Ecohydrological energy inputs in semiarid coniferous gradients: Responses to management- and drought-induced tree reductions

Large-scale, rapid reductions in forest and woodland tree cover caused by fire, drought-induced die-off, or wildfire-mitigating thinning prescriptions, all three of which differentially affect canopy structure, are increasingly altering coniferous-dominated landscapes across extensive regions such as western USA. These types of reductions in canopy cover can result in substantial increases in near-ground solar radiation, which in turn drive numerous ecological processes. However, existing relationships for how reductions in canopy cover translate into changes in incoming near-ground solar radiation do not account for the ways in which fire, die-off, and thinning differentially alter either or both the foliar and woody components of canopy architecture and the degree to which such alterations depend on foliar density. We systematically quantified trends in near-ground solar radiation for a broad range of canopy cover for two of the most extensive semiarid coniferous forest and woodland vegetation types in the western USA: those dominated by a combination of piñon and juniper (using Pinus edulis and Juniperus monosperma as representative species) or by ponderosa pine (Pinus ponderosa). We used hemispherical photography to account for how canopy architecture affected mean and variance in near-ground solar radiation over a broad range of canopy cover (from as low as ∼5% to as high as ∼85%). For both vegetation types, we evaluated four disturbance types: undisturbed, controlled burns, drought- and beetle-induced die-off, and prescriptive thinning treatments. We also assessed near-ground solar radiation for undisturbed vegetation spanning an elevation continuum that included both piñon-juniper and ponderosa pine vegetation types. Our results quantify how near-ground solar radiation varies substantially and systematically among forest gradient types and as a function of forest disturbance type. Trends in near-ground solar radiation differed among gradients associated with fire, die-off, or thinning, dependent on how each affects the foliar and/or woody components of canopy architecture. Deviations from undisturbed conditions for remaining disturbed tree cover were greatest for burned, intermediate for die-off and least for thinned. The differences in microclimate quantified here and how they vary with type of tree reduction are relevant for assessing vegetation responses following reductions in tree cover. In addition, the differences are large enough to require consideration in evaluating land surface interactions of forests and woodlands with the atmosphere (e.g., increases of >40Wm^-2 relative to undisturbed conditions). Our results provide a means to enable managers to rapidly relate readily-obtainable field estimates of canopy cover to approximate estimates of near-ground solar radiation.