Ongoing anthropogenic changes, such as land-use change, climate changes, and rising CO₂, can alter vegetation distribution and function. Ecotones and biome boundaries, such as at the boundary between savannas and closed forests, may be the most vulnerable to these environmental changes. Here, I use both historical survey data and tree ring records to explore the impacts of ~150 years of past environmental changes on the savanna-forest boundary region of the Midwest US. We find that past vegetation had a multiple stable vegetation states of open savanna and closed forests that were likely maintained by disturbance-vegetation feedbacks, but ongoing land use changes have collapsed the modern vegetation into a single closed forest state. While modern systems are currently maintained as closed forests, hotter and drier future conditions could threaten these forests. Tree ring data indicate that rising atmospheric CO₂ could help alleviate drought stress, because it increases plant water use efficiency (WUE), resulting in decreased sensitivity to precipitation. However, we find that the increasingly negative impacts of high summer temperatures outweigh any positive impacts of higher WUE on tree growth. Overall, the historical perspective provided here documents non-linearities in vegetation-environment relationships that are difficult to forecast, and indicates that even mesic systems may be susceptible to higher future temperatures.