Open access publications in the SLU publication database

Abstract

Variation in ring width in tree ring series of mountain birch (Betula pubescens ssp. czerepanovii) was analyzed and related to climate and herbivory. The effects of different defoliation levels were studied from natural outbreaks of the autumnal moth (Epirrita autumnata) and from experimental defoliations. Short-term variation in ring width was mainly positively correlated with the current year July followed by June temperature. A warm May resulted in wider rings, probably due to earlier budburst. Low defoliation levels (<12%) and 50% experimental defoliation had no effect on ring widths. The remaining foliage in the experiment showed increased photosynthetic capacity related to increased nitrogen content; this is presumably an important factor in maintaining growth performance after defoliation. Even after a defoliation level of 84% only a one-year reduction in stem growth was observed, but a slightly higher defoliation level (93%) resulted in a growth reduction during at least 4 years, which indicates that the defoliation-growth relationship is non- linear. Tree ring series display long-term dynamics characterized by regular periods of increased growth. These growth increases are preceded by outbreaks of Epirrita with complete defoliation and mortality of old stems and branches, and subsequent rejuvenation by means of sprouts at the tree base and new branches in the crown. This results in a long period of better growth of the surviving and new stems. The period of rapid growth is followed by a growth decline. Competition between stems within trees seems to be an important factor causing this decline, together with other consequences of tree size. It is suggested that an increased stem and branch surface area/leaf area ratio makes old stands susceptible for Epirrita outbreaks.