Reflectance of Alaskan black spruce and white spruce foliage in relation to elevation and latitude

Leaf reflectance at visible and near-infrared wavelengths (400-1000 nm) is related primarily to pigmentation, leaf structure and water content, and is an important tool for studying stress physiology and relationships between plants and their growth environment. We studied reflectance of two co-occurring Alaskan conifers, black spruce (Picea mariana (Mill.) BSP) and white spruce (Picea glauca (Moench) Voss), at elevations from 60 to 930 m a.s.l. along a latitudinal gradient from 61 degrees to 68 degrees N. Black spruce samples were collected from 24 sites and white spruce from 30 sites. Overall, reflectance spectra of the two species were similar, but from 400 to 700 nm, needle reflectance was consistently higher in black spruce than in white spruce (all P < or = 0.05). This difference is probably related to differences in epicuticular wax morphology or amount, and may represent a photoprotective mechanism in black spruce. Reflectance at visible wavelengths generally increased with elevation and latitude in both species, consistent with a general stress response. However, in a multiple regression, latitude and elevation explained only 25-45% of the total variation in the indices studied. Reflectance indices suggested that needle yellowness increased, whereas chlorophyll content and photochemical efficiency decreased with both elevation and latitude. These trends were consistent between species, but white spruce generally showed a much smaller (and insignificant) reflectance response to latitude compared with black spruce. Differences between species could be related to black spruceʾs ability to colonize more stressful sites and white spruceʾs greater competitiveness on less stressful sites, coupled with the effects of drainage and microtopography (which may vary less predictably with latitude than elevation) on species distribution. The black spruce results suggest that a 1000-m increase in elevation is roughly comparable with a 6 degrees increase in latitude.