Individual-Based Modeling of Amazon Forests Suggests That Climate Controls Productivity While Traits Control Demography

Abstract

Fauset et al. Climate, Traits, and Forest Processes Climate, species composition, and soils are thought to control carbon cycling and forest structure in Amazonian forests. Here, we add a demographics scheme (tree recruitment, growth, and mortality) to a recently developed non-demographic model-the Trait-based Forest Simulator (TFS)-to explore the roles of climate and plant traits in controlling forest productivity and structure. We compared two sites with differing climates (seasonal vs. aseasonal precipitation) and plant traits. Through an initial validation simulation, we assessed whether the model converges on observed forest properties (productivity, demographic and structural variables) using datasets of functional traits, structure, and climate to model the carbon cycle at the two sites. In a second set of simulations, we tested the relative importance of climate and plant traits for forest properties within the TFS framework using the climate from the two sites with hypothetical trait distributions representing two axes of functional variation ("fast" vs. "slow" leaf traits, and high vs. low wood density). The adapted model with demographics reproduced observed variation in gross (GPP) and net (NPP) primary production, and respiration. However, NPP and respiration at the level of plant organs (leaf, stem, and root) were poorly simulated. Mortality and recruitment rates were underestimated. The equilibrium forest structure differed from observations of stem numbers suggesting either that the forests are not currently at equilibrium or that mechanisms are missing from the model. Findings from the second set of simulations demonstrated that differences in productivity were driven by climate, rather than plant traits. Contrary to expectation, varying leaf traits had no influence on GPP. Drivers of simulated forest structure were complex, with a key role for wood density mediated by its link to tree mortality. Modeled mortality and recruitment rates were linked to plant traits alone, drought-related mortality was not accounted for. In future, model development should focus on improving allocation, mortality, organ respiration, simulation of understory trees and adding hydraulic traits. This type of model that incorporates diverse tree strategies, detailed forest structure and realistic physiology is necessary if we are to be able to simulate tropical forest responses to global change scenarios.

Tree mode of death and mortality risk factors across Amazon forests

“…Together, the species traits and tree-level predictors identified here can provide a robust empirical underpinning for simulating tree mortality in the Amazon. The empirical relationships found here can be directly incorporated into individual-based sizestructured vegetation models, such as done by Fauset et al 43 . The linkage between mortality probability and individual relative growth can also be readily incorporated into the size-cohortbased vegetation models, which are becoming increasingly widespread 6 , replacing widely applied theoretical approaches, which are hard to parameterise in practice 44 .…”

Species Matter: Wood Density Influences Tropical Forest Biomass at Multiple Scales

“…Thus, the species traits of Amazon forests do not strongly control the rate at which carbon is being cycled by the forest (Fig. 7, and see Fauset et al 2019). Yet they are associated with the rates are which individual trees are cycled—stem mortality rates are clearly linked to the wood density of the forest, confirming that the lower the stand-level wood density is, the more rapidly the trees die (Fig.…”

Soil properties explain tree growth and mortality, but not biomass, across phosphorus-depleted tropical forests

“…There is clear evidence for natural gradients in soil nutrient availability impacting forest productivity across large-scale gradients spanning chrono-sequences in geologic and pedogenic timescales 11,29,30 . However, support for this paradigm within more limited biogeographic regions of the tropics remains elusive 6,31 .…”