Effects of Warming on the Phenology of Photosynthesis of Eastern White Pine and Implications for the use of Remote Sensing in a Changing Climate

In northern trees, temperature and photoperiod control the beginning and end of the photosynthetically active season. Climate warming could extend the length of the photosynthetically active season, while photoperiod could potentially prevent trees from benefitting from warmer temperatures. Changes in the length of the photosynthetically active season can be estimated with the photochemical reflectance index (PRI), which detects xanthophyll cycle pigments involved in non-photochemical quenching (NPQ), a process affecting photosynthetic efficiency. In overwintering conifers, sustained NPQ replaces xanthophyll cycle-mediated NPQ, likely causing seasonal variations in the relationship between PRI and photosynthetic efficiency. To better understand the impacts of climate warming on the spring and autumn photosynthesis of conifer forests, we investigated 1) the potential for enhanced photosynthesis during warmer springs and autumns in Pinus strobus L. and 2) seasonal variations in the relationship between PRI and the photosynthetic efficiency of needles. During simulated summer-autumn and winter-spring transitions, PRI variation mainly reflected constitutive carotenoid pigment pool adjustments. During spring, differences in the timing of pigment pool adjustments and photosynthetic recovery impaired the relationship between PRI and photosynthesis. NPQ mechanisms and energy sinks undetected by PRI, such as electron transport around photosystem I, also impaired the relationship between PRI and photosynthesis. In P. strobus seedlings from provenances located along a 1000 km latitudinal gradient, we observed differences in the phenology of photosynthesis, suggesting genetic controls of photosynthesis by photoperiod, with stronger controls during autumn than spring. The PRI could effectively track seasonal variations in energy partitioning, as well as carotenoid pigment pool sizes indicative of cold stress. Photosynthesis in P. strobus might be enhanced during warmer springs and autumns, however, photoperiodic regulation of photosynthesis is to be expected during autumn. The interpretation of the PRI in evergreens should take into account alternative energy sinks, especially under cold spring and autumn conditions.Ph.D.2020-03-21 00:00:0