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

Understanding the mechanisms that produce novel ecosystems is of joint interest to conservation biologists and paleoecologists. 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. 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 predicto...