Variation in Canopy Structure, Light and Soil Nutrition Across Elevation of a Sri Lankan Tropical Rain Forest

Background

This study aims to examine differences in resource availability across elevation and geology in mixed dipterocarp forest by measuring light, soil nutrition and soil water availability in relation to forest structure in southwest Sri Lanka.

Research Goals & Methods

Eighteen canopy gaps caused by natural disturbances were randomly selected within primary rain forest that ranged across 100–1200 m elevation (lowland 100 m amsl, middle elevation 300–900 m amsl, lower montane 1200 m amsl) Measurements were taken of forest structure (basal area, canopy height, canopy cover index [CCI]), shade (photosynthetically active radiation [PAR], global site factor [GSF]) and soil nutrition (pH, Al, K, Mg Ca, N, and P). Soil moisture was measured at bi-weekly intervals for five years across middle elevation sites only (300– 900 m amsl). Linear regression was used to model a straight line fit for 1) plot measures to elevation/geology, 2) basal area, CCI, and gap area to understory canopy height, 3) understory PAR and GSF to canopy height, CCI, and basal area of understory plots, and 4) gap PAR and GSF to gap area.

Conclusions & Takeaways

The results demonstrate strong differentiation in soil and light resources with elevation that appears related to size of tree-fall disturbance, stature of the forest, topographic position and underlying geology and soil-weathering environment. The authors especially highlight the demonstrated direct relationship between tree height and gap size with elevation and potentially associated changes in geology: there is a decline in canopy opening size (and therefore size of tree fall) with an increase in elevation. They recommend that forest management practices develop tailored techniques that emulate and/or account for change in elevation, geology and topographic position.