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Semi-arid woodlands have expanded and contracted across the Great Basin and Colorado Plateau throughout the Pleistocene and Holocene.� Large scale expansions typically occurred during mild-moist climate conditions and contractions during the transition from mild-moist to dry.� There is strong evidence that both tree infill into existing stands and expansion were slowly increasing prior to Eurasian settlement.� However, expansion and infill rapidly accelerated during settlement in the late 1800s, peaking in the early 1900s.� Causes attributed to woodland expansion and infill since the late 1800s have been widely debated, and are most frequently attributed climate, livestock grazing, altered fire regimes, and/or changes in atmospheric CO2.� There is considerable evidence that climate has been a primary driver of woodland expansion, contraction, and infill over the past tens of thousands of years and was likely a major factor during the persistent wet period throughout the West in the early 1900s.� However, the effects of climate on woodland dynamics since Eurasian settlement cannot be separated from anthropogenic factors such as altered fire regimes, grazing, and elevated CO2 levels.� There is also considerable debate as to the proportion of woodland expansion versus regeneration, and old-growth or persistent woodlands versus recently converted sagebrush-steppe and savannas.� This in part can be attributed to regional differences in disturbance regimes and ecological site characteristics, which can lead to regional biases when interpreting woodland dynamics across the Great Basin and Colorado Plateau.� Managing these woodlands takes careful consideration of the ecological site components, potential and current vegetation, and the resilience and resistance to invasive annuals for the site of interest.� Introduced invasives are a considerable threat in replacing both recently encroached and persistent (old-growth) woodlands throughout the West and can be linked to warming climate, fire, and elevated CO2.