Decoupling of the relationship between canopy and stem growth in broadleaf trees under humid conditions

Abstract

Tree stems act as the primary carbon storage organs, with leaf growth variability regulating storage intensity. However, how environmental factors influence carbon storage via leaf growth remains unclear. This study examines the relationship between tree stem growth (tree-ring width index, RWI) and leaf growth (normalized difference vegetation index, NDVI) across different forest types globally. We analyzed the potential influencing factors of this relationship and identified the pathway from environmental conditions to leaves and then stems. We found that MODIS NDVI, with higher spatial resolution, better reflects stem growth in broadleaf forests, while GIMMS NDVI, with a longer time series, is more effective in characterizing stem growth in coniferous forests. Forest type and moisture availability are the dominant factors shaping the leaf-stem growth relationship. Coniferous trees growing under increasingly drier conditions, with higher tree density and poorer soil fertility, tend to exhibit a stronger coupling between leaf and stem growth. Further analysis showed that in moist environments, especially in broadleaf forests, leaf and stem growth become increasingly decoupled. In contrast, conifers maintain stronger coordination. This may be related to trade-offs in resource allocation driven by the varying relative importance of external environmental stress and internal biological connections affecting leaf and stem growth under different moisture conditions. Leaf growth is influenced by soil moisture, soil temperature, and air temperature, but their relative contributions vary across forest types and moisture regimes. These findings suggest caution when using leaf growth (e.g., NDVI) as a proxy for forest productivity in humid broadleaf ecosystems, where decoupling may mask actual carbon allocation patterns.