Gap Dynamics Following Forest Decline: A Case Study of Red Spruce Forests

Abstract

Forest decline is perceived as a threat to forest health in many regions of the world. The decline of red spruce in the mountains of the northeastern United States is one well-known example. We evaluated the impact of spruce decline by considering its effect on the prevailing gap dynamics. We compared plant composition, height growth, and indices of resource availability in gaps to the forest as a whole in four old-growth stands. In our study, there were no appreciable compositional differences between gaps and the forest as a whole. However, the relative height growth rate of saplings and seedlings was greater in gaps and was significantly different in three of the four old-growth communities studied. There were also significant differences in the response of species to gaps; birches grew faster than balsam fir which grew faster than spruce. The initial stage of vegetation recovery in the spruce–fir forest was dominated by advance regeneration of balsam fir. Much of the recovery in the hardwood–conifer transition zone will depend on the response of spruce, and therefore the closure rate of gaps will be constrained by the slow growth of spruce advance regeneration. In both forest types, the shortage of mineral soil seedbed restricted birch establishment and limited its role in vegetation recovery. Throughout the subalpine community, a large fraction of the potential radiation reached the understory regardless of canopy status (means ranged from 19% to 32% of potential radiation). In fact, gaps received less light on average than random points in the understory. Judging from indices of fine root growth, belowground resources were enriched in gaps relative to the forest as a whole. The lack of quantitative light gaps and the higher fine-root density in gaps suggest that the observed response of the trees resulted from changes in belowground resources or qualitative changes in the light regime. The phenomenology of gap formation influenced the gap environment and consequently the course and rate of recovery. Many red spruce trees died standing. These standing dead trees continued to intercept an important fraction of available light. Damage to understory plants and disruption of the forest floor were minimal. These aspects of the gap regime favored recovery via upgrowth of advance regeneration rather than the establishment and growth of seedlings. General recommendations when evaluating other instances of decline include: (1) assess the impact of decline relative to the existing disturbance regime; (2) consider the influence of the phenomenology of decline on the recovery process; (3) recognize that vegetation recovery also depends on the inherent community organization of the affected forests.