Irena Šímová and colleagues recently had their paper on large-scale forest productivity published in Journal of Ecology. Lead author Irena provides some background on the study below…

Climate is commonly considered the main driver of forest biomass production. Nevertheless, it has recently been suggested that climatic factors affect biomass production mostly indirectly via local adaptations, soil nutrients or regional land-use history mirrored in characteristics of coexisting species. Using data on species occurrence and functional trait databases for North and South America, we assessed the extent to which climate conditions affect large-scale forest net primary productivity (hereafter ‘productivity’) indirectly through their effect on tree size and leaf functional traits.

Figure 1: Simplified conceptual path diagram demonstrating hypothesized causal relationships of climate conditions, disturbances, tree height, leaf traits and soil nutrient concentrations (‘Soil’) in explaining net primary productivity (‘NPP’) of forest assemblages. Solid black lines represent our predictions about the indirect effect of climatic variables, disturbances and soil nutrient concentrations on NPP via tree size and leaf traits whereas dashed lines represent alternative direct effects of these variables on NPP.

We found forest productivity to be determined by both climatic variables and tree functional traits. However, the importance of each variable is not universal, as different processes matter in different environmental contexts. In tropical and temperate forests, productivity increases with increasing tree height, in agreement with current theory and evidence from smaller spatial scales that tree size is a central trait affecting the flux of carbon and biomass production. Furthermore, productivity in temperate forests increases rapidly with increasing concentrations of nitrogen in leaves whereas in tropical forests it strongly increases with leaf phosphorus concentrations. These results support field-based evidence that the limiting effect of nitrogen on productivity is more pronounced in temperate than in tropical regions, where phosphorus is the key nutrient influencing it. Productivity of boreal forests, by contrast, is best explained by a single effect of temperature, and neither tree height nor leaf traits have a significant influence.

Figure 2: The metabolic scaling theory for forest structure and dynamics predicts that the largest individual in the stand is the central predictor of variation in whole-stand biomass production. The beech-maple forests of the Warren Woods State Park in Michigan are home to some of the tallest specimens of the North American beech (Fagus grandifolia) and the sugar maple (Acer saccharum). (Photo: Benjamin Blonder)

Understanding how environmental changes alter the composition of plant assemblages and how this in turn affects ecosystem processes is key to improving our ability to predict the ecological impacts of global climate change. Our finding that plant size and leaf traits link species composition to large-scale productivity might considerably help improve models predicting the dynamics of the global carbon cycle. An important next step will be to explore the roles of plant functional traits in other ecosystem processes such as the ability of trees to regulate regional climates via rates of transpiration.

Irena Šímová, Charles University, Czech Republic

Read the full paper online: The relationship of woody plant size and leaf nutrient content to large‐scale productivity for forests across the Americas