Effects of species and hardwood–softwood mix on the balance of growth and mortality in old stands in New Brunswick, Canada

• Growth and mortality of older stands were studied in 602 mature-overmature PSPs.
• More than half of the PSPs had increasing volume over the 20-year period.
• Stands dominated by balsam fir had highest mortality and minimum PAI.
• Wind related causes resulted in more than 56% of mortality for all stand types.
• Species (balsam fir) had more effect on stand dynamics than stand category.

Growth and mortality patterns influencing dynamics of older stands were studied in 602 ‘mature-overmature’ (mean age 84–108 years) permanent sample plots (PSPs) representing 12 stand types categorized as hardwood (HW), mixedwood (MW) and softwood (SW) in New Brunswick, Canada. Plots were grouped based on stand type and volume development pattern over 20 years (decreasing, fluctuating, and increasing net volume change classes). Among all PSPs, 58% increased in volume from 1987 to 2007, 17% decreased, and 25% had fluctuating (variable patterns over time) volume change. Percentage of plots with decreasing volume over the 20-year period averaged 7–8% for HW and MW, versus 17% for SW. Poor site spruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.)-spruce had the highest decreasing volume plots at 30% and 28% respectively. Mortality averaged 7.5, 4.4, and 2.2 m3 ha−1 yr−1 for decreasing, fluctuating, and increasing volume change classes, respectively. Stands dominated by balsam fir in the decreasing volume change class sustained cumulative mortality of 118–134 m3 ha−1 over 20 years due to short longevity, vulnerability to insect disturbance, and, versus only 10–40 m3 ha−1 of mortality in increasing volume change plots. Windthrow caused 56% of tree mortality. Increment of surviving trees was relatively constant among stand types and volume change classes, ranging from 2.0 to 6.1 m3 ha−1 yr−1. Regression tree analyses showed that % basal area of balsam fir, ecoregion, and quadratic mean diameter were the most influential variables determining mortality and periodic annual increment. More than half of the PSPs in 11 of the 12 stand types had increasing volume over the 20-year period, but mortality exceeded increment in older balsam fir dominated MW and SW stands. Results show that balsam fir and spruce species were more important than broad stand types (HW, MW, SW) in determining stand dynamics, and indicate the carbon sequestration potential of older stands of different stand types.