As China's natural forests age, canopy structure becomes an ever-stronger engine of biomass accumulation

China is investing heavily in restoring its natural forests, and those forests are quietly changing how they build carbon. A new study of 30 years of forest regeneration across the country shows that as stands age, the architecture of the canopy becomes an increasingly decisive partner in biomass accumulation — and the environmental levers that govern recovery change with it. Forest restoration is a leading nature-based solution for climate mitigation, but assessments often rely on static assumptions: that a young forest and an old one respond to their environment in the same way. Testing that assumption at national scale has been difficult because field plots capture only a fraction of the landscape. A team at Peking University led by corresponding author Qinghua Guo combined satellite-derived maps of naturally regenerating forest with spaceborne lidar measurements of canopy structural complexity (CSC) — a measure of how intricately a canopy is arranged in three dimensions — and estimates of aboveground biomass density (AGBD). They divided 30 years of recovery into three stages: 1–10, 11–20, and 21–30 years, and analyzed 15 of China's major forest biomes separately. The result is unambiguous: canopy structure was the core determinant of biomass density throughout recovery, and its influence grew stronger with age (interaction with stand age, P < 0.001; model R² = 0.76). In the first decade, young trees race to occupy horizontal space, and structure adds relatively little biomass. After canopy closure, competition shifts vertically and complex canopies translate into markedly higher biomass — the slope of the structure–biomass relationship rises from 0.46 to 0.52 across stages. Each forest biome also carries an early "structural fingerprint": tropical and subtropical forests build complexity faster than temperate and cold-temperate ones, and these early differences persist and widen through recovery, providing measurable, biome-specific benchmarks for restoration. The controls themselves move in stages. In young stands, temperature — especially its seasonal range — is the dominant filter, together with topography and soil. As canopies close, their buffering microclimate weakens the direct climatic signal, and soil resources become the chief constraint. In mature, high-leaf-area forests, precipitation, particularly annual totals, emerges for the first time as the dominant national-scale driver, setting an upper limit on mature forests' biomass carrying capacity. The authors caution that their analysis covers the first 30 years of recovery and draws on public remote-sensing products, so older stands and fine-scale processes remain to be tested. Even so, the message for managers is concrete: restoration targets and carbon accounting should switch their attention from temperature to soil to water as a forest matures — one-size-fits-all benchmarks will misread both young and old forests.