Topography-mediated microclimates give rise to opposing hillslope ecosystems in north facing and south facing slopes of a semiarid basin in central New Mexico. It is hypothesized that large solar irradiance differences are a driving mechanism in the observed vegetation patterns. Using a distributed solar radiation model, we explore the topography-vegetation controls on annual and seasonal irradiance in the basin. Three digital elevation models (DEMs), ranging from 1m to 10m resolution, and a digital surface model with tree canopies, from Light Detection and Ranging (LiDAR), are used to assess improvements in capturing irradiance differences in the opposing slopes. Remarkably reduced irradiance is found in the north facing slope throughout the year. This suggests that terrain aspect is a first-order control on the spatial distribution of irradiance, with terrain slope leading to differences within each aspect. Tree cover and its spatial arrangement are important second-order controls on irradiance which can overwhelm topographic effects in specific locations and times of year. For example, differences between north facing and south facing slopes are maximized in the spring equinox, rather than the winter solstice, when tree shading and reflection are accounted for. North facing trees also diminish intercanopy radiation, depending on tree cover and vegetation albedo, with important ecological implications for the conifer-grass association. Solar irradiance analysis helps identify the underlying topographic and vegetation controls on microclimate in the opposing hillslope ecosystems, suggesting a feedback mechanism that helps reinforce the differences in vegetation establishment.