Abstract
Abstract. Bookkeeping models are used to estimate land-use and land-cover change (LULCC) carbon fluxes (ELULCC). The uncertainty of bookkeeping models partly arises from data used to define response curves (usually from local data) and their representativeness for application to large regions. Here, we compare biomass recovery curves derived from a recent synthesis of secondary forest plots in Latin America by Poorter et al. (2016) with the curves used previously in bookkeeping models from Houghton (1999) and Hansis et al. (2015). We find that the two latter models overestimate the long-term (100 years) vegetation carbon density of secondary forest by about 25 %. We also use idealized LULCC scenarios combined with these three different response curves to demonstrate the importance of considering gross forest area changes instead of net forest area changes for estimating regional ELULCC. In the illustrative case of a net gain in forest area composed of a large gross loss and a large gross gain occurring during a single year, the initial gross loss has an important legacy effect on ELULCC so that the system can be a net source of CO2 to the atmosphere long after the initial forest area change. We show the existence of critical values of the ratio of gross area change over net area change (γAnetAgross), above which cumulative ELULCC is a net CO2 source rather than a sink for a given time horizon after the initial perturbation. These theoretical critical ratio values derived from simulations of a bookkeeping model are compared with observations from the 30 m resolution Landsat Thematic Mapper data of gross and net forest area change in the Amazon. This allows us to diagnose areas in which current forest gains with a large land turnover will still result in LULCC carbon emissions in 20, 50 and 100 years.
Highlights
The global carbon flux from land-use and land-cover change (ELULCC) represents a net source of carbon to the atmosphere of 0.9 ± 0.5 Gt C yr−1 during the last decade (Ciais et al, 2013; Le Quéré et al, 2015)
Based on ELULCC calculated using a bookkeeping approach and several idealized scenarios constructed to have different gross forest area changes but with the same net area change (Sect. 3.2), we show the existence of a critical ratio of gross to net forest area change above which cumulative ELULCC remains a net source after initial land-use and landcover change (LULCC) because carbon losses from deforestation are not compensated for by carbon gains from secondary forest growth (Sect. 3.3)
Due to the higher carbon density of primary compared to secondary forest and the identical time at which both loss curves reach zero in Houghton (1999) and Hansis et al (2015), the loss curves for a cleared primary forest are steeper than those for a cleared secondary forest (Fig. 1a, b)
Summary
The global carbon flux from land-use and land-cover change (ELULCC) represents a net source of carbon to the atmosphere of 0.9 ± 0.5 Gt C yr−1 during the last decade (Ciais et al, 2013; Le Quéré et al, 2015). Most DGVMs (e.g. in the TRENDY project; Sitch et al, 2015) estimate emissions due only to net area changes between different landuse and land-cover types in a grid cell. Efforts are being made to incorporate gross land-use and landcover change (LULCC) in these models, that is for DGVMs the sub-grid transitions that sum up to net changes (Bayer et al, 2017). The bookkeeping model of Houghton (1999) includes emissions from both net area changes and gross LULCC from shifting cultivation, previously on the scale of large regions (Houghton, 2003), and more recently for each country (Houghton and Nassikas, 2017). Consider a region with co-existing forest and cropland where 20 % of Published by Copernicus Publications on behalf of the European Geosciences Union
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