Evaluating photosynthesis in boreal forest species with fluorescence measurements

The utility of fluorescence measurements in identifying changes in physiological response associated with seasonal change in the photosynthesis process was investigated in several boreal forest species during the summer and fall of 1994, as part of the Boreal Ecosystem-Atmospheres Study (BOREAS) in Saskatchewan, Canada. Four overstory species were examined: jack pine (Pinus banksiana), black spruce (Picea mariana BSP), white spruce (Picea glauca), and quaking aspen (Populus tremuloides). The aspen understory shrub, hazelnut (Corylus americana), was also examined. Stands of different ages, mature and young, were examined for the pine and aspen; the black spruce stand was mature, whereas the white spruce stand was young. For the conifers, needles on the same shoot from separate age classes were evaluated separately, but combined here. Canopy averages were computed from field and laboratory measurements made on shoots or leaves from the upper and lower tree layers of the tree overstory (and hazelnut understory). In situ gas exchange measurements were made in the field. Photosynthetic capacity was determined by oxygen evolution under controlled conditions in a field laboratory. Extracts of leaves/needles were later utilized to determine pigment content from spectral absorption coefficients, and fluorescence emission spectra between 360-780 nm (excitation at 340 nm). A qualitatively different pattern between coniferous and deciduous species was observed in the seasonal shifts in fluorescence: conifers exhibited a general fluorescence decrease in autumn from higher summer values at all wavelengths, whereas the deciduous species exhibited lower (higher) red/far-red (blue) fluorescence in fall. In situ photosynthesis was significantly higher for aspen than its hazelnut understory or the conifers during midsummer. However, photosynthetic capacity determined under controlled laboratory conditions was 4-6 times greater than field values for all species, indicating strong stomatal limitations to ambient gas exchange. A red/blue fluorescence ratio (680 nm/440 nm) was correlated with photosynthetic capacity.