Spatial variation in climate modifies effects of functional diversity on biomass dynamics in natural forests across Canada

Aim Forest net biomass change (Delta AGB; the difference between biomass gain from growth and loss through mortality) determines how forests contribute to the global carbon cycle. Understanding how plant diversity affects Delta AGB in diverse abiotic conditions is crucial in the face of anthropogenic environmental change. Recent studies have advanced our understanding of the effects of plant diversity on growth dependent on the abiotic context, either supporting or rejecting the stress gradient hypothesis. However, we know little about how diversity influences mortality, which prevents us from knowing how diversity affects Delta AGB in diverse abiotic conditions. Location Across Canada (43-60 degrees N, 52-133 degrees W). Time period 1951-2016. Major taxa studied Ninety-three tree species. Methods We modelled the relationships of growth, mortality and Delta AGB with functional diversity that represented niche complementarity, while simultaneously accounting for the influence of functional identity and stand age. Results Growth and mortality increased, on average, with functional diversity, but the magnitude of the increase in growth was greater than that of mortality, resulting in an increase of Delta AGB. The positive relationship between growth and functional diversity was more prominent in more humid sites than in drier sites. Mortality increased with functional diversity in drier sites but did not increase in wetter sites. The positive relationship between Delta AGB and functional diversity was strengthened with water availability. Moreover, the positive relationship between growth and functional diversity became stronger with temperature, but the positive associations of diversity with mortality and Delta AGB were consistent across the gradient of temperature. Main conclusions Our results suggest that higher functional diversity leads to an increase in forest biomass accumulation owing to a greater positive effect of functional diversity on productivity than on mortality. However, in contrast to the stress gradient hypothesis, our findings show that the positive effect of functional diversity is more pronounced in an environment favourable for growth.