Land-surface microclimate is an important driver of vegetation and of feedbacks to climate, and is particularly important through its role in driving the dynamics of soil moisture and associated ecohydrological processes. Land-surface microclimate is affected directly by shrub or tree cover, and recent modeling studies have predicted trends in microclimate as a function of the amount and stature of woody plant cover. Here we evaluate previous predictions of near-ground solar radiation as a function of amount of woody plant cover and assess associated trends in soil temperature that drive soil evaporation rates. We focus on piñyon-juniper ecosystems, which are extensive and dynamic throughout North America, and characterized spatially as heterogeneous; ranging in woody plant canopy cover from savannah with less than 5%, to dense woodlands with greater than 60%. We established a series of research transects in piñyon-juniper ecosystems on a mesa north of Flagstaff that ranged in cover from 5% to 65% and estimated near-ground solar radiation using hemispherical photographs and measured soil temperature dynamics at 5 cm.

Results/Conclusions

Consistent with previous model predictions, solar radiation exhibited a near-linear decrease with increasing cover, and a peak variance at less than 50%.  Soil temperature changed systematically from low to high cover values, with the magnitude of difference varying with season, and with the greatest difference between low and high cover sites occurring when temperatures were hottest.  Our results provide a basis for assessing ecohydrological changes in soil evaporation as a function of woody plant canopy cover.  In addition, our results provide a baseline set of relationships against which the effects of disturbances that alter canopy cover, such as fire, drought-triggered tree die-off and forest thinning, can be evaluated.  More generally, they provide general insights about trends in land surface microclimate along gradients of vegetation that can span a continuum from grassland to forest.