Ecosystem age-class dynamics and distribution in the LPJ-wsl v2.0 global ecosystem model

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.

Simulating Global Dynamic Surface Reflectances for Imaging Spectroscopy Spaceborne Missions: LPJ‐PROSAIL

“…(2003). An age‐class specific land use change and landcover (LUCLC) module was created and implemented by Calle and Poulter (2021) within LPJ but was not run for this initial integration with PROSAIL.…”

“…The LPJ DGVM was developed in 2003 and couples a dynamic representation of vegetation demography with a mechanistic representation of plant physiology and biogeochemistry at a daily time step (Calle & Poulter, 2021; Sitch et al., 2003). LPJ simulates soil hydrology and vegetation dynamics in 0.5° grid cells where climate, atmospheric CO 2 , and soil texture are prescribed from different driver datasets (Figure 1).…”

“…Within LPJ, vegetation is categorized into 10 plant functional types (PFTs), namely tropical broad-leaved evergreen (TrBE), tropical broad-leaved raingreen (TrBR), temperate needle-leaved evergreen (TeNE), temperate broad-leaved evergreen (TeBE), temperate broad-leaved summergreen (TeBS), boreal needle-leaved evergreen Disturbances include fire, with the sub-module based on the Glob-FIRM model by Thonicke et al (2001) and implementation is described in Sitch et al (2003). An age-class specific land use change and landcover (LUCLC) module was created and implemented by Calle and Poulter (2021) within LPJ but was not run for this initial integration with PROSAIL.…”

Dynamic Global Vegetation Models: Searching for the balance between demographic process representation and computational tractability

“…When this occurs patches are merged, with the new functional traits being the area-weighted average. JSBACH4 [137] and LPJ-wsl2.0 models [138], use "vector tracking of fractional transitions". In this approach, there is a fixed number of age classes containing several sub-divisions.…”

Simulating global dynamic surface reflectances for imaging spectroscopy spaceborne missions - LPJ-PROSAIL

“…The LPJ dynamic global vegetation model was developed in 2003 and couples dynamic representation of vegetation demography with a mechanistic representation of plant physiology and biogeochemistry at a daily time step (Sitch et al, 2003;Calle & Poulter, 2021). Photosynthesis is represented by the Farquhar biochemical model, determined as a co-limited process between maximum carboxylation capacity (Vcmax), electron transport (Jmax), and sucrose utilization (Farquhar et al, 1980).…”