Shifts in trait means and variances in North American tree assemblages: species richness patterns are loosely related to the functional space

the co-occurrence of species that are more ecologically similar than expected from a random sample of the species that could potentially colonize the study site (Keddy 1992). Filtering is thought to regulate species richness both locally and regionally, with a potentially stronger eff ect of climate at regional and continental scales, and topography, soil characteristics and land management at fi ner scales (Algar et al. 2011, Freschet et al. 2011, Lessard et al. 2012, de Bello et al. 2013; see also Terborgh 1973). Although there are numerous Ecography 37: 001–010, 2014 doi: 10.1111/ecog.00867 © 2014 Th e Authors. Ecography © 2014 Nordic Society Oikos Subject Editor: Zhiheng Wang. Editor-in-Chief: Carsten Rahbek. Accepted 1 Spetember 2014