Stand age structures of single and mixed-species of fire-origin boreal stands in central Canada

Abstract

Keywords : biodiversity, bimodal, boreal forest, cover type, developmental stage, old-growth, reverse-J, stand age structure, time since fire, wildlife habital, [disturbances, spruce budworm, wind, harvesting, logging].

Stand age structure has been linked to wildlife habitat and can affect biodiversity. Further, certain stand age structures (such as those associated with old-growth) are valuable from an aesthetic and fiber supply perspective. A review and synthesis of literature indicated that boreal forest stand age structure is influenced by time since stand replacing fire, stand composition, and disturbances such as spruce budworm (Choristoneura fumiferana) and wind. Silvicultural options and implications for managing age structure at the stand and landscape level are discussed. My objectives were to examine how stand age structure changes among stand developmental stages and stand cover types. I sampled living trees in 32 stands representing typical conifer, mixedwood, and hardwood stand types in the central boreal forest region o f North America representing the stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamic, and gap dynamic stages o f stand development. Using a 400 m2 plot in each stand as the sampling unit, the diameter at breast height (DBH) o f all canopy trees (DBH >10 cm) was measured. Five canopy trees in each diameter class ((1) 10-14.9 cm, (2) 15-19.9 cm, (3) 20-24.9 cm, (4) 25-29.9 cm, and (5) > 30 cm) for each species were randomly selected, and the height o f each tree was measured and an increment core was taken and aged. Three circular 25 m subplots were then randomly established within each 400m plot and the diameter at root collar and height of all seedlings and saplings (DBH <10 cm) were measured and recorded. A disk was then taken at root collar and aged for 5 seedlings and saplings in each height class. Canopy trees and regeneration were then summed to the plot level and scaled to per hectare. The results suggest that stand age structure is variable with stand developmental stages and stand cover type. Age distribution in conifer stands was bimodal in the stem exclusion/canopy transition, bimodal in the canopy transition, reverse-J in the canopy transition/gap dynamics, and finally bimodal in the gap dynamics. In the mixedwood stands, similar to the conifers, stand age structure was bimodal in all the stand developmental stages. In the hardwood stands, age structure was bimodal in the stem exclusion/canopy transition, unimodal in the canopy transition stage, reverse-J in the canopy transition/gap dynamics stage and finally bimodal in the gap dynamics stage. Forest management activities such as partial harvesting, selection harvesting, and seed-tree systems may diversify stand age structures at the stand and landscape levels, benefiting wildlife and creating characteristics of old-growth. It is recommended that quantitative old-growth be defined. I suggest using the following criteria to determine old-growth in the boreal forest: (1) canopy breakdown of pioneering cohort is complete and the stand is dominated by later successional tree species (2) the age structure of the stand is bimodal, with dominating canopy trees that fall within a relatively narrow range o f age and height classes and a significant amount of understory regeneration.