Climate and Spatially Heterogeneous Abiotic Site Constraints Shape Temperate Tree Species Abundance at Northern Range Limits

Abstract

Aim

Northern populations of temperate tree species evolve under climatic, topographic and edaphic conditions that approach the limits of their ecological tolerance. However, the environmental constraints acting on species abundance are not spatially uniform, and their relative importance can shift markedly from core to northern populations. We aimed to identify the environmental drivers that control temperate tree abundance across Quebec, determine how their influence varies across space, and identify the key constraints at their northern range limit.

Location

Meridional Quebec (Canada).

Time Period

1970–2024.

Major Taxa Studied

Temperate tree species.

Methods

We analysed tree species abundance data from 115,035 inventories derived from permanent and temporary plots established by the Quebec government. Using a random forest model, we modelled the abundance of 17 temperate species based on climate variables, topographic variables, soil characteristics and stand dynamics. We then analysed SHAP (SHapley Additive exPlanations) values to evaluate the importance, direction and spatial variability of each explanatory variable's contribution to species abundance.

Results

Model performance was strong across species, with R² values ranging from 0.18 to 0.57. Climate variables exerted the strongest influence on abundance for most species, while others, such as Acer saccharum, were more affected by edaphic and topographic variables, with a moderate influence from climate. The spatialized analysis of SHAP values indicates that colder temperatures during the growing season generally have a negative effect on the abundance of northern populations. In contrast, edaphic and topographic variables reveal strong interspecific and spatial heterogeneity in environmental constraints, associated with the structuring of environmental gradients across Québec. Importantly, the relative influence of environmental drivers was strongly spatially heterogeneous.

Main Conclusions

Our results show that abundance at range edges is not governed by a single limiting factor, but by spatially variable climatic, edaphic and topographic constraints operating across the temperate–boreal transition.