Modeling and empirical validation of long‐term carbon sequestration in forests (France, 1850–2015)

Abstract

The development of appropriate tools to quantify long‐term carbon (C) budgets following forest transitions, that is, shifts from deforestation to afforestation, and to identify their drivers are key issues for forging sustainable land‐based climate‐change mitigation strategies. Here, we develop a new modeling approach, CRAFT (CaRbon Accumulation in ForesTs) based on widely available input data to study the C dynamics in French forests at the regional scale from 1850 to 2015. The model is composed of two interconnected modules which integrate biomass stocks and flows (Module 1) with litter and soil organic C (Module 2) and build upon previously established coupled climate‐vegetation models. Our model allows to develop a comprehensive understanding of forest C dynamics by systematically depicting the integrated impact of environmental changes and land use. Model outputs were compared to empirical data of C stocks in forest biomass and soils, available for recent decades from inventories, and to a long‐term simulation using a bookkeeping model. The CRAFT model reliably simulates the C dynamics during France's forest transition and reproduces C‐fluxes and stocks reported in the forest and soil inventories, in contrast to a widely used bookkeeping model which strictly only depicts C‐fluxes due to wood extraction. Model results show that like in several other industrialized countries, a sharp increase in forest biomass and SOC stocks resulted from forest area expansion and, especially after 1960, from tree growth resulting in vegetation thickening (on average 7.8 Mt C/year over the whole period). The difference between the bookkeeping model, 0.3 Mt C/year in 1850 and 21 Mt C/year in 2015, can be attributed to environmental and land management changes. The CRAFT model opens new grounds for better quantifying long‐term forest C dynamics and investigating the relative effects of land use, land management, and environmental change.

Carbon benefits from Forest Transitions promoting biomass expansions and thickening

“…More recently, Le Noë et al. (2020) showed that during the last 50 years the carbon stock of forests in France has grown from increases in biomass density. They also discussed how trees serve as a pathway for carbon from the atmosphere into soils.…”

“…We propose that Forest Transitions played a growing role, not yet fully appreciated in accelerating the carbon sink in terrestrial ecosystems. Across the large areas of global lands where Forest Transition has occurred (Meyfroidt & Lambin, 2011), vegetation biomass carbon builds up responding to an expansion of both forest area and forest density (Le Noë et al., 2020).…”

“…A first documentation of Forest Transition was published for France showing a turn-around from shrinking to expanding forest area in the early 19th century (Mather, Fairbairn, & Needle, 1999). More recently, Le Noë et al (2020) showed that during the last 50 years the carbon stock of forests in France has grown from increases in biomass density. They also discussed how trees serve as a pathway for carbon from the atmosphere into soils.…”

“…Initially, Forest Transition referred to land area as the landscape attribute (Mather, 1992). More recently, it has been applied also to forest biomass density as a shift from degrading to becoming denser (Kauppi et al., 2006; Le Noë et al., 2020).…”

Unprecedented pluri-decennial increase in the growing stock of French forests is persistent and dominated by private broadleaved forests

“…Thus, natural ageing of these broadleaved production systems might counterbalance GS expansion resulting from active coniferous afforestation programs. A full understanding of causal forest management processes of this expansion requires mandatory explorations of diameter-structured budgets of GS and growth/harvest flux balance across ownership categories, making more global analyses insufficient (Le Noë et al 2020). Such analyses at hand with NFI data are underway.…”