Amazon forest structure generates diurnal and seasonal variability in light utilization
The complex three-dimensional (3-D) structure of tropical forests generates a diversity of light environments for canopy and understory trees. Understanding diurnal and seasonal changes in light availability is critical for interpreting measurements of net ecosystem exchange and improving ecosystem...
Main Authors: | , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-04-01
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Series: | Biogeosciences |
Online Access: | http://www.biogeosciences.net/13/2195/2016/bg-13-2195-2016.pdf |
Summary: | The complex three-dimensional (3-D) structure of tropical forests generates a
diversity of light environments for canopy and understory trees.
Understanding diurnal and seasonal changes in light availability is critical
for interpreting measurements of net ecosystem exchange and improving
ecosystem models. Here, we used the Discrete Anisotropic Radiative Transfer
(DART) model to simulate leaf absorption of photosynthetically active
radiation (<i>l</i>APAR) for an Amazon forest. The 3-D model scene was developed from
airborne lidar data, and local measurements of leaf reflectance, aerosols,
and PAR were used to model <i>l</i>APAR under direct and diffuse illumination
conditions. Simulated <i>l</i>APAR under clear-sky and cloudy conditions was
corrected for light saturation effects to estimate light utilization, the
fraction of <i>l</i>APAR available for photosynthesis. Although the fraction of
incoming PAR absorbed by leaves was consistent throughout the year
(0.80–0.82), light utilization varied seasonally (0.67–0.74), with minimum
values during the Amazon dry season. Shadowing and light saturation effects
moderated potential gains in forest productivity from increasing PAR during
dry-season months when the diffuse fraction from clouds and aerosols was
low. Comparisons between DART and other models highlighted the role of 3-D
forest structure to account for seasonal changes in light utilization. Our
findings highlight how directional illumination and forest 3-D structure
combine to influence diurnal and seasonal variability in light utilization,
independent of further changes in leaf area, leaf age, or environmental
controls on canopy photosynthesis. Changing illumination geometry
constitutes an alternative biophysical explanation for observed seasonality
in Amazon forest productivity without changes in canopy phenology. |
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ISSN: | 1726-4170 1726-4189 |