Anatomy of white rings in trembling aspen and reconstruction of forest tent caterpillar outbreaks in the Duck Mountain region, Manitoba

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Sutton, Alanna C.
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Insect outbreaks in forest ecosystems are an important natural disturbance, and the source of much timber loss. The forest tent caterpillar (Malacosoma disstria Hubner) is a major defoliator throughout the distribution of trembling aspen (Populus tremuloides Michx.), its preferred host. Forest tent caterpillar outbreaks have been shown to cause reduced growth and even mortality in the past, and recently, the presence of 'white' rings in trembling aspen have been linked to severe early season defoliation. While a good correspondence between the formation of white rings and severe defoliation has been found, white rings are, as yet, not described anatomically. In addition, white rings are presumed to be useful in the reconstruction of past forest tent caterpillar outbreaks. A study was undertaken to determine the difference between white rings and 'normal' rings in trembling aspen. White rings were also used in the spatiotemporal reconstruction of forest tent caterpillar outbreaks in the Duck Mountain Provincial Forest. To assess the difference in white ring formation and other forest tent caterpillar characteristics among vegetation and age classes, sites were sampled within four vegetation types and two age classes. White ring development in the tree stem was found to occur uniformly around the stem from the base, up to approximately 70% of the stem height at the time of white ring formation. White rings were also found to be narrower and less dense than normal rings and to have thinner fiber cell walls, smaller fiber diameter and a higher proportion of fiber lumen than normal rings. It is speculated that the growth hormones within the stem, and the re-allocation of reserves for re-foliation are affected by severe defoliation, causing a structural sacrifice. White rings were associated with all major forest tent caterpillar outbreaks in the Duck Mountain Provincial Forest, as well as many of the smaller, suspended outbreaks. Major outbreaks in the Duck Mountain Provincial Forest occurred between 1939 and 1948, 1961 and 1965 and 1982 and 1985, with another suspected major outbreak occurring during the 1870s. Smaller possible outbreaks were also observed in the 1950s 1970s and the 1990s. These outbreaks were also recorded in the balsam poplar (Populus balsamifera L.) and paper birch (Betula papyrifera Marsh.) chronologies, and white rings observed in these two species also corresponded with these periods of forest tent caterpillar outbreaks. The 1960s outbreak was different than all other outbreaks, occurring directly after a year of extreme drought, and spreading to the entire Duck Mountain Provincial Forest within a single year. The other major outbreaks were not associated with extreme drought events, and did not cause growth suppression in trembling aspen as severe as that produced during the 1960s outbreaks. In addition, during the 1960s outbreak, sites which were composed of trembling aspen and jack pine (Pinus banksiana Lamb.) recorded more severe growth suppression and white ring formation than any other stand type. The increased severity of forest tent caterpillar outbreaks in dry sites may become more important in a warmer climate, as has already been predicted. White rings, while being useful in the reconstruction of forest tent catepillar outbreaks, must be used cautiously and in addition to other markers like growth suppression, as they were not always present in trees from the same sites, nor did they occur in all sites in the Duck Mountain Provincal Forest.