Ecosystem age-class dynamics and distribution in the LPJ-wsl v2.0 global ecosystem model
Authors: Calle Leonardo , Poulter Benjamin 
Published: May 2021

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.