Using Root Carbohydrates Reserves as an Indicator of Vulnerability to Defoliation in Trembling Aspen

Tent caterpillar (Malacosoma disstria [H¸bner]) and large aspen tortrix (Choristoneura conflictana [Walker]) are native defoliators of trembling aspen (Populus tremuloides Michx.) in the boreal forests of North America. Defoliation events can be sporadic and localized but there can be large scale outbreaks covering hundreds of square kilometers. Large outbreaks are thought to occur in 10-year cycles and can last several years. Generally it is thought that defoliation events have short-term effects on aspen productivity but do not result in significant mortality, as aspen reflushes after these spring defoliation events. However, in combination with other stressors such as drought, it has been observed that aspen clones can be weakened and are more susceptible to stem dieback or even clone mortality. Over a period of 8 years we determined seasonal carbohydrate reserves of different tissues in aspen clones. Non-structural carbohydrate reserves were determined in twig, stem and root samples from 9 different clones. During the collection period some of the aspen clones were defoliated in 2000 and/or 2007. After defoliation, tissue carbohydrate reserves in stems and twigs recovered by the end of the same summer. In contrast, in roots, carbohydrates reserves (particularly starch) were still depressed the second summer after defoliation, relative to clones that were not defoliated. After only one defoliation event starch reserves in the roots were close to zero, suggesting that repeated defoliations could have significant impacts on the survival of aspen clones. The research indicates that root reserves are severely impacted by defoliation and that clones with already low carbohydrate reserves are likely at a higher risk of dieback and mortality and could function as a valuable indicator to assess risks of clonal dieback in aspen