Differing climatic mechanisms control transient and accumulated vegetation novelty in Europe and eastern North America

Conservation biologists, tasked with stewarding biodiversity during the Anthropocene, face the challenge of managing systems during a time of rapid environmental change and the shifting of the climate system to a state unlike any in societal experience. One expected outcome of current climate trends is the rapid reshuffling of species into novel mixtures, and understanding the mechanisms that cause the emergence of novelty is of vital importance. Here, we define and differentiate transient from accumulated novelty and evaluate four climatic mechanisms proposed to cause species to reshuffle into novel assemblages: high climatic novelty, high spatial rates of change (displacement), high variance among displacement rates for individual climate variables, and divergence among displacement vector bearings. Transient novelty better captures the short term emergence of novel communities during periods of rapid environmental change, while accumulated novelty integrates the long term tendencies over increasingly distant time periods. We use climate simulations to quantify climate novelty, displacement, and divergence across Europe and eastern North America (ENA) from the last glacial maximum (LGM) to present and fossil pollen records to quantify vegetation novelty. Transient climate novelty is consistently the strongest predictor of transient vegetation novelty, while displacement rates (mean and variance) are equally important in Europe. However, transient vegetation novelty is lower in Europe and its relationship to climatic predictors is the opposite of expectation. For both continents, accumulated novelty is greater than transient novelty, and climate novelty is the strongest predictor of accumulated ecological novelty. These results suggest that controls on novel ecosystems vary with timescale and among continents, and that the 21^st-century emergence of novel ecosystems will be driven by both rapid rates of climate change and the emergence of novel climate states.