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

Abstract

Old-growth forest ecosystems comprise a mosaic of patches in different successional stages, with the fraction of the landscape in any particular state relatively constant over large temporal and spatial scales. The size distribution and return frequency of disturbance events, and subsequent recovery processes, determine to a large extent the spatial scale over which this old-growth steady state develops. Here, we characterize this mosaic for a Central Amazon forest by integrating field plot data, remote sensing disturbance probability distribution functions, and individual-based simulation modeling. Results demonstrate that a steady state of patches of varying successional age occurs over a relatively large spatial scale, with important implications for detecting temporal trends on plots that sample a small fraction of the landscape. Long highly significant stochastic runs averaging 1.0 Mg biomass·ha −1 ·y −1 were often punctuated by episodic disturbance events, resulting in a sawtooth time series of hectare-scale tree biomass. To maximize the detection of temporal trends for this Central Amazon site (e.g., driven by CO 2 fertilization), plots larger than 10 ha would provide the greatest sensitivity. A model-based analysis of fractional mortality across all gap sizes demonstrated that 9.1-16.9% of tree mortality was missing from plot-based approaches, underscoring the need to combine plot and remote-sensing methods for estimating net landscape carbon balance. Old-growth tropical forests can exhibit complex large-scale structure driven by disturbance and recovery cycles, with ecosystem and community attributes of hectare-scale plots exhibiting continuous dynamic departures from a steady-state condition.biodiversity | community composition | gap dynamics | NEP NEE NBP A common assumption in old-growth forest studies is that, in the absence of a directional forcing, ecosystem characteristics and tree species composition should exhibit some type of steady-state behavior (1). Thus, plot-based studies in old-growth tropical forests that observe changing tree species composition (2), increased liana abundance (3), faster turnover rates (4), and forest biomass accumulation (5, 6), are viewed as surprising departures from an expected steady-state condition. However, disturbance events can create a landscape with patches of varying successional age, and the extent to which forest plots representatively sample this mosaic remains an open question. An important issue is how to distinguish directional trends driven by a warming climate, or rising atmospheric CO 2 concentration, from smaller-scale stochastic patterns driven by disturbance and recovery cycles (7,8).Over long time periods, the disturbance regime of a forested region creates a shifting steady-state mosaic, represented by patches of different successional ages, with the fraction of the landscape in any particular state remaining relatively constant over time (9,10). In many tropical forests, gaps created by the windthrow of canopy trees is a major mode of disturb...

Windthrow Variability in Central Amazonia

“…For instance, the southeastern corner of the Landsat image has high anthropogenic activity and therefore few windthrows were mapped here. The histogram of windthrow sizes was skewed to the left indicating that the smallest windthrows were the most frequent, which is consistent with previous studies [7,8]. Windthrows in bins (data in intervals) of 20, 40, and 60 ha represented 42%, 28%, and 12% of windthrows events, respectively.…”

“…Only windthrows located >2 km away from urban areas and larger than 5 ha were considered in our analysis. Examples of windthrows over our study area have been published in our previous studies (e.g., Figure 2 in [3] and Figure S5 in [8]). To identify windthrows, a spectral mixture analysis (SMA) [46,47] was applied to the Landsat images.…”

“…Windthrows were identified by their fan-shaped form [4] and high NPV values, as we have shown in previous studies [3,7,8]. We visually inspected each image that preceded a windthrow image to ensure that the prior image showed undisturbed forest.…”

“…These gaps vary in size from a single tree to thousands of hectares of forest [4,[6][7][8]. Windthrows affect the residence time of woody biomass, which, in turn, affects patterns of productivity and biomass [8,9], floristic composition [10][11][12], and soil composition [13] in the basin. Recent studies using demographic models have shown that an increase in windthrow frequency promotes a decrease in biomass and leaf area index in Central Amazonia [14].…”

“…They are produced by downbursts [1][2][3], which are strong descending winds associated with severe convective storms [1][2][3][4][5] that create gaps of uprooted or broken trees [6]. These gaps vary in size from a single tree to thousands of hectares of forest [4,[6][7][8]. Windthrows affect the residence time of woody biomass, which, in turn, affects patterns of productivity and biomass [8,9], floristic composition [10][11][12], and soil composition [13] in the basin.…”