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Abstract
Abstract. How carbon (C) is allocated to different plant tissues (leaves, stem, and roots) determines how long C remains in plant biomass and thus remains a central challenge for understanding the global C cycle. We used a diverse set of observations (AmeriFlux eddy covariance tower observations, biomass estimates from tree-ring data, and leaf area index (LAI) measurements) to compare C fluxes, pools, and LAI data with those predicted by a land surface model (LSM), the Community Land Model (CLM4.5). We ran CLM4.5 for nine temperate (including evergreen and deciduous) forests in North America between 1980 and 2013 using four different C allocation schemes: i. dynamic C allocation scheme (named "D-CLM4.5") with one dynamic allometric parameter, which allocates C to the stem and leaves to vary in time as a function of annual net primary production (NPP); ii. an alternative dynamic C allocation scheme (named "D-Litton"), where, similar to (i), C allocation is a dynamic function of annual NPP, but unlike (i) includes two dynamic allometric parameters involving allocation to leaves, stem, and coarse roots; iii.–iv. a fixed C allocation scheme with two variants, one representative of observations in evergreen (named "F-Evergreen") and the other of observations in deciduous forests (named "F-Deciduous"). D-CLM4.5 generally overestimated gross primary production (GPP) and ecosystem respiration, and underestimated net ecosystem exchange (NEE). In D-CLM4.5, initial aboveground biomass in 1980 was largely overestimated (between 10 527 and 12 897 g C m−2) for deciduous forests, whereas aboveground biomass accumulation through time (between 1980 and 2011) was highly underestimated (between 1222 and 7557 g C m−2) for both evergreen and deciduous sites due to a lower stem turnover rate in the sites than the one used in the model. D-CLM4.5 overestimated LAI in both evergreen and deciduous sites because the leaf C–LAI relationship in the model did not match the observed leaf C–LAI relationship at our sites. Although the four C allocation schemes gave similar results for aggregated C fluxes, they translated to important differences in long-term aboveground biomass accumulation and aboveground NPP. For deciduous forests, D-Litton gave more realistic Cstem ∕ Cleaf ratios and strongly reduced the overestimation of initial aboveground biomass and aboveground NPP for deciduous forests by D-CLM4.5. We identified key structural and parameterization deficits that need refinement to improve the accuracy of LSMs in the near future. These include changing how C is allocated in fixed and dynamic schemes based on data from current forest syntheses and different parameterization of allocation schemes for different forest types. Our results highlight the utility of using measurements of aboveground biomass to evaluate and constrain the C allocation scheme in LSMs, and suggest that stem turnover is overestimated by CLM4.5 for these AmeriFlux sites. Understanding the controls of turnover will be critical to improving long-term C processes in LSMs.
Highlights
Over the last half century, on average a little more than a quarter of global CO2 emissions were absorbed by terrestrial carbon (C) sinks (Le Quéré et al, 2015), with forests accounting for most (Malhi et al, 2002; Bonan, 2008; Pan et al, 2011; Baldocchi et al, 2016)
Our results highlight the importance of evaluating the C allocation scheme and the stem turnover in land surface model (LSM) using measures of C stocks in addition to flux data
The four C allocation schemes translated to important long-term differences in C accumulation in aboveground biomass, but gave similar results for short-term C fluxes
Summary
Over the last half century, on average a little more than a quarter of global CO2 emissions were absorbed by terrestrial carbon (C) sinks (Le Quéré et al, 2015), with forests accounting for most (Malhi et al, 2002; Bonan, 2008; Pan et al, 2011; Baldocchi et al, 2016). The mechanism by which forests accumulate C is through photosynthetic uptake and allocation of the C to biomass in different plant pools (leaf, stem, and root). How long-lived different plant pools are (leaf, stem, and root) influences whether ecosystems are projected to act as C sources or sinks (Delbart et al, 2010; Friend et al, 2014). Once C is taken up by the plant, the carbon is allocated either to short-lived leaf or fine-root tissues, or to longer-lived woody tissues. Ecological theory suggests that variation in C allocation to different plant pools is governed by functional trade-offs (Tilman, 1988), with plants investing in either aboveground or belowground tissues depending on which strategy would maximize growth and reproduction. If the functional trade-off hypothesis is relevant on forest or regional scales, land surface models (LSMs) for forests should represent it using dynamic C allocation schemes, which are responsive to above- (e.g., light) and belowground (e.g., water or nutrients) factors that limit growth
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