OOS 36-1 - Physiology from afar: Using tree canopy reflectance to quantify post-fire physiological performance

Fire is a major cause of tree injury and mortality worldwide, yet our current understanding of fire effects is largely based on ocular estimates of stem charring and foliage discoloration, which are error prone and provide little information on underlying tree function. Accurate quantification of physiological performance is a research and forest management need given declining performance could help identify mechanisms of—and serve as an early warning sign for—mortality. Many previous efforts have been hampered by the inability to quantify the heat flux that a tree receives during a fire, given its highly variable nature in space and time. Here, we used a dose-response approach to elucidate fire impacts on tree physiology by subjecting saplings of four conifer species (Larix occidentalis, Pinus contorta, Pinus monticola, Pseudotsuga menziesii) to surface fires of known intensity and measuring short term post-fire physiological performance in photosynthetic rate, stomatal conductance, and chlorophyll fluorescence. We evaluated the ability of spectral reflectance indices to quantify change in physiological performance at the individual tree crown and stand scales. Photosynthesis, stomatal conductance, and chlorophyll fluorescence declined with increasing fire intensity in all species. Generally, crown-scale spectral indices were more accurate at quantifying physiological performance than those acquired at the stand-scale. The Photochemical Reflectance Index outperformed other indices at quantifying photosynthesis and chlorophyll fluorescence, highlighting its potential use to quantify crown scale physiological performance. These results suggest that current high spatial resolution sensors could utilize such spectral indices for tree crown-level assessments. Furthermore, repeated observations may help identify early indicators of tree death, such as decreasing trends or high variability in spectral index values over time.