Seasonal variability in foliar characteristics and physiology for boreal forest species at the five Saskatchewan tower sites during the 1994 Boreal Ecosystem‐Atmosphere Study

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Abstract
Leaf‐level measurements of gas exchange, chemistry, morphology, and spectral optical properties were acquired at the five instrumented tower sites during the three 1994 growing season intensive field campaigns (IFCs) conducted near Prince Albert, Saskatchewan, as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Stands included old and young aspen (OA, YA) associated with the hazelnut shrub, old and young jack pine (OJP, YJP) stands, and an old black spruce (OBS) stand; white spruce (at YA) and an understory herb (dogbane, at OJP) were also examined. Midsummer peak photosynthesis for aspen leaves in the field (A, light saturated) and laboratory (Amaxlight and CO2saturated) was ∼12.6 and 33–41 μmol CO2m−2s−1. Black spruce exhibited the lowest A, 3 μmol CO2m−2s−1. Jack pine and black spruce attained their highestAmax(17–20 μmol CO2m−2s−1) in late summer/early fall. Gas exchange by white spruce was significantly higher and stomatal limitation lower than for other conifers, at levels comparable to broadleaf responses. White spruce foliage had the highest chlorophyll content in fall (∼41 μg cm−2), followed by aspen (OA) and hazelnut (YA) in midsummer (∼31 μg cm−2). Specific leaf mass of aspen, hazelnut, and conifer foliage was 86, ∼47, and ∼174 g m−2, respectively. Leaf nitrogen content of broadleaves (18–40 g N g−1dry wt) was 2–3 times greater than conifer needles (8–12 g N g−1). Significantly larger needles were produced at OJP versus YJP, but needle number per age class was greater at YJP. The absorbed photosynthetically active radiation fraction (fAPAR) in June/July averaged ∼80% for broadleaves and ∼83% in conifer needles. The simple ratio (SR, near‐infrared/red ratio) calculated from foliar transmittances was more strongly related tofAPAR than SR calculated from reflectances, with stronger correlation for broadleaves (r=0.92) than for conifers (r=0.78).