Reducing Solar Heat Gain during Winter: The Role of White Bark in Northern Deciduous Trees

• Published in: Arctic 2003-06, Vol.56 (2), p.168-174
• Main Authors: Karels, Tim J., Boonstra, Rudy
• Format: Article
• Language: English
• Subjects: Air temperature; Ambient temperature; Aspen; Bark; Birch; Birch trees; Boreal forests; Cambium; Cooling; Deciduous trees; Environmental aspects; Freezing; Hardwood trees; Heat; Marshes; Optical reflection; Plant species; Risk reduction; Stems; Sun; Thawing; Trees
• ISSN: 0004-0843; 1923-1245
• DOI: 10.14430/arctic612

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 papyrifera Marsh.) near Timmins, Ontario, were higher for brownpainted 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 whitepainted 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. /// Dans toute la forêt boréale de l'Amérique du Nord, les arbres à feuilles caduques ont une écorce plus claire que celle des espèces dont l'habitat est limité aux forêts plus au sud. On a testé l'hypothèse selon laquelle, au cours de l'hiver, l'écorce claire minimise le gel-dégel du cambium qui pourrait contribuer à une blessure d'insolation. Au milieu de l'hiver, à midi, près de Timmins, en Ontario, la température maximale du cambium de l'écorce orientée au sud du bouleau à papier (Betula papyrifera Marsh.) était plus élevée pour l'écorce peinte en brun (+ 1,6°C) que pour l'écorce naturelle (-9,4°C) et celle peinte en blanc (-12,1°C). Après midi, quand les arbres étaient à l'ombre, le taux de baisse de la température était plus rapide pour l'écorce peinte en brun (0,06°C/min) que pour l'écorce naturelle (0,03°C/min) et pour cel