Establishment and persistence of early successional pine species in late-successional landscapes of the Great Lakes Region.

摘要

Early-successional tree species can be relatively common even where return intervals for stand replacing disturbances are long relative to the species lifespan. The mechanisms by which these species maintain a presence in such late-successional landscapes are unclear. The goal of this project was to assess potential mechanisms of stand and landscape-level persistence for early-successional tree species, in particular white pine (Pinus strobus), in hemlock-hardwood forests. Specific research objectives were to: (1) relate fine-scale distributions of early-successional tree species to potential landscape persistence strategies, (2) investigate impacts of habitat variability on establishment and persistence of pines, and (3) quantify effects of stand and site characteristics on growth and competitive success of white pines planted into hemlock-hardwood stands. To address these objectives, a broad array of data sources and analyses were applied, including analysis of species distributions in relation to habitat in 19th U.S. public land survey records, analysis of stand history and age structure in remnant pine-hemlock-hardwood stands using dendrochronological data (Including development of a methodology to account for uncertainty in age determinations), and assessment of restoration-focused planting of white pines in hemlock-hardwood forests.;Early-successional tree species in the hemlock-hardwood landscape were strongly associated with marginal habitats such as lake edges and sandy inclusions, and >70% of the landscape was within 200m of such refuge habitats (the common maximum dispersal distance for the pine species). Based on dendrochronological analysis, most pine-hemlock-hardwood stands in mesic habitats were established by severe, stand-replacing disturbances and were primarily even-aged, while those in refuge habitats had more age classes (2.8 vs. 1.7) and greater age diversity (D' 0.36 vs. 0.13) and were established by a range of disturbance severities. In general, these findings are consistent with a mechanism in which refuge habitats act as a seed source for early-successional tree species in the mesic landscape. Analysis of planting experiments suggests that reintroduction of white pine to the mesic stands may be possible, but that the success of such management may be largely dependent of control of overstory light conditions (to at least 35% light availability) and removal of pre-existing shade-tolerant understory tree layers where they exist.