Abstract
To study the effects of stand development and overstory composition on stand age structure, we sampled 32 stands representing conifer, mixedwood, and hardwood stand types, ranging in ages from 72 to 201 years on upland mesic sites in northwestern Ontario. We defined the stages of stand development as: stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamics, and gap dynamics. Stand age structure of conifer stands changed from bimodal, bimodal, reverse-J, and bimodal, respectively, through the stages of stand development. Mixedwood and hardwood stands revealed similar trends, with the exception of missing the canopy transition/gap dynamic stage in mixedwoods. Canopy transition/gap dynamic stage in hardwoods showed a weaker reverse-J distribution than their conifer counterparts. The results suggest that forest management activities such as partial and selection harvesting and seed-tree systems may diversify standard landscape-level age structures and benefit wildlife, hasten the onset of old-growth, and create desired stand age structures. We also recommend that the determination of old-growth using the following criteria in the boreal forest: 1) canopy breakdown of pioneering cohort is complete and stand is dominated by later successional tree species, and 2) stand age structure is bimodal, with dominating canopy trees that fall within a relatively narrow range of age and height classes and a significant amount of understory regeneration.
Highlights
Forest stands have long been described by their age structure and diameter distribution
The younger cohort was composed of mainly balsam fir and black spruce which recruited in the understory at various times after the standreplacing fire
The age structure in the canopy transition stage of stand development was similar to the stem exclusion/canopy transition stage with a bimodal age structure as well (Figure 1(b))
Summary
Forest stands have long been described by their age structure and diameter distribution. Ecosystem dynamics in the Canadian boreal forest are closely tied to natural fire regimes. Intensity and time since fire (TSF) are fundamental to forest species composition, age structure and forest succession [1,2,3]. Age structures of natural forest stands change over time [4,5]. Research has found that forest stands change from an even-aged, relatively homogeneous tree height structure to a two-cohorts, bimodal height structure to where tree heights are relatively heterogeneous as TSF increases [5,6,7,8]. As stand age increases, stands become increasingly composed of later successional species that can establish under a closed canopy with limited light
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