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Harvard Forest Symposium Abstract 2011

• Title: Establishing a physiological basis for camera phenology measurements
• Primary Author: Josh McLaren (Harvard University)
• Additional Authors: J. William Munger (Harvard University); Andrew Richardson (Northern Arizona University); Leland Werden (Harvard University); Steven Wofsy (Harvard University)
• Abstract:

  Phenological data offers key insight into ecosystem change, and cameras placed on eddy-flux towers (PhenoCams) are well suited for monitoring canopy phenology at relevant spatial and time scales. PhenoCam data have yet to be linked to ecosystem development or function, at either the leaf or canopy levels. A distinct recurring canopy color change is observable every spring in the PhenoCam data from multiple deciduous forest sites, and our overarching goal has been to determine whether this event is ecologically significant, allowing us to monitor future physiological changes at forested research sites.
  
  
  
  Weekly monitoring of leaf-level properties (leaf mass per area (LMA), foliar nitrogen content, and leaf reflectance) for three dominant deciduous tree species at the Harvard Forest EMS tower was initiated in April 2010. These leaf level changes in structure and function were compared with PhenoCam data, leaf area index (LAI), and carbon flux. To support our interpretation of the leaf level data, we also compared PhenoCam data with canopy photosynthetic capacity for 2008-2010.
  
  
  
  The comparison of 2010 LMA and LAI shows that the seasonal increase in dry mass lags well behind leaf expansion, and that LMA and LAI peak at roughly the same time. The camera reflectance peak corresponded with a concurrent increase in dry mass and decline in leaf expansion, suggesting that tissue development is influencing canopy reflectance. But most importantly, the PhenoCam data during leaf darkening, after the peak in reflectance, correlates with LMA very well.
  
  
  
  Closely associated with this is the finding that a stark increase in canopy photosynthetic capacity occurs within several days of the camera reflectance peak for each year we have data, and that changes in capacity lagged significantly behind camera bud-break; a greater than 14 day lag during each study year. We believe this correspondence of color change, increased photosynthetic activity, and leaf development suggests that photosynthetic competence and full photo-protection develop simultaneously, as reported in several recent studies, leading us to hypothesize that the rapid change in reflectance (i.e.. the darkening) is partly due to a decline in the synthesis of protective pigments (e.g., Anthocyanins).
  
  
  
  This implies that the appearance of green foliage (i.e., bud-break) in a forest canopy is a poor predictor of changes in photosynthetic activity, and that the leaf darkening associated with tissue development is a better indicator of the physiological developments relevant to photosynthetic function.

• Research Category: Forest-Atmosphere Exchange


• Figures:
LMA_LAI.png
RGB_stack.png
flux_stack.png
flux_cam_corr.png