Seasonal and drought‐related changes in leaf area profiles depend on height and light environment in an Amazon forest

Abstract

Summary Seasonal dynamics in the vertical distribution of leaf area index (LAI) may impact the seasonality of forest productivity in Amazonian forests. However, until recently, fine‐scale observations critical to revealing ecological mechanisms underlying these changes have been lacking. To investigate fine‐scale variation in leaf area with seasonality and drought we conducted monthly ground‐based LiDAR surveys over 4 yr at an Amazon forest site. We analysed temporal changes in vertically structured LAI along axes of both canopy height and light environments. Upper canopy LAI increased during the dry season, whereas lower canopy LAI decreased. The low canopy decrease was driven by highly illuminated leaves of smaller trees in gaps. By contrast, understory LAI increased concurrently with the upper canopy. Hence, tree phenological strategies were stratified by height and light environments. Trends were amplified during a 2015–2016 severe El Niño drought. Leaf area low in the canopy exhibited behaviour consistent with water limitation. Leaf loss from short trees in high light during drought may be associated with strategies to tolerate limited access to deep soil water and stressful leaf environments. Vertically and environmentally structured phenological processes suggest a critical role of canopy structural heterogeneity in seasonal changes in Amazon ecosystem function.

Functional composition of epiphyte communities in the Colombian Andes

“…The prevalence of tougher leaves at sites with homogeneous height distributions indicates that these sites may be more homogeneously "canopy-like," as is expected in shorter forests at higher elevations. penetrate toward the forest floor, and have been shown to structure tree biomass responses to the macroenvironment (Stark et al 2012, Smith et al 2019. Future studies should investigate the role of forest structural attributes, such as canopy gap fraction and vertical biomass distributions, which determine the extent to which canopy-like conditions FIG.…”

“…Strong heterogeneity in sun exposure at a given height, depending on canopy rugosities and gap distributions (Smith et al 2019), largely controls water availability, a key factor for epiphyte survival and establishment. This conclusion is based on the fact that the PCA 1 and PCA2 derived from traits and relative epiphyte height of attachment, respectively, followed a similar trend.…”

Phenological and structural linkages to seasonality inform productivity relationships in the Amazon Rainforest

“…Within the Amazon, leaf area (measured as leaf area index or LAI) may remain fairly constant across the year (Wu et al ., ) yet show seasonal differences in its vertical distribution within the canopy (Tang & Dubayah, ). In this issue of New Phytologist , Smith et al . (pp. 1284–1297) link these seasonal changes in the vertical distribution of LAI to the seasonality of Amazonian forest productivity.…”

“…In Smith et al ., vegetation structure is quantified using a ground‐based LiDAR system that samples a vertical plane of the canopy along a transect. LiDAR pulse returns are converted to estimates of leaf area density (LAD) and LAI (Stark et al ., ) allowing for characterization of the vertical and horizontal distribution of vegetation.…”

“…Smith et al . linked canopy structure to productivity by comparing annual ranges of total LAI, as well as LAI in upper and lower canopy strata (e.g.…”