Light-energy processing and freezing-tolerance traits in red spruce and black spruce: species and seed-source variation

Red spruce (Picea rubens Sarg.) and black spruce (Picea mariana (Mill.) B.S.P.) are genetically and morphologically similar but ecologically distinct species. We determined intraspecific seed-source and interspecific variation of red spruce and black spruce, from across the near-northern margins of their ranges, for several light-energy processing and freezing-tolerance adaptive traits. Before exposure to low temperature, red spruce had variable fluorescence (F-v) similar to black spruce, but higher photochemical efficiency (F-v/F-m), lower quantum yield, lower chlorophyll fluorescence (%), and higher thermal dissipation efficiency (q(N)), although the seed-source effect and the seed-source x species interaction were significant only for Fv/Fm. After low-temperature exposure (-40 degreesC), red spruce had significantly lower F-v/F-m, quantum yield and q(N) than black spruce, but higher chlorophyll, fluorescence and relative fluorescence. Species, seed-source effect, and seed-source x species interaction were consistent with predictions based on genetic (e.g., geographic) origins. Multi-temperature exposures (5, -20 and -40 degreesC) often produced significant species and temperature effects, and species x temperature interactions as a result of species-specific responses to temperature exposures. The inherent physiological species-specific adaptations of red spruce and black spruce were largely consistent with a shade-tolerant, late-successional species and an early successional species, respectively. Species differences in physiological adaptations conform to a biological trade-off, probably as a result of natural selection pressure in response to light availability and prevailing temperature gradients.