Isolating the effects of forest regrowth and functional adjustments upon global change impacts on Yucatán's forest biomass

Tropical forests hold large stocks of carbon in biomass and face pressures from changing climate and anthropogenic disturbance. Understanding the impact of these pressures on biomass is vital for effective forest management and conservation over the next century. Forests' capacity to store biomass under future conditions and accumulate biomass during regrowth after clearance are major knowledge gaps. Here we use chronosequence data, satellite observations and a C-cycle process model to diagnose woody C dynamics in two major dry forest ecotypes (semi-deciduous and semi-evergreen) in Yucatán, Mexico. Woody biomass differences between mature semi-deciduous (90 MgC/ha) and semi-evergreen (175 MgC/ha) forest landscapes are mostly explained by differences in climate (c. 60%), particularly temperature, humidity and soil moisture effects on production. Functional variation in foliar phenology, woody allocation, and wood turnover rate explained c. 40% of biomass differences between ecotypes. Modelling experiments explored varied forest clearance and regrowth cycles, under a range of climate and CO2 change scenarios up to 2100. Climate scenario projections indicate that production and steady state biomass in both ecotypes were reduced by forecast warming and drying (mean biomass 2021-2100 reduced 16-19% compared to 2001-2020), but compensated by fertilisation from rising CO2. Functional analysis indicates that trait adjustments could amplify biomass losses by c. 70%. Experiments with disturbance and recovery across historically reported levels for the Yucatán indicate reductions to mean forest biomass stocks over 2021-2100 similar in magnitude to climate change impacts (10-19% reductions for disturbance with recovery). Forest disturbance without regrowth amplifies biomass loss by three- or four-fold. Our results identify the potential for functional adjustments, hypothesised to limit climate risks, to magnify biomass reductions over the coming century. However, the range of impacts of land use and land use change are as, or more, substantive than the totality of direct and indirect climate impacts. The dataset is related to the upcoming publication "Isolating the effects of regrowth and functional adjustments on climate and land use impacts on Yucatán's forest biomass in the twenty-first century" (in review).