Jinhyuk Kim Dissertation Defense

Title: Understanding the influence of fire on photosynthesis in North American Arctic-boreal ecosystems

Abstract: The global carbon cycle is ever in flux and more uncertain as climate driven increases of wildfire burn through terrestrial ecosystems. While fires are important for ecosystem health, Arctic-boreal ecosystems have experienced an increasing number of wildfires in recent years pushing them into an unstable state. These ecosystems are important carbon sinks and understanding both the immediate and long-term impacts on ecosystem fluxes is important to understanding the persistence of the carbon sink. In my dissertation, I focused on better understanding how fires affect the largest terrestrial carbon flux, photosynthesis. Using an array of satellite platforms, eddy covariance flux measurements, and simple statistical models, I explored how disturbances affect landscape photosynthesis, gross primary production (GPP). In chapter 2, my co-authors and I explored post-fire mechanisms that drive multidecadal enhancements to photosynthesis in the Alaskan tundra using the Anaktuvuk River fire as a case study. The enhancement of photosynthesis could be seen at individual sites using multiple eddy covariance flux towers, and across the burn scar using satellite observations of solar-induced fluorescence (SIF). I found that more severe fires led to the deepening of the active layer from in-situ measurements. The deepening of the active layer can then support greater growth of shrubs and consequently enhanced photosynthesis. In Chapter 3 I explored the impacts of fires on photosynthesis across the western North American boreal forests by analyzing historical fire perimeters and satellite data. I found fires are reducing the extent of evergreen conifer forests and shifting the landscape to early successional species. These early successional species enhance the seasonality of SIF for multiple decades. Then using a simple stochastic model, I find that increased disturbance can cause a long-term positive trend in the seasonality of photosynthesis. In Chapter 4, I developed a simple GPP model using the near-infrared reflectance of vegetation (NIRv), photosynthetically active radiation, and GPP measurements from a network of eddy covariance flux towers. I scaled the model globally and found global GPP to be roughly 160 Pg C yr-1. This estimate is more in line with top-down constraints of GPP. Using this simple GPP model, I find that fires in North American Arctic-boreal ecosystems strongly influence seasonal anomalies of GPP and can suppress the additional benefits of warming. Overall I find that fires lead to long-term enhancements of photosynthesis through post-fire successional processes. These mechanisms along with the immediate impacts from fires have strong controls over GPP trends and interannual variability in Arctic-boreal ecosystems.