Abstract
Abstract. Forest canopy structure is strongly influenced by environmental factors and disturbance, and in turn influences key ecosystem processes including productivity, evapotranspiration and habitat availability. In tropical forests increasingly modified by human activities, the interplay between environmental factors and disturbance legacies on forest canopy structure across landscapes is practically unexplored. We used airborne laser scanning (ALS) data to measure the canopy of old-growth and selectively logged peat swamp forest across a peat dome in Central Kalimantan, Indonesia, and quantified how canopy structure metrics varied with peat depth and under logging. Several million canopy gaps in different height cross-sections of the canopy were measured in 100 plots of 1 km2 spanning the peat dome, allowing us to describe canopy structure with seven metrics. Old-growth forest became shorter and had simpler vertical canopy profiles on deeper peat, consistent with previous work linking deep peat to stunted tree growth. Gap size frequency distributions (GSFDs) indicated fewer and smaller canopy gaps on the deeper peat (i.e. the scaling exponent of Pareto functions increased from 1.76 to 3.76 with peat depth). Areas subjected to concessionary logging until 2000, and illegal logging since then, had the same canopy top height as old-growth forest, indicating the persistence of some large trees, but mean canopy height was significantly reduced. With logging, the total area of canopy gaps increased and the GSFD scaling exponent was reduced. Logging effects were most evident on the deepest peat, where nutrient depletion and waterlogged conditions restrain tree growth and recovery. A tight relationship exists between canopy structure and peat depth gradient within the old-growth tropical peat swamp forest. This relationship breaks down after selective logging, with canopy structural recovery, as observed by ALS, modulated by environmental conditions. These findings improve our understanding of tropical peat swamp ecology and provide important insights for managers aiming to restore degraded forests.
Highlights
The structure of forest canopies is a determinant of fundamental ecological processes governing productivity, nutrient cycling and turnover across tropical landscapes (Asner et al, 1998; Brokaw, 1982; Denslow, 1987; Kellner et al, 2009; Prescott, 2002; Vitousek and Denslow, 1986)
Along the whole peat depth gradient and in both old-growth and logged plots, canopy top height decreased by 1 m for each metre of added peat depth (Fig. 3a, Supplement)
The canopy shape, derived from the complete airborne laser scanning (ALS) point cloud, did not change along the peat depth gradient in old-growth forest suggesting that the height of the main canopy volume decreased in parallel to canopy top height (Fig. 3a)
Summary
The structure of forest canopies is a determinant of fundamental ecological processes governing productivity, nutrient cycling and turnover across tropical landscapes (Asner et al, 1998; Brokaw, 1982; Denslow, 1987; Kellner et al, 2009; Prescott, 2002; Vitousek and Denslow, 1986). The interception and processing of light, and primary production, is affected by total leaf area and by the layering, positioning and angle of leaves within the canopy (Asner et al, 1998; Ellsworth and Reich, 1993; Montgomery and Chazdon, 2001; Stark et al, 2012); evapotranspiration is affected by the internal length of hydraulic pathways and roughness of the canopy (Costa and Foley, 1997; Malhi et al, 2002). In human-modified tropical forests the interplay between environmental factors and disturbance legacies on forest canopy structure is practically unexplored. In the biodiversity hotspot of Borneo, more than 30 % of forest cover has been lost over the past 40 years, 46 % of remaining forests have been selectively logged
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