PARENT SESSION
Oral Session 8: Forest Ecology I: Dynamics and Succession.
Presiding: A Fiala and D Kashian
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room B 117.

Structure and floristic composition of selection-managed vs. ancient forest remnants in a northern hardwood forest.

Thomas, Sean^*,1, ¹ Faculty of Forestry, Toronto, ON, Canada

ABSTRACT- Selection system silviculture -- which maintains high canopy cover and a balanced distribution of tree sizes -- has increasingly been emphasized as a form of "ecologically sustainable" forestry; however, its long-term effects on structure and species composition are not well documented. Selection-managed stands ranging from 1-40 years post-harvest were compared to ancient forest remnants with no evidence of prior harvest activity in a northern hardwood forest in central Ontario, Canada. The managed stands showed reduced basal area of live trees and snags, and reduced tree species richness and diversity compared to ancient forest remnants. Relative changes in species abundance were related to species-specific light thresholds for tree regeneration: species of intermediate shade tolerance, such as Betula alleghaniensis and Tilia cordata decreased in abundance, whereas shade tolerant species such as Fagus grandifolia and Acer saccharum increased in abundance in selection-managed stands. Stands 30-40 years post harvest were more similar in structure and species composition, but had not yet converged with ancient forest remnants. Understory light levels were lower in managed stands than in ancient forest remnants on average, but peak light levels were higher in managed stands. Thus, considered as an ensemble, selection-managed stands showed greater heterogeneity in understory light levels than did ancient forest remnants. There was limited recruitment of mid-tolerant trees in either forest type, suggesting that in the absence of human disturbance, mid-tolerant species are maintained mainly by infrequent large-scale disturbances. These observations suggest that the main mechanism for reduced abundance of mid-tolerant tree species under selection management is "accelerated succession" resulting from the truncation of the life-cycle of individual trees, rather than the failure of selection management harvests to emulate natural gap-phase dynamics.

Key words: silviculture, Northern hardwood forests, forest management, tree diversity