Changes in Spectral Properties, Chlorophyll Content and Internal Mesophyll Structure of Senescing Populus balsamifera and Populus tremuloides Leaves

In this paper we compare leaf traits and spectral reflectance for sunlit and shafded leaves of Populus tremuloides and Populus balsamifera during autumn senescence using information derived from an Analytical Spectral Devise (ASD) Full Range spectrometer. The modified simple ratio (mSR(705)) and modified normalized difference index (mND(705)) were effective in describing changes in chlorophyll content over this period. Highly significant (P<0.01) correlation coefficients were found between the chlorophyll indices (mSR(705), mND(705))) and chlorophyll a, b, total chlorophyll and chlorophyll a/b. Changes in mesophyll structure were better described by the plant senescence reflectance index (PSRI) than by near-infrared wavebands. Overall, P. balsamifera exhibited lower total chlorophyll and earlier senescence than P. tremuloides. Leaves of P. balsamifera were also thicker, had a higher proportion of intercellular space in the spongy mesophyll, and higher reflectance at 800 nm. Further research, using larger sample sizes over a broader range of sites will extend our understanding of the spectral and temporal dynamics of senescence in P. tremuloides and P. balsamifera and will be particularly useful if species differences are detectable at the crown level using remotely sensed imagery.