Abstract
Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8–32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.Synthesis. During succession, light conditions changed linearly, which should lead to a continuous and constant replacement of species. Especially in later successional stages, stronger vertical light gradients can limit the regeneration of light‐demanding pioneer species and increase the proportion of shade‐tolerant late‐successional species under the canopy. These changes in light conditions were largely driven by the successional changes in forest structure, as basal area strongly determined the height where most light is absorbed, whereas crown area, and to a lesser extent crown length, determined light distribution.
Highlights
| INTRODUCTIONLight is a key resource for plants, affecting their recruitment, growth and survival (Chen et al, 2014)
We found a rapid increase in stand size and stand differentiation, which leads to fast changes in light conditions and more light heterogeneity
This study aims to evaluate how changes in forest structural attributes lead to changes in vertical and horizontal light heterogeneity during tropical forest succession
Summary
Light is a key resource for plants, affecting their recruitment, growth and survival (Chen et al, 2014). From the forest canopy to the forest floor, irradiance shows a marked exponential decline, as light is intercepted by successive leaf layers (Kitajima et al, 2005; Yoshimura et al, 2008) This vertical light gradient has a strong effect on light competition among trees and other plant growth forms, and increasing size inequality (i.e. stand differentiation) and mortality risk of suppressed individuals (Weiner, 1990; Yoda et al, 1963), a process called stand thinning. We addressed two questions: (a) How, when and where do forest structural attributes and the vertical and horizontal light heterogeneity change during succession? We predict that forest height and stand size increase during early stage of secondary succession, which is followed by an increase in light interception at the top of the stands, and increases vertical light heterogeneity rapidly and decreases the horizontal light heterogeneity in the understorey. (b) Which, when and how do forest structural attributes shape vertical and horizontal light heterogeneity? We hypothesize that during succession (a) vertical light heterogeneity increases because of an increase in basal area of larger trees as basal area scales strongly with leaf area (Shinozaki et al, 1964), and the higher inequality of basal area at later successional stages leads to a large amount of light interception in the top of the stand and less light transmission to the forest floor and (b) horizontal light heterogeneity in the understorey decreases because of more homogeneous crown distributions within the stand leading to a lower frequency of vertical and lateral gaps
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