Large-Scale Wind Disturbances Promote Tree Diversity in a Central Amazon Forest

Abstract

Canopy gaps created by wind-throw events, or blowdowns, create a complex mosaic of forest patches varying in disturbance intensity and recovery in the Central Amazon. Using field and remote sensing data, we investigated the short-term (four-year) effects of large (>2000 m2) blowdown gaps created during a single storm event in January 2005 near Manaus, Brazil, to study (i) how forest structure and composition vary with disturbance gradients and (ii) whether tree diversity is promoted by niche differentiation related to wind-throw events at the landscape scale. In the forest area affected by the blowdown, tree mortality ranged from 0 to 70%, and was highest on plateaus and slopes. Less impacted areas in the region affected by the blowdown had overlapping characteristics with a nearby unaffected forest in tree density (583±46 trees ha−1) (mean±99% Confidence Interval) and basal area (26.7±2.4 m2 ha−1). Highly impacted areas had tree density and basal area as low as 120 trees ha−1 and 14.9 m2 ha−1, respectively. In general, these structural measures correlated negatively with an index of tree mortality intensity derived from satellite imagery. Four years after the blowdown event, differences in size-distribution, fraction of resprouters, floristic composition and species diversity still correlated with disturbance measures such as tree mortality and gap size. Our results suggest that the gradients of wind disturbance intensity encompassed in large blowdown gaps (>2000 m2) promote tree diversity. Specialists for particular disturbance intensities existed along the entire gradient. The existence of species or genera taking an intermediate position between undisturbed and gap specialists led to a peak of rarefied richness and diversity at intermediate disturbance levels. A diverse set of species differing widely in requirements and recruitment strategies forms the initial post-disturbance cohort, thus lending a high resilience towards wind disturbances at the community level.

Windthrow Variability in Central Amazonia

“…It should be emphasized that the interaction of wind and trees is nonlinear and involves complex processes and the integration of disciplines such as soil science, physiology, ecology, biomechanics, and meteorology [6,11,83,84]. Wind, wind loading, gusts of winds, tree stature, tree species (and associated characteristics such as tree crown shape and density, root architecture and shape, and wood density), topography, soil, cumulative processes, etc.…”

“…Wind, wind loading, gusts of winds, tree stature, tree species (and associated characteristics such as tree crown shape and density, root architecture and shape, and wood density), topography, soil, cumulative processes, etc. all play a role in the production of windthrows [11,[83][84][85][86][87][88] resulting in tree failure at a lower wind speed than expected [1,3]. Amazonia covers about 5.3 million km 2 , and has many environmental and functional gradients [89,90].…”

“…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].…”

Windthrows increase soil carbon stocks in a central Amazon forest

“…Our results indicate that in central Amazon terra firme forests, vegetation dynamics can also influence soil attributes at the landscape level. In this region, the observed organic carbon enrichment derived from widespread tree mortality might also be related to the fast establishment and growth of pioneer species in heavily disturbed areas (Chambers et al, 2009;Marra et al, 2014).…”

“…The widespread tree mortality creates canopy gaps with a wide range of sizes (from few square meters up to thousands of hectares; Nelson et al, 1994;Negrón-Juárez et al, 2010 and affects forests at the landscape level . It has been reported that these large gaps have a potential effect on carbon cycling (Chambers et al, 2013) and can promote tree species diversity by allowing a diverse cohort of species with a broad range of life history strategies (Chambers et al, 2009;Marra et al, 2014) and allometry .…”