopy cover phenology along a continuum of semi-arid deciduous vegetation: Pulsed heterogeneity effects on soil evaporation

Background/Question/Methods
Evapotranspiration compromises the majority of the water budget in semiarid ecosystems. The effect of vegetation presence and dynamics in the partitioning of total evapotranspiration into transpiration and evaporation has important ecological implications. Soil evaporation is affected by the presence of woody plant canopy cover, as previously demonstrated for evergreen systems, but the interactive effects of canopy cover and changes in phenology associated with deciduous vegetation on soil evaporation dynamics are less clear and have not been tested systematically. We established a gradient spanning 6 transects with canopy covers of the deciduous woody plant mesquite (Prosopis velutina) ranging from 2 to 73% at the Santa Rita Experimental Range in southern Arizona. On each transect, we selected 5 canopy and 5 intercanopy locations and deployed 2 microlysimeters at each: one containing bare soil and the other with soil covered at the surface with a litter layer. The microlysimeters were built using a well mixed soil from the study site. During each of the main phenological stages, a pulse of moisture equivalent to 20 mm was added to each mycrolysimeter. Soil evaporation rates were calculated from changes in gravimetric soil moisture content, intensively measured through the duration of the experiments. In addition, we estimated incoming solar radiation using hemispherical photography. Results/Conclusions

Our results quantify the degree to which soil evaporation rate depends interactively on the amount of woody plant cover and the presence of a litter layer in the soil, and how rates differ for canopy/intercanopy locations along a gradient of cover that can be viewed as part of a grassland-forest continnum. In particular, the results quantify differences between canopy and intercanopy locations, as well as with phenological stage, with differences in evaporation rates being greatest at lower levels of canopy cover. In addition, our results quantify differences in evaporation rates associated with the presence of a litter layer, which are greatest for intercanopy locations at lower levels of canopy cover. Our results highlight that the presence and seasonality of vegetation influences the ecohydrological dynamics of soil evaporation not only by means of modifying the surface energy balance, reflected on soil temperature, but also by increasing the presence of a litter layer in the surface of the soil. They also highlight systematic and interactive effects of canopy cover and phenology along gradients of vegetation cover in hydrological variables that will, in turn, influence ecosystem processes and properties such as plant-water use, germination and recruitment.