Distinguishing mature and immature trees allows estimating forest carbon uptake from stand structure

Abstract. Relating forest productivity to local variations in forest structure has been a long-standing challenge. Previous studies often focused on the connection between forest structure and stand-level photosynthesis (gross primary production – GPP). However, biomass production (net primary production – NPP) and net ecosystem exchange (NEE) are also subject to respiration and other carbon losses, which vary with local conditions and life history traits. Here, we use a simulation approach to study how these losses impact forest productivity and reveal themselves in forest structure. We fit the process-based forest model FORMIND to a 25 ha inventory of an old-growth temperate forest in China and classify trees as “mature” (fully grown) or “immature” based on their intrinsic carbon use efficiency. Our results reveal a strong negative connection between the stand-level carbon use efficiency and the prevalence of mature trees: gross primary production (GPP) increases with the total basal area, whereas net primary production (NPP) and NEE are driven by the basal area of immature trees. Accordingly, the basal area entropy, a structural proxy for the prevalence of immature trees, correlated well with NPP and NEE and had a higher predictive power than other structural characteristics, such as the Shannon diversity and height standard deviation. Our results were robust across spatial scales (0.04–1 ha) and yield promising hypotheses for field studies and new theoretical work.