Long-term changes in belowground and aboveground resource allocation of boreal forest stands

Abstract

Age-related decline of forest stand growth is a common phenomenon, but the associated physiological causes remain uncertain. This study investigated a possible mechanism that could explain stand growth decline observed after canopy closure. We hypothesised that the proportion of resource allocation to roots increases with stand age as a response to a decrease in nutrient availability, which is related to the long-term accumulation of organic matter in boreal forests. Proxies based on soil respiration measurements and stem biomass production were used to describe temporal changes in the proportion of carbon allocated to belowground and aboveground stand components along a 1067-year post-fire chronosequence. The proportion of resources that were allocated belowground increased in the first 200years following fire and declined thereafter. The inverse pattern was observed for the organic matter decomposition rate. Stand-level decline in wood productivity that was observed during the first 60-year period after fire can be attributed to a greater proportion of carbohydrates being allocated to roots in response to a decrease in nutrient availability. However, the relatively low productivity of old-growth stands was not associated with high belowground allocation, suggesting that other mechanisms operating at the tree- or stand-level may be involved.