Abstract
Abstract. Forest ecosystem processes follow classic responses with age, peaking production around canopy closure and declining thereafter. Although age dynamics might be more dominant in certain regions over others, demographic effects on net primary production (NPP) and heterotrophic respiration (Rh) are bound to exist. Yet, explicit representation of ecosystem demography is notably absent in many global ecosystem models. This is concerning because the global community relies on these models to regularly update our collective understanding of the global carbon cycle. This paper aims to present the technical developments of a computationally efficient approach for representing age-class dynamics within a global ecosystem model, the Lund–Potsdam–Jena – Wald, Schnee, Landschaft version 2.0 (LPJ-wsl v2.0) dynamic global vegetation model and to determine if explicit representation of demography influenced ecosystem stocks and fluxes at global scales or at the level of a grid cell. The modeled age classes are initially created by simulated fire and prescribed wood harvesting or abandonment of managed land, otherwise aging naturally until an additional disturbance is simulated or prescribed. In this paper, we show that the age module can capture classic demographic patterns in stem density and tree height compared to inventory data, and that simulated patterns of ecosystem function follow classic responses with age. We also present two scientific applications of the model to assess the modeled age-class distribution over time and to determine the demographic effect on ecosystem fluxes relative to climate. Simulations show that, between 1860 and 2016, zonal age distribution on Earth was driven predominately by fire, causing a 45- to 60-year difference in ages between older boreal (50–90∘ N) and younger tropical (23∘ S–23∘ N) ecosystems. Between simulation years 1860 and 2016, land-use change and land management were responsible for a decrease in zonal age by −6 years in boreal and by −21 years in both temperate (23–50∘ N) and tropical latitudes, with the anthropogenic effect on zonal age distribution increasing over time. A statistical model helped to reduce LPJ-wsl v2.0 complexity by predicting per-grid-cell annual NPP and Rh fluxes by three terms: precipitation, temperature, and age class; at global scales, R2 was between 0.95 and 0.98. As determined by the statistical model, the demographic effect on ecosystem function was often less than 0.10 kg C m−2 yr−1 but as high as 0.60 kg C m−2 yr−1 where the effect was greatest. In the eastern forests of North America, the simulated demographic effect was of similar magnitude, or greater than, the effects of climate; simulated demographic effects were similarly important in large regions of every vegetated continent. Simulated spatial datasets are provided for global ecosystem ages and the estimated coefficients for effects of precipitation, temperature and demography on ecosystem function. The discussion focuses on our finding of an increasing role of demography in the global carbon cycle, the effect of demography on relaxation times (resilience) following a disturbance event and its implications at global scales, and a finding of a 40 Pg C increase in biomass turnover when including age dynamics at global scales. Whereas time is the only mechanism that increases ecosystem age, any additional disturbance not explicitly modeled will decrease age. The LPJ-wsl v2.0 age module represents another step forward towards understanding the role of demography in global ecosystems.
Highlights
Forest ecosystem production follows predictable patterns with time since disturbance
Simulations demonstrate that fire and land-use change and land management (LUCLM) have been driving the latitudinal age distribution towards younger states in contemporary times (Fig. 8), suggesting an increasing role of age dynamics in global ecosystem functioning
Small-scale logging activity is a dominant disturbance in the southeastern US (Williams et al, 2016) but it is underestimated by the LUCLM driver data in this study (LUHv2; Hurtt et al, 2020); otherwise, the simulated age of secondary forests in this region (∼ 100 years) would be lower and closer to inventory-based age estimates of these forests (< 50 years; Fig. 4 in Pan et al, 2011b)
Summary
Forest ecosystem production follows predictable patterns with time since disturbance. A multimodel global regrowth analysis with demographically enabled dynamic global vegetation models (DGVMs), to which Lund–Potsdam–Jena – Wald, Schnee, Landschaft version 2.0 (LPJ-wsl v2.0) contributed, estimated that post-disturbance regrowth comprised a large global regrowth sink of 0.3 to 1.1 Pg C yr−1 due to demography alone over the years 1981– 2010 (Pugh et al, 2019b). Much of the focus of these global modeling studies has been on the effect of natural and anthropogenic disturbances on the carbon dynamics in old-growth versus second-growth forests (Gitz and Ciais, 2003; Shevliakova et al, 2009; Kondo et al, 2018; Yue et al, 2018; Pugh et al, 2019b) but lack finer distinction of demographic effects at different age classes. Following a call to the science community to improve demographic representation in models (Fisher et al, 2016), there is a growing list of global models that are capable of simulating global ecosystem demographics (Gitz and Ciais, 2003, OSCAR; Shevliakova et al, 2009, LM3V; Haverd et al, 2014, CABLE-POP; Lindeskog et al, 2013, LPJ-GUESS; Yue et al, 2018, ORCHIDEE MICT; Nabel et al, 2020, Jena Scheme for Biosphere Atmosphere Coupling in Hamburg version 4 – JSBACH4), more models need the capability to represent landscape heterogeneity in forest structure and function
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