Seasonally Pulsed Heterogeneity in Microclimate: Phenology and Cover Effects along Deciduous Grassland-Forest Continuum

Much of the terrestrial biosphere can be viewed as part of a gradient, with varying amounts of woody plant cover ranging from grassland to forest--the grassland–forest continuum. Woody plant cover directly impacts the soil microclimate through modifications of near-ground solar radiation and soil temperature, and these interactive effects are relevant for key ecohydrological processes such as soil evaporation. Trends in how increasing woody plant cover affect soil surface microclimate have recently been evaluated for gradients of evergreen woody plants, but analogous trends for deciduous plants, where phenology should be influential, are lacking. We evaluated season-dependent changes in soil microclimate along a deciduous grassland–forest continuum of velvet mesquite (Prosopis velutina Wooton) using repeated hemispherical photography and continuous soil temperature measurements at the 5-cm depth. Both near-ground solar radiation and soil temperature decreased with increasing canopy cover, even during the leafless season. The trends varied substantially among seasons, however, with differences between canopy and intercanopy patches readily evident only during the period of full leaf-out, during which the correlation between near-ground solar radiation and soil temperature was strongest. Our results provide a more comprehensive understanding about the interactions of canopy cover, canopy structure attributes, and plant phenology that produce seasonally pulsed heterogeneity in the soil surface microclimate. Notably, our results add a new dimension to the moisture "pulse dynamics" perspective commonly applied to dryland ecohydrology, highlighting seasonally pulsed heterogeneity in soil microclimate that could influence soil moisture dynamics in drylands.