COS 300-2 - Biotic interactions moderate the climate-vegetation relationship over the last 2,000 years of the pre-Industrial Holocene in the Upper Midwest, U.S.

Abstract: Change in forest aboveground biomass, a major component of the terrestrial carbon sink, is driven by a combination of environmental variables and biotic interactions between trees (e.g., competition), and these processes can be correlated in time (i.e., resulting in time lags). Because of the long lifespans of forest trees, the contributions of biotic interactions and time lags to forest aboveground biomass only become fully apparent over long time periods (decades to centuries). As a consequence of the lower availability of time series data spanning decades to centuries, processes occurring over long time spans are often overlooked. Investigating the degree to which biotic interactions and time lagged effects moderate the relationship between climate and forest vegetation is crucial to making accurate long-term predictions of the carbon cycle.

The objective of this research was to estimate the relationship between climate drivers and forest community composition over the last 2,000 years of the Holocene in the Upper Midwest, U.S. Our approach was to statistically reconstruct forest community composition from fossil pollen proxy data. Then, we combined our estimates of community composition with reconstructions of average annual temperature and total annual precipitation in a hierarchical process model. The process model quantified processes corresponding to the effect of environmental drivers on forest community composition, biotic interactions, and time lagged effects. We quantified the relationship between environmental drivers and forest community composition via a Bayesian generalized linear regression model, with species covariance allowing us to estimate biotic interactions. We incorporated time lags by generalizing our linear regression model to an autoregressive model.

We found that the forest community was on average more sensitive to temperature than precipitation, despite the presence of an ecotone with more mesic forest communities in the east and more xeric prairie communities in the west. Moreover, there were strong correlations between taxa in our study region, with a particularly strong negative correlation between the oak taxon and nearly every other taxon (correlation between oak and maple taxa: R = -0.63). In this region, oak dominance is often associated with drier environments (e.g., oak savanna ecosystems). Our results imply that the maintenance of the prairie-savanna-forest ecotone was a result of not only taxon-specific sensitivity to a precipitation gradient, but also the strong relationships between taxa, resulting in potential climate disequilibrium along the ecotone.