Geology and climate conditions affect more humus forms than forest canopies at large scale in temperate forests

We investigated by linear gradient analysis (RDA) the relationships between forest humus forms (9 humus forms and the Humus Index) and 148 variables describing geology, climate, soil type, geography and the floristic composition of forest canopies, using 3441 plots of the EcoPlant database covering the whole French territory. Among these variables, geology (alkaline vs acidic substrate) and climate (warm/dry vs cold/rainy) were the major determinants of humus forms, scaling mull humus forms from eumull to dysmull and opposing them to mor/moder, while the contribution of tree canopies was negligible. This trend was verified by partial RDA with environment or abundance of tree species from forest canopy as co-factors. The original position of amphi was confirmed: it was the only humus form not included in the gradient of increasing biological activity ordinated according to climate and geology. Results and possible forecasts of humus forms according to global warming were discussed to the light of existing knowledge.