Impacts of climate warming on biomass proportion of global forest carbon stocks

A large amount of carbon is stored in global forests. However, the fraction of carbon stored as plant biomass vs. soil organic carbon (SOC) varies among forest types. The extent to which biomass and SOC pools may change over the 21st century is uncertain, yet important to carbon cycle interactions with climate change. Here, we used data derived from inventories and remote sensing and CMIP6 models to examine the current and 21st century dynamics in the proportion of biomass and SOC across global forests. Our results show contrasting ecosystem carbon pools of forests in colder vs. moist warmer climates. Boreal forests currently store only 14±7% of their ecosystem carbon as plant biomass compared to 50 ±18% for moist tropical forests. We found that annual precipitation, topography, soil, and wildfire were the primary controls of these differences in forest carbon pool fractions. Under the Shared Socioeconomic Pathway (SSP5-8.5) climate scenario, CMIP6 models project that the ratio of biomass to ecosystem carbon in global forests will increase across the 21st century, with the largest increases in boreal forests compared to moist tropical forests. Changes in forest ecosystem carbon pools resulting in greater biomass fraction will affect the surface energy balance, disturbance regime, wildfire fuel loads, and ecosystem carbon balances, all of which interact with the climate system.