Many landscapes can be viewed as gradients of tree cover, the physiognomy and continuity of which influence atmospheric and land surface interaction. Tree canopy cover influences several key environmental properties, with amount and spatial variation of overstory-attenuated radiation near the land surface being of particular importance. Large-scale, rapid reductions in 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. Solar radiation penetration through canopy overstory should increase as canopy cover decreases and changes in spatial variability, but how such response functions change with vegetation type, canopy cover, and type of canopy reduction remains unclear. We quantified trends in mean and spatial variance in near-ground radiation over a wide range of canopy cover (~5% to ~85%) using hemispherical photographs for reference gradients for piñon-juniper woodlands and ponderosa pine forests, as well as an elevational gradient that spanned both vegetation types. We compared changes associated withfire, die-off, and thinning for both vegetation types relative to reference gradients.

Results/Conclusions

Trends for reference cover gradients differed between vegetation types, and were consistent with previous, limited modeling assessments that did not account for canopy foliar structure. Trends in near-ground solar radiation differed among gradients associated with fire, die-off, or thinning. In several cases near-ground solar radiation increased relative to reference condition by > 40 W/m ² — large enough to be significant for atmospheric and land surface interactions and for management of coniferous-dominated landscapes.