A new national assessment published in Ecological Processes finds that China's forest light use efficiency, a measure of how well trees convert sunlight into biomass, rose by 0.003 grams of carbon per megajoule per year from 2000 to 2023. The increase covered 94.2% of forested land, with 72% showing statistically significant gains. The study used 1,710 ground observations, remote sensing, and machine learning to map efficiency across the country, revealing higher values in southern and southwestern forests and lower values in the north and northeast. The key finding is that biological factors, such as forest type, stand age, and canopy structure, account for about 69% of the observed efficiency gains in expanding areas, while rising atmospheric CO2 contributes roughly 16% and land-surface environmental factors about 15%. This challenges older assumptions that warming-driven longer growing seasons were the main driver. The authors note that stand age and forest type strongly modulate responses to moisture and temperature, with mature forests showing the only significant positive response to increased soil moisture. For carbon accounting and reforestation planning, this suggests that forest age and composition should be central to any carbon sink projection or offset methodology. Ignoring biological structure could lead to over or under crediting of forest-based climate projects. The study also highlights regional differences, such as water limitation in most temperate forests but a negative moisture effect in northeastern deciduous needleleaf forests due to permafrost hydrology.
