Simulated Importance of Dispersal, Disturbance, and Landscape History in Long-Term Ecosystem Change in the Big Woods of Minnesota

Dynamic relationships among climate, disturbance, and vegetation affect the spatial configuration and composition of ecological communities. Paleoecological records indicate the importance of such relationships in Minnesota's Big Woods (BW) region, where isolated hardwood forest populations expanded to regional dominance after AD 1250. We used LANDIS-II to model the BW forest expansion, and conducted simulation experiments that isolated the important ecological factors in this regional change. In our simulations, BW forest expanded at approximately 15 m per year to achieve regional dominance within 600 years, which is comparable to empirical records. The distribution of the BW depended on the locations of scattered pre-existing tree populations that were sheltered from previous severe fire regimes by firebreaks. During the simulated spread of the tree populations, however, the presence or absence of firebreaks did not further influence vegetation pattern. When we assumed a fire rotation of 10-13 years in grasslands/woodlands and more than 400 years in BW, the feedback between fire-resistant BW fuels and fire severity caused fire severity to decline in a time frame consistent with sedimentary data. In our simulations, seed dispersal from core initial populations caused forest expansion, changed fuel loads, and thus reduced fire severity-not the other way around as has been commonly proposed. Forest expansion was slowed by fire, but species' life history attributes, namely seed dispersal distances and maturity ages, asynchronous successional dynamics across many stands, and landscape history were at least as important in the temporal and spatial patterns of the regional response to climate change.