Early response of the herbaceous layer to harvesting in a mixed coniferous-deciduous forest in New Brunswick, Canada

We studied plant communities before and after two types of clearcutting to determine their response and their relationships to disturbance intensity and to make management recommendations. Permanent plots sampled before and for 2 years after harvesting were used to compare changes in the herbaceous layer for 2 years after harvest in three treatments: clearcutting with natural regeneration (C), clearcutting with mechanical site preparation and planting (CS) and uncut control (UC). The greatest compositional changes occurred in the CS treatment, where 16 species were lost and 20 new species were gained compared to 14 lost and 16 gained in the C treatment. The number of forest habitat species decreased 20-24% by the second year after harvest in both treated areas. Species that were lost included Cypripedium acaule, Monotropa hypopithys, Orthilia secunda, Ribes lacustre, Streptopus amplexifolius, Thelypteris noveboracensis and Vaccinium vitis-idaea. The number and cover of disturbance-adapted species increased about 250% in the CS treatment compared to 200% for the C treatment. Mean total species richness per plot, Shannon-Wiener, Simpson and evenness diversity indices were significantly greater (P < 0.05) the second year after harvest than before hat-vest in the two harvest treatments. Richness of forest species significantly declined in both treated areas, Sorensen's similarity index (each post-harvest year versus pre-harvest) increased with time in all treatments and was lowest in the CS treatment. Cover of most growth forms, especially bryophytes, decreased after harvesting in the two harvest areas, whereas small trees/shrubs and graminoids increased, particularly in the CS treatment. Cover of conifer seedlings and bryophytes greatly decreased in the C treatment where coniferous stand types dominated before harvest. Four non-native taxa (Plantago major, Ranunculus acris, Taraxacum officinale and Veronica officinalis) were encountered in the study. In 1997, total cover of non-natives was 0.2% in the CS treatment and <0.05% in the C and UC treatments. We conclude that the more dramatic changes in species composition and diversity in the CS treatment are the result of greater forest floor disturbance caused by site preparation. Protection of forest understory plants can be improved by minimizing forest floor disturbance, maintaining shade cover and leaving uncut strips or patches in areas containing species that are sensitive to harvesting. (C) 2002 Elsevier Science B.V. All rights reserved.