Do physiological changes at leaf level explain seasonal changes in remotely sensed canopy greenness?
Abstract
The PhenoCam (http://phenocam.sr.unh.edu) network uses digital cameras to observe phenological events and track seasonal changes in forest canopy greenness. As a near surface remote sensing platform it acts as an intermediary between leaf level measurements, typically made by a human observer, and satellite based remote sensing products. The cameras typically document a rapid increase in canopy greenness after leaf out, which peaks in early summer and then gradually declines before a rapid decline corresponding to autumn senescence and abscission. Open questions remain, however, as to whether the observed changes in canopy greenness are directly related to changes in leaf physiology and pigmentation, changes in canopy structure (leaf size, shape, and orientation), or some combination thereof. The goal of this study was to investigate how leaf-level structure and function relate to canopy greenness as measured by the cameras in an oak-dominated temperate forest. Sampling was conducted at the Harvard Forest, in central Massachusetts USA. We sampled upper-canopy leaves of three dominant deciduous tree species red oak (Quercus rubra), red maple (Acer rubrum) and yellow birch (Betula alleghaniensis) on a weekly basis for a full growing season, from leaf out to leaf drop. Leaf mass per area, nitrogen content, and chlorophyll fluorescence were measured for each leaf, along with spectral reflectance and transmission at wavelengths from 350 to 2500 nm. Leaf gas exchange measurements were also made weekly and used to derive leaf photosynthetic parameters. Results show that changes in leaf mass per area and photosynthetic capability at leaf-level lag initial increases in greenness measured by the cameras. Spectral indices related to chlorophyll content such as the photochemical reflectance index (PRI) and chlorophyll normalized difference index (Chl NDI), along with chlorophyll fluorescence indicate that chlorophyll content continues to increase after greenness measured by the cameras begins to decline. These results provide insights into understanding corresponding changes in greenness observed at diverse sites across the PhenoCam network, and contribute to improving our interpretation of remotely sensed satellite products.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B43A0276D
- Keywords:
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- 0439 BIOGEOSCIENCES / Ecosystems;
- structure and dynamics;
- 0476 BIOGEOSCIENCES / Plant ecology;
- 0480 BIOGEOSCIENCES / Remote sensing