Abstract
Abstract. Poorly constrained rates of biomass turnover are a key limitation of Earth system models (ESMs). In light of this, we recently proposed a new approach encoded in a model called Populations-Order-Physiology (POP), for the simulation of woody ecosystem stand dynamics, demography and disturbance-mediated heterogeneity. POP is suitable for continental to global applications and designed for coupling to the terrestrial ecosystem component of any ESM. POP bridges the gap between first-generation dynamic vegetation models (DVMs) with simple large-area parameterisations of woody biomass (typically used in current ESMs) and complex second-generation DVMs that explicitly simulate demographic processes and landscape heterogeneity of forests. The key simplification in the POP approach, compared with second-generation DVMs, is to compute physiological processes such as assimilation at grid-scale (with CABLE (Community Atmosphere Biosphere Land Exchange) or a similar land surface model), but to partition the grid-scale biomass increment among age classes defined at sub-grid-scale, each subject to its own dynamics. POP was successfully demonstrated along a savanna transect in northern Australia, replicating the effects of strong rainfall and fire disturbance gradients on observed stand productivity and structure. Here, we extend the application of POP to wide-ranging temporal and boreal forests, employing paired observations of stem biomass and density from forest inventory data to calibrate model parameters governing stand demography and biomass evolution. The calibrated POP model is then coupled to the CABLE land surface model, and the combined model (CABLE-POP) is evaluated against leaf–stem allometry observations from forest stands ranging in age from 3 to 200 year. Results indicate that simulated biomass pools conform well with observed allometry. We conclude that POP represents an ecologically plausible and efficient alternative to large-area parameterisations of woody biomass turnover, typically used in current ESMs.
Highlights
Changes in woody biomass storage in forest and savanna ecosystems, including woody ecosystems regenerating on abandoned agricultural lands, are the major driver of the terrestrial carbon sink, which currently amounts to around a quarter of anthropogenic emissions, mitigating climate change (Ahlström et al, 2012; Pan et al, 2011; Le Quéré et al, 2013)
Such ecosystem dynamics and their feedbacks to atmospheric carbon content and radiative forcing are represented in Earth system models (ESMs) by incorporating dynamic vegetation models (DVMs)
The first-generation DVMs adopted by most current ESMs (Arora et al, 2013) employ large-area parameterisations designed for application on the scales of grid cells 10s to 100s of kilometres on a side
Summary
Changes in woody biomass storage in forest and savanna ecosystems, including woody ecosystems regenerating on abandoned agricultural lands, are the major driver of the terrestrial carbon sink, which currently amounts to around a quarter of anthropogenic emissions, mitigating climate change (Ahlström et al, 2012; Pan et al, 2011; Le Quéré et al, 2013). Haverd et al.: A stand-alone tree demography and landscape structure module for ESM parameterisations treat carbon flows associated with respiration and mortality as first-order decay processes, expressed as products of pool biomasses and bulk rate parameters independent of age structure (the “big wood” approximation; Wolf et al, 2011) These are computationally efficient – an important consideration for global-scale applications – but have the disadvantage of not resolving underlying population and community processes such as recruitment, mortality and competition between individuals and species for limiting resources It has been suggested that the representation of forest dynamics in ESMs may be one of the greatest sources of uncertainty in future climate projections (Purves and Pacala, 2008)
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