DISSERTATION RESEARCH: Investigating the resiliency of the savanna-forest biome to environmental change

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Today, closed forests dominate the non-agricultural land in the North American Midwest, but data from before European settlement, less than 200 years ago, indicate that both open savanna and closed forests co-existed in the region. Such changes in forests are often attributed to changes in fire occurrence and frequency, but analyses of tree rings suggest other environmental changes such as rising CO2 and changes in weather patterns that may have contributed to shifts in forests and savannas. In this Doctoral Dissertation Improvement Grant (DDIG) award funds will be used to measure tree ring carbon stable isotopes to estimate annual trends in water use efficiency across a network of savanna and forest sites to determine the mechanism for changes in tree growth and the trends observed in historical survey and ring-width data. A better understanding of the many factors responsible for changing forest cover in the past will help in understanding how anticipated future environmental changes might affect Midwestern forests: Hotter temperatures and drought stress could drive tree decline, pushing today's forests towards the savanna state. Alternatively, increases in CO2 may confer drought resilience by enhancing tree growth, and favoring forest expansion. These alternative trajectories for the Midwest create uncertainty in predicting future vegetation distribution and make it difficult for local land and forest managers to develop effective resource management plans. The award funds will be used to test the hypothesis that increased atmospheric CO2 has driven past changes in tree growth and altered forest resilience to future changes. This research advances scientific understanding of how environmental changes affect Midwestern forest resilience and growth, and more broadly identifies the conditions where tree growth is resilient to environmental changes. This award builds on a network of sites where the researchers have developed annual tree ring growth records from 1850-2015 for oak trees in both closed forest and open savannas. From the annual trends in tree ring growth across these sites, the researchers have identified places where tree growth has increased since 1950 and where the growth-climate relationships change over time. Such shifts are consistent with CO2 driven enhancement of tree water use efficiency (carbon uptake by the tree per unit of water needed), which can theoretically drive increases in tree growth and improve drought tolerance in trees. However, growth data alone cannot determine if CO2 fertilization is the mechanism for these changes. Thus, researchers will use stable carbon isotopes in tree rings, which are related to water use efficiency, to determine the extent to which rising CO2 can explain changes in tree growth. Overall, this project will test whether CO2 enhancement played a role in stabilizing trends in Midwest forest cover, and will provide scientific insight into the conditions necessary for CO2 enhancement of tree growth.