The effect of species diversity on tree growth varies during forest succession in the boreal forest of central Canada

Abstract

Although major advances have demonstrated that species diversity has a general positive effect on forest ecosystem productivity, some studies report negligible or even negative effects, highlighting remaining uncertainty in our knowledge of the ecological mechanisms that influence diversity–productivity relationships. In particular, ecological succession is postulated to drive temporal shifts in the strength and direction of diversity–productivity relationships, but few studies have explicitly tested this hypothesis because long-term succession data (from forest initiation to eventual climax) are rare.Using a detailed, replicated chronosequence (space-for-time substitution) study design of 53 natural forest stands (ages 8 to 210 years) in the boreal forests of central Canada, we investigated the relationship between neighbourhood species diversity and tree growth of five dominant boreal tree species, covering entire, long-term secondary successional sequences following stand-replacing wildfire.We found compelling evidence that the strength of the relationship between species diversity and tree growth changes over the course of secondary succession, following a general “hump-shaped” pattern, with mid-succession stages of higher functional diversity exhibiting the strongest growth–diversity relationships. However, tree species exhibited individualistic responses to succession-driven changes in species diversity, with broadleaf species (e.g., Populus tremuloides) generally showing negative responses, whereas conifers (e.g., Pinus banksiana) responded more favorably to higher neighbourhood diversity. Furthermore, our results show the effect of individual tree size on the relationship between species diversity and tree growth to be highly variable, contradicting the hypothesis that larger trees benefit more from complementarity due to size-asymmetric competitive ability. These results contribute to disentangling the mechanisms that link species diversity to forest growth and function, which is important to sustainable forest management planning and for predicting the consequences of global biodiversity loss, especially for the boreal forest, which plays a critical role in controlling global carbon flux and climate.