Plant community responses to harvesting and post-harvest manipulations in a Picea-Larix-Pinus wetland with a mineral substrate

Forested wetlands in the Northern Great Lakes Region are becoming increasingly used as a timber resource. Yet, limited information is available on the effects of harvesting and post-harvest manipulations (site preparation and fertilization) on tree and ground vegetation in these wetland communities. The objective of this study was to examine production changes and species diversity in the vascular plant community four years after a forested, mineral wetland in Northern Michigan was whole-tree harvested, site prepared (bedded or trenched), and fertilized (N, P, N + P). The wetland had an original overstory of black spruce (Picea mariana), tamarack (Larix laricina), and jack pine (Pinus banksiana), with a significant cover of Sphagnum and Ericaceous shrubs. Site preparation techniques were done immediately after harvesting. The site was then planted with jack pine seedlings (1-0 stock). Fertilization occurred four years after harvesting and site preparation. Results indicate that trees in bedded areas with N fertilizer applied had significantly greater total seedling height, basal diameter and height increment when compared with those from harvest-only or trenched areas. On harvest-only areas, seedling production was greater with P and N + P fertilizers than with N fertilizer alone. Fertilizer responses were attributed to which type of site preparation (bedding versus trenching) was used and the degree of organic matter and Sphagnum incorporated into the mineral soil. Only site-preparation treatments (not fertilization treatments) had significant effects on numbers and cover of vascular plant groups (woody, herbaceous, and grass/sedge). Number of species and total cover of all vascular plants were significantly greater on the harvest-only areas than on trenched, bedded, or uncut areas. As expected, relative cover of the grass/sedge group increased with increasing site disturbance (bedded and trenched, mainly due to disturbance and lack of the thick Sphagnum mat. When compared to the adjacent uncut area, relative cover of herbaceous species was significantly reduced on treated areas, In future years, if the significant effects of manipulation treatments on tree productivity and vascular plants continue, the resulting community may be different than the successional sequence witnessed by the original forest. This will, however, depend on the rate of crown closure and the invasion of bryophyte species.