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Reducing solar heat gain during winter: the role of white bark in northern deciduous trees
Deciduous tree species throughout the boreal forest of North America have lighter-coloured bark than do species restricted to more southern forests. We tested the hypothesis that light-coloured bark minimizes the thawing and freezing of cambium tissue during winter that could contribute to sunscald injury. During mid-winter, maximum midday cambium temperatures of south-exposed bark of white birch (Betula papyriferaMarsh.) near Timmins, Ontario, were higher for brown-painted bark (+1.6 °C) than for natural bark (-9.4 °C) and white-painted bark (-12.1 °C). Rates of temperature decrease after trees were shaded at midday were more rapid for brown-painted bark (0.06 °C/min) than for natural bark (0.03 °C/min) and white-painted bark (0.03 °C/min). When stems of white birch, trembling aspen (Populus tremuloides Michx.), yellow birch (B. alleghaniensis Britton), and largetooth aspen (P. grandidentata Michx.) were illuminated and subsequently shaded at -10 °C ambient air temperature, maximum cambium temperatures and rates of cambium cooling increased with decreasing measures of whiteness. For trembling aspen in the southwest Yukon, we found that after two years, brown-painted trees had a higher incidence (35%) of wounding that resembled sunscald injury than did white-painted trees (2.5%) and natural trees (4.5%). Therefore, we suggest that light-coloured bark reduces the risk of winter sunscald injury, probably by protecting the cambium from solar heat gain in subfreezing temperatures. This physical mechanism for reducing sunscald risk may explain why the deciduous trees at the northern limit of tree growth are those with highly reflective bark.