Anthromes, CO2 and Terrestrial Carbon – Session 3: The role of forest demography & succession in the carbon cycle
Authors: Anderson-Teixeira Kristina , Hogan J. Aaron , Heinrich Viola , Fricke Evan , Au Tsun Fung 
Published: March 2023

3.00 – 3.20: Carbon cycling in mature and regrowth forests globally, Kristina Andersen-Teixeira

3.20 – 3.40: Aging forests and weak growth enhancement drive biomass dynamics in US forests, J.Aaron Hogan

3.40 – 4.00: The carbon sink of recovering degraded and secondary forests across the tropics: Science, Satellites and Policy, Viola Heinrich

4.00 – 4.20: Disruption of animal seed dispersal reduces carbon accumulation during tropical forest regrowth, Evan Fricke

4.20 – 4.40: Forest demographic shifts alters drought responses and associated carbon sequestration, Tsun Fung Au

Carbon cycling in mature and regrowth forests globally

Anthropogenic disturbances have impacted a large and growing proportion of Earth's forests, dramatically altering the global forest carbon (C) budget. As society increasingly seeks to make forests an important component of achieving net-zero C emissions, it is essential to understand how forest C cycles change with stand age as forests recover from stand-clearing disturbances. Here, we draw from the Global Forest Carbon Database, ForC, to provide a macroscopic overview of C cycling in the world's forests, giving special attention to stand age-related variation. Specifically, we use 11,923 ForC records for 34 C cycle variables from 865 geographic locations to characterize ensemble C budgets and quantify trends with stand age for four broad forest types. C cycling rates generally decreased from tropical to temperate to boreal in both mature and regrowth forests, whereas C stocks showed less directional variation. Mature forest net ecosystem production did not differ significantly among biomes. The majority of flux variables, together with most live biomass pools, increased with stand age. This synthetic global overview of C stocks and fluxes across biomes and stand ages provides a baseline for understanding and managing the carbon dynamics of forests in the face of accelerating global change.

Climate change determines the sign of productivity trends in US forests,

U.S. forests offset about 10% of annual U.S. carbon emissions, potentially slowing climate change. Forest recovery from past land use is known to influence the U.S. carbon sink, however, previous studies on forest responses to CO2 fertilization show mixed evidence. Therefore, the contributions of forest recovery and growth enhancement (ge, defined as the change in biomass stock/production at a given stand age) to the US forest carbon sink remain uncertain. Using non-linear models applied to national forest inventory data (2000-2022), we quantified ge by ecoprovince in non-plantation forests. We explored a variety of model forms and data subsets (e.g., excluding selectively-harvested plots). In most analyses, ge ranged from -1.7 to +2.1%/year, being positive in the East but negative in the Interior West and Pacific. Stand age-dependent biomass change was always positive, ranging from 0.11 Mg/ha/yr in Nevada-Utah Coniferous Forest to 1.35 Mg/ha/yr in Cascade Mixed Forest. These results suggest that disturbances in the western U.S. are causing biomass reductions at a given stand age, rather than reductions in reported stand age. While aging stands contribute significantly to forest carbon balance across the entire USA, growth enhancement is greatest where non-stand-age-resetting disturbances (e.g., pests/pathogens, minor wind/fire, warming, drying) are absent.

The carbon sink of recovering degraded and secondary forests across the tropics: Science, Satellites and Policy

Tropical forests play a key role in climate change mitigation. Recovering, degraded and secondary forests are becoming more dominant in tropical landscapes and yet large uncertainties exist regarding their carbon sink and storage. I will present the results of our recently accepted paper, where we combined satellite-based datasets of different forest types and aboveground carbon to model the regional carbon recovery in degraded and secondary forests across the Amazon, Central Africa, and Borneo, according to different environmental variables and disturbances. Between 1984 and 2018 recovering forests offset a quarter of carbon emissions from tropical forest loss, indicating the mitigation potential of protecting them, alongside old-growth forest conservation.

Disruption of animal seed dispersal reduces carbon accumulation during tropical forest regrowth

Over two thirds of aboveground carbon in tropical forests is contained in plant species that rely on animal seed dispersers for regeneration. Animal biodiversity declines and movement restrictions due to human activities including land use change may therefore negatively affect natural forest regrowth and carbon storage following deforestation or degradation. We estimated the effect of seed dispersal disruption on aboveground carbon accumulation in tropical forests. We modelled land use-dependent seed dispersal using data from thousands of field studies, and then used records of forest growth over time to assess whether variation in seed dispersal is related to differences in carbon accumulation among sites. We found that sites with high seed dispersal integrity accumulated more carbon more quickly than did sites where seed dispersal is strongly disrupted. Differences in seed dispersal integrity explained >3-fold variation in carbon accumulation rates. Our findings identify areas with greatest potential for natural regrowth as an alternative to tree planting and highlight the potential for the restoration of vertebrate biodiversity to act as a natural climate solution by amplifying carbon storage during natural forest regrowth. Overall, the analysis highlights a strong relationship between animal biodiversity and carbon storage in tropical forests.

Forest demographic shifts alters drought responses and associated carbon sequestration

As forest demographics are altered by the global decline of old trees and reforestation efforts, younger trees are expected to have an increasingly important influence on carbon sequestration and forest ecosystem functioning. However, the relative resilience of these younger trees to climate change stressors is poorly understood. Here we examine age-dependent drought sensitivity of over 20,000 individual trees across five continents and show that younger trees in the upper canopy layer have larger growth reductions during drought. Angiosperms show greater age differences than gymnosperms, and age-dependent sensitivity is more pronounced in humid climates compared with more arid regions. However, younger canopy-dominant trees also recover more quickly from drought. The future combination of increased drought events and an increased proportion of younger canopy-dominant trees suggests a larger adverse impact on carbon stocks in the short term, while the higher resilience of younger canopy-dominant trees could positively affect carbon stocks over time.