All climate predictors were derived from WorldClim version 2 climate data, downloaded from http://worldclim.org/version2.org/version2
Climate predictors are abbreviated as in Additional file 1: Table S1.
In the two sub-periods 1902 1955 and 1956 2009, drought signals are evident as well, but are expressed through different climate predictors.
We identified the most influential climate predictors for spruce and fir, and found a distinct unimodal response of SI to some of these predictors.
Using NPMR, species distribution responses were predicted as a function of multiplicative interactions among climate predictors, using an iterative algorithm that maximizes fit by analyzing target points in a local neighborhood in predictor space via distance-weighted smoothing functions.
This bioclimatic approach has two assumptions: 1. Species distributions are at equilibrium with, and represent direct sensitivity, to macroclimate; 2. Relationships among climate predictors remain comparable when extrapolating from baseline to climate change scenarios.
The presented null models consider constant tree-ring indices p1-p3 climate predictors 1 to 3, c1-c3 coefficients of fixed effects, ΔAIC c corrected AIC value (AICc) of the null model minus the AICc of the tested model, R 2 marginal explained variance.
NPMR may have advantages over other niche modeling methods (Lintz et al. 2011), including the ability to identify non-linear, multiplicative effects of climate predictors on species occurrence patterns, improving the potential to predict "multi-niche" space (e.g., bimodal species responses).
Climate predictors had a more significant influence on Holocene chironomid assemblages, especially July insolation at 60 °N, estimates of regional precipitation and estimates of northern hemisphere temperature, while only the development of the taiga biome had a significant impact on these primary consumers.
We also tested some of the closely correlated alternative climate predictors in this way, but none gave a better fit than the set we had already chosen.
