Response of tree biomass and wood litter to disturbance in a Central Amazon forest

Abstract

We developed an individual-based stochasticempirical model to simulate the carbon dynamics of live and dead trees in a Central Amazon forest near Manaus, Brazil. The model is based on analyses of extensive field studies carried out on permanent forest inventory plots, and syntheses of published studies. New analyses included: (1) growth suppression of small trees, (2) maximum size (trunk base diameter) for 220 tree species, (3) the relationship between growth rate and wood density, and (4) the growth response of surviving trees to catastrophic mortality (from logging). The model simulates a forest inventory plot, and tracks recruitment, growth, and mortality of live trees, decomposition of dead trees (coarse litter), and how these processes vary with changing environmental conditions. Model predictions were tested against aggregated field data, and also compared with independent measurements including maximum tree age and coarse litter standing stocks. Spatial analyses demonstrated that a plot size of ~10 ha was required to accurately measure wood (live and dead) carbon balance. With the model accurately predicting relevant pools and fluxes, a number of model experiments were performed to predict forest carbon balance response to perturbations including: (1) increased productivity due to CO2 fertilization, (2) a single semi-catastrophic (10%) mortality event, (3) increased recruitment and mortality (turnover) rates, and (4) turnover, increased tree growth rates, and decreased mean wood density of new recruits. Results demonstrated that carbon accumulation over the past few decades observed on tropical forest inventory plots (~0.5 Mg C ha"^ year"') is not likely caused by CO2 fertilization. A maximum 25% increase in woody tissue productivity with a doubling of atmospheric CO2 only resulted in an accumulation rate of 0.05 Mg C ha"' year"' for the period 1980-2020 for a Central Amazon forest, or an order of magnitude less than observed on the inventory plots. In contrast, model parameterization based on extensive data from a logging experiment demonstrated a rapid increase in tree growth following disturbance, which could be misinterpreted as carbon sequestration if changes in coarse litter stocks were not considered. Combined results demonstrated that predictions of changes in forest carbon balance during the twenty-first century are highly dependent on assumptions of tree response to various perturbations, and underscores the importance of a close coupling of model and field investigations.

The steady-state mosaic of disturbance and succession across an old-growth Central Amazon forest landscape

“…Episodic disturbance events with return frequencies greater than ∼30 y are not well represented in the existing Amazon network where plots were monitored for 4.0-21.7 y (mean of 10.9 y) up to the early 2000s (5). A plot-based study in the Central Amazon, for example, found that the largest gap occurring on 56 separate census intervals varying from 1 to 5 y on 21 single hectare permanent plots included only one eight-tree blowdown cluster, and only seven events exceeding six trees per cluster (24,25). Thus, a plot-based mortality rate estimate for the Central Amazon of 1.02% stems·y −1 (24) was entirely missing the mortality contribution of events exceeding eight trees per cluster.…”

“…Episodic disturbances create patches with a large amount of coarse woody debris (CWD), and the time required for the decomposer community to consume this CWD and release CO 2 is much faster in Amazon forests than biomass recovery following disturbance (24). Thus, recent tree fall gaps in the early stages of succession will act as large carbon sources, followed by decades of slow carbon accumulation as trees fill the gap and species community composition changes over time.…”

“…Calculating a landscape-scale mortality rate presented a number of challenges. First, over the spatial and temporal domains sampled in the permanent forest plots used in this study, mortality estimates included only a single eight-tree cluster, and only seven events were recorded that exceeded six trees per cluster (24). Thus, our plot-based rates were entirely missing episodic events exceeding eight trees per cluster.…”

“…1 and Fig. S1), and the individual-based tropical tree ecosystem and community simulator (TRECOS) model (24). To simulate the entire disturbance gradient from individual tree falls to large blowdowns, TRECOS was modified to use a gap size probability distribution function (PDF) generated from merging forest plot data and Landsat image analyses ( Fig.…”

Widespread Amazon forest tree mortality from a single cross‐basin squall line event

“…Detectable highmortality areas ranged from 5-10 fallen trees (Text S2) occupying single pixels, to blowdown patches of ∼30 ha in size. The disturbance produced by the squall line across the Manaus region encompassed 2668 ha causing estimated mortality of 0.32 ± 0.05 (SD) million trees, equivalent to a total biomass loss of 85 ± 25 (SD) Gg C which will be respired to the atmosphere over an ∼18 year period [Chambers et al, 2004]. The MC model predicted a mortality of 0.5 ± 0.056 million trees, equivalent to a total biomass loss of 128 ± 14 Gg C (using Figure S5).…”