Understanding the patterns and controls on the phenology of tropical vegetation photosynthesis is a pressing question in tropical ecosystem ecology. Few ground-based studies have documented seasonal changes in leaf area index (LAI) in tropical forests, and yet it is a key driver of forest productivity. Furthermore, prior studies were unable to monitor vertical changes in leaf area, a potentially key driver of changes in ecosystem light use efficiency, which also varies seasonally. We present results from the first large-scale ground-based survey to document the vertical distribution of seasonal LAI changes in a tropical forest. Ground-based LiDAR transects were employed to measure seasonal changes in forest leaf area at two sites in the Tapajós National Forest: Km-67 and Km-72, Pará, Brazil. We address the following questions: how does leaf area vary seasonally at Km-67; do different vertical canopy strata exhibit different phenological patterns; and is the phenological pattern observed at Km-67 part of a larger, regional trend (i.e. do we see the same patterns at a site 5 km away—Km-72)? LiDAR surveys—4 km at the Km-67 site and 2 km at the Km-72 site—were conducted throughout the 2012 dry season (August to December) and are on-going at Km-67.
Results/Conclusions
LAI increased during the dry season, decreased suddenly at the onset of the wet season (late November/early December) and increased again through the first half of the wet season; the magnitude of LAI change was similar to previous studies (1 m2/m2). Low, mid and upper canopy strata showed similar patterns and timing of leaf area changes, however, the mid canopy showed the greatest absolute increase and decrease in leaf area, and the upper canopy, which consists of emergent canopy trees, showed the greatest proportional change. This suggests that although the leaves at the top of the canopy don’t show the greatest absolute changes in LAI, large trees may be the most responsive to seasonality—large trees both experience the greatest magnitude of change in light and possibly the strongest change in hydrological stress across seasons. The phenological leaf area pattern is consistent between Km-67 and Km-72 sites, suggesting a broad-scale trend since these sites were separated by 5 km. We conclude that changes in not just the total LAI but also its vertical distribution are essential to understand ecosystem seasonality in the Amazon. This approach can provide essential information to validate air and space-borne remote detection of leaf area phenology.