Spatial Contagiousness of Canopy Disturbance in Tropical Rain Forest: An Individual‐tree‐based Test

Abstract

Spatial contagiousness of canopy dynamics-the tendency of canopy disturbances to occur nearby existing canopy openings due to an elevated risk of tree fall around gaps-has been demonstrated in many temperate-zone forests, but only inferentially for tropical forests. Hypothesized mechanisms increasing the risk of tree fall around tropical forest gaps are (1) increased tree exposure to wind around gaps, (2) reduced stability of trees alongside gaps due to crown asymmetry, or (3) reduced tree health around gaps due to damage from prior disturbances. One hypothesized consequence of elevated disturbance levels around gaps would be that gap-edge zones offer relatively favorable prospects for seedling recruitment, growth, and survival. We tested whether disturbance levels are indeed elevated around natural canopy gaps in a neotropical rain forest in French Guiana, and more so as gaps are larger. We followed the fate of 5660 trees >10 cm stem diameter over five years across 12 ha of old-growth forest and analyzed the risk and magnitude of canopy disturbance events in relation to tree diameter and the proximity and size of natural canopy gaps. We found that the cumulative incidence of disturbance over the five-year survey was not significantly elevated around preexisting gaps, and only weakly related to gap size. Also, neither the risk nor the magnitude of canopy disturbances increased significantly with the proximity of gaps. Moreover, canopy disturbance risk around gaps was independent of gap size, while the magnitude of disturbance events around gaps was weakly related to gap size. Tree size was the major driver of disturbance risk as well as magnitude. We did find an elevated incidence of disturbance inside preexisting gaps, but this "repeat disturbance" was due to an elevated disturbance risk inside gaps, not around gaps. Overall, we found no strong evidence for canopy dynamics in this rain forest being spatially contagious. Our findings are consistent with the traditional view of tropical rain forests as mosaics of patches with predictable regeneration cycles.

Pattern and dynamics of biomass stock in old growth forests: The role of habitat and tree size

“…As shown by our model, the inherent variations of forest AGB stock and mortality are quite different, and more sampling efforts are required to obtain a reliable AGB dynamic than standing stock. Similar results were found by Jansen et al (2008), who indicated that at least 9-ha area would be needed to estimate AGB increment but at least 40 ha to reliable estimate AGB loss. Compared with mortality of seedling and small trees, disturbance-driven deaths of dominant, big trees that have greater impact on stand biomass accumulation are rare and intermittent (Berger et al, 2004).…”

Forest Canopy Gap Distributions in the Southern Peruvian Amazon

“…square meters of canopy loss). As such, it remains unknown whether any given gap is shrinking due to regeneration or growing due to adjacency effects from neighboring individuals [59], [60]. The best way around this problem is to image canopy disturbances directly to derive recent disturbance events.…”

Detecting and projecting changes in forest biomass from plot data

“…Our own analyses found no spatial autocorrelation in biomass change within the plot (Figure 14.2), suggesting that this assumption is appropriate for our site. The direct causal influence between patches is, in general, likely to be small most of the time; although gaps are commonly believed to be contagious and may be modelled as such (Sole & Manrubia 1995), a recent study found that areas next to existing canopy gaps have disturbance rates that are similar to those of areas far from existing gaps (Jansen, Van der Meer & Bongers 2008). On the other hand, large-scale disturbances, e.g.…”