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

TL;DR

This study demonstrates that forest structure, especially the prevalence of immature trees, can be used to estimate forest carbon uptake, revealing key relationships between tree maturity, stand structure, and carbon fluxes.

Contribution

It introduces a novel approach linking forest structural proxies to carbon fluxes, emphasizing the importance of immature trees in predicting NPP and NEE.

Findings

Basal area of immature trees predicts NPP and NEE effectively.

Basal area entropy correlates with forest productivity metrics.

Results are consistent across multiple spatial scales.

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 (GPP). However, biomass 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 25ha inventory of an old-growth temperate forest in China and classify trees as "mature" (full-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: GPP increases with the total basal area, whereas 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 higher predictive power than other structural characteristics such as Shannon diversity and height standard deviation. Our results were robust across spatial scales (0.04-1ha) and yield promising hypotheses field studies and new theoretical work.