Effects of topography, soil type and forest age on the frequency and size distribution of canopy gap disturbances in a tropical forest

J. W. Dalling,E. Lobo

doi:10.5194/bg-10-6769-2013

Abstract

Abstract. Treefall gaps are the major source of disturbance in most tropical forests. The frequency and size of these gaps have important implications for forest ecosystem processes as they can influence the functional trait distribution of tree communities, stand-level aboveground biomass and productivity. However, we still know little about the relative importance of environmental drivers of gap disturbance regimes because existing studies vary greatly in criteria used for defining gaps, in the spatial extent of the study area, and the spatial resolution of canopy height measurements. Here we use lidar (light detecting and ranging) to explore how forest age, topography and soil type affect canopy disturbance patterns across a 1500 ha tropical forest landscape in central Panama. We characterize disturbance based on the frequency distribution of gap sizes (the "gap size distribution"), and the area of the forest affected by gaps (the "gap area fraction"). We found that slope and forest age had significant effects on the gap size distribution, with a higher frequency of large gaps associated with old-growth forests and more gentle slopes. Slope and forest age had similar effects on the gap area fraction, however gap area fraction was also affected by soil type and by aspect. We conclude that variation in disturbance patterns across the landscape can be linked to factors that act at the fine scale (such as aspect or slope), and factors that show heterogeneity at coarser scales (such as forest age or soil type). Awareness of the role of different environmental factors influencing gap formation can help scale up the impacts of canopy disturbance on forest communities measured at the plot scale to landscape and regional scales.

Highlights

In most tropical forests, the main cause of disturbance is the creation of openings in the forest canopy when one or multiple trees fall (Swaine and Whitmore 1988)
Comparisons across forests indicate that differences may exist in the area of the forest affected by gap disturbance, and in the frequency distribution of gap sizes (Fisher et al, 2008; Baker et al, 2005; Chambers et al, 2009; Foster et al, 2008; Lertzman et al, 1996)
The diversity of effects on the metrics of gap disturbance shown here suggests that variation in disturbance patterns across the landscape is a result of multiple factors acting at the same time and in different ways

Summary

Introduction

In most tropical forests, the main cause of disturbance is the creation of openings in the forest canopy when one or multiple trees fall (Swaine and Whitmore 1988). The environmental factors that lead to variation in the gap area fraction and frequency distribution remain unclear. In part, this is because published studies vary in the criteria used for defining gaps (range 2–15 m minimum canopy height), in the spatial extent of the study area (0.5– 100 ha), and in the sampling intensity and spatial resolution of canopy height measurements (Kellner et al, 2009; Marthews et al, 2008; Brokaw 1985; Lawton and Putz 1988; Chandrashekara and Ramakrishnan 1994; Jans et al, 1993; Ferreira de Lima et al, 2008; Kapos et al, 1990). We still know little about the relative importance of the environmental drivers of variation in canopy disturbance

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Discussion

Conclusion