Shade tolerance and the functional trait - demography relationship in temperate and boreal forests

Despite being instrumental in forest ecology, the definition and nature of shade tolerance are complex and not beyond controversy. Moreover, the role it plays in the trait-demography relationship remains unclear. Here, we hypothesize that shade tolerance can be achieved by alternative combinations of traits depending on the species' functional group (evergreen vs. deciduous species) and that its ability to explain the array of traits involved in demography will also vary between these two groups. We used dimension reduction to identify the main trait spectra for 48 tree species, including 23 evergreens and 25 deciduous - dispersed across 21 genera and 13 families. We assessed the relationship between functional traits, shade tolerance, and demographic performance at high and low light using structural equation modelling. The dimensions found corresponded to the trait spectra previously observed in the literature and were significantly related to measures of demography. However, our results support the existence of a divergence between evergreen and deciduous species in the way shade tolerance relates to the demography of species along light gradients. We show that shade tolerance can be attained through different combination of traits depending on the functional and geographical context, and thus, its utilization as a predictor of forest dynamics and species coexistence requires previous knowledge on the role it plays in the demographic performance of the species under study. The authors acknowledge Dr. Yoshida and Dr. Evans for kindly providing the SORTIE parameters for species from Japan and United Kingdom, respectively. We are also grateful to Timothy Paine and two anonymous referees for their constructive comments on an earlier version of this manuscript. This research was supported by a Juan de la Cierva Fellowship to A.A. (FJCI‐2014‐20739), a NSERC Strategic Grant to D.G. and the NSERC industrial Chair programme to C.M. Additional support came from the CERCA Programme of the Generalitat de Catalunya. The authors declare no conflict of interests.