Rate and causes of decline of mature and overmature balsam fir and spruce stands in New Brunswick, Canada.

Data from 585 balsam fir (Abies balsamea (L.) Mill.) and spruce (Picea spp.) permanent sample plots (PSPs) >50 years old in New Brunswick, Canada, measured between 1987 and 1998, were used to quantify the timing, magnitude, and causes of decline in stand growth, measured as negative net volume increment. Plots were subdivided into classes based on species group, ecoregion, and volume-development pattern (decreasing, fluctuating, stable, or increasing volume with time). Decline began at an earlier age in balsam fir – spruce (BFSP) PSPs (~70 years old) than in spruce – balsam fir (SPBF) PSPs (~120 years). One-third of plots (29%) fell into the decreasing volume-development category. Variation in the proportion of decreasing plots occurred among ecoregions (range 0%–60%) and, to a lesser extent, among species groups (26%–33%). Decreasing BFSP plots exhibited double the rate of volume loss of their SPBF counterparts (–7.4 and –3.9 m3·ha–1·year–1, respectively). Decline was caused by an increased rate of mortality, which was highest in the decreasing volume-development classes (7.5 m3·ha–1·year–1) and lowest in the increasing volume-development classes (1.4 m3·ha–1·year–1). Growth of surviving trees remained relatively stable (2.5–4.2 m3·ha–1·year–1) among volume-development classes. Blowdown was the biggest cause of tree mortality, accounting for approximately 50% of dead trees in BFSP and SPBF plots. The rate of mortality was influenced by ecoregion, species, and the combined effect of previous insect-outbreak and wind disturbances. [ABSTRACT FROM AUTHOR]