Fig. 1: A conceptual framework for assessing how phenotypic traits mediate effects of climate on population growth rate.

a General framework. Variables and paths shown in black were included in path analysis models: the effect of climate C on population growth rate G can be mediated by the considered trait Z (black solid line) and by other non-considered traits or direct effects (black dotted line, path “CG”, read as “the effect of climate on growth”). Population size P is included to account for potential confounding effects (e.g., density dependence; black dashed line, path “PG”). Notation next to each arrow provides the name of the estimated path coefficient. Time was used outside of the path analyses to detrend the climate variable obtained with the sliding window analysis, prior to fitting the path analyses (dashed grey arrow from T to C, “TC”—see “Methods”). b–d show relations behind each path coefficient. The relations in b–d reflect our predictions for spring phenological traits. We also calculated the trait-mediated effect of climate on population growth rate (“CZG”, i.e., the path from C to G that is mediated by Z and is found by multiplying the coefficients CZ and ZG for the two black solid paths from C to Z and Z to G) and the total effect of C on G (TotalCG, sum of the trait-mediated and other effects, see “Methods”).