1.Tall-shrub expansion into low-statured communities, a hallmark of recent vegetative change across tundra ecosystems, involves three major genera: Alnus, Betula, and Salix. Which genus expands most into tundra landscapes will determine ecosystem properties. 2.We show that Alnus and Salix shrubs segregate thermal space (elevation x insolation) and colonize tundra landscapes differently in response to climate warming, thereby replacing different tundra types. 3.Vegetative change estimated from repeat photography should account for hill-slope. Methodologically, slope determines surface area estimated from orthophotos as projected pixel area times secant of pixel slope. Ecologically, the change in thermally-responsive vegetative area is sensitive to terrain steepness, scaling as the cosecant of hill-slope, so that studies should expect more shrub expansion in areas of shallow slopes than steep slopes. 4.Repeat aerial photography in Alaska's Chugach Mountains from 1972-2012 orthorectified on high-resolution lidar DEM indicated tall Salix was rare in 1972 and colonized warmer slopes by 2012. Tall Alnus colonized steeper, cooler slopes both by 2012 and by 1972. Salix and forest colonized similar thermal space. Colonization probability for both shrub genera was maximized at intermediate elevations. 5.Alnus colonization adjacent to dwarf-shrub tundra was twenty-times as likely as Salix colonization. Salix colonization adjacent to low-shrub/herbaceous tundra was three-times as likely as Alnus colonization. Replacement of dwarf-shrub tundra by Alnus and of low-shrub/herbaceous communities by Salix will affect herbivores and soil properties. 6.Good agreement between observations of plant functional type and multinomial predictions in a thermal space defined by elevation and insolation suggested that these two variables were sufficient for forecast modeling. Spatially explicit, climate-driven GLM multinomial and random forest classification models in available thermal space forecast surface areas of forest, Alnus, Salix, and tundra over a range of warming, modeled as upward shifted isotherms, including expected IPCC scenarios. Both modeling approaches indicated that shrubs may respond non-linearly to warming. 7.Synthesis The provision of taxon-specific coefficients for climate-driven, spatially-explicit models using high resolution digital elevation models is necessary for accurately forecasting vegetative change due to climate warming in montane and arctic regions.