Spectral reflectance and photosynthetic properties of Betula papyrifera (Betulaceae) leaves along an elevational gradient on Mt. Mansfield, Vermont, USA.

We studied relationships between spectral reflectance and photosynthesis of mountain paper birch, Betula papyrifera var. cordifolia (Regel) Fern., leaves from three different elevations on Mt. Mansfield (summit elevation 1339 m above sea level) in the Green Mountains of Vermont, USA. The different reflectance indices we used all suggested progressively increasing stress with increasing elevation. The photochemical reflectance index (PRI) indicated lower photosynthetic radiation use efficiency at higher elevations, the red edge position (λRE) indicated lower chlorophyll concentrations at higher elevations, and the structure-independent pigment index (SIPI) indicated a higher carotenoid : chlorophyll a ratio at higher elevations. The rate of change in these indices with changes in elevation was much higher than we have observed in our studies of red spruce and balsam fir reflectance along a similar elevational gradient; we take this to be an indicator of the greater susceptibility of paper birch to elevation-related stressors compared to the very stress-tolerant conifers. At all light levels, photosynthesis decreased with increasing elevation; this pattern was most noticeable in the light-saturated rate of photosynthesis (A(sat)), which was nearly twice as high in low-elevation leaves (17.0 ± 1.0 μmol·m(-2)·s(-1)) than in high-elevation leaves. The quantum yield of photosynthesis (Φ) exhibited a similar trend. Furthermore, the highest elevation leaves showed a much sharper transition from the light-limited to the light-saturated part of the light response curve than did the lowest elevation leaves. The photochemical reflectance index was highly correlated with A(sat) (r(2) = 0.99) and Φ (r(2) = 0.96). In addition to contributing to our knowledge of the ecophysiology of paper birch along a steep environmental gradient, these results are further evidence of the usefulness of reflectance measures for the rapid and noninvasive detection of plant stress, especially when used in conjunction with direct measurements of photosynthesis.