Seasonal photosynthetic activity in evergreen conifer leaves monitored with spectral reflectance

Annual trends of the Photochemical Reflectance Index (PRI) and pigment content in evergreen conifer leaves were monitored over two years from 2011 to 2013. During the second year, chlorophyll fluorescence and gas exchange were included to examine the spring recovery of photosynthesis. All the metrics indicated large seasonal changes in photosynthetic activity, with a sharp transition in the spring and a more gradual transition in the autumn. The PRI was primarily driven by changes in carotenoid:chlorophyll pigment levels (constitutive processes) that correlated with seasonal photosynthetic activity, with a much smaller variation caused by diurnal changes in xanthophyll cycle activity (facultative processes). Additionally, a previously unrecognized shift in spectral reflectance also affected the PRI under deep cold temperatures. Together, these findings indicate that evergreen conifers photosynthetic system possesses a remarkable degree of resilience in response to large temperature changes across seasons, and that optical remote sensing can be used to observe the seasonal effects on photosynthesis and productivity.