Influence of legacy-tree retention on regeneration dynamics and harvest opening persistence in northern hardwoods

Background/Question/Methods

We examined the influence of initial opening size and legacy-tree retention on regeneration dynamics and opening persistence in a well-replicated, long-term silvicultural experiment.   Forty-nine harvest gaps with a centrally retained yellow birch (Betula alleghaniensis) and twenty closed-canopy reference sites were established during the winter of 2003/04 at the Ford Research Forest near Alberta, MI.  Opening and legacy tree attributes and regeneration response were measured in 2004/05 and 2011/12. In addition to plot sampling, the location of each yellow birch and eastern hemlock (Tsuga canadensis) in the regenerating cohort was mapped in 2012.

Results/Conclusions

Projections based on observed gap closure rates suggest that openings > 500 m² should persist long enough for mid-tolerant tree species to successfully recruit into the overstory.  Centrally located legacy trees had only a minor impact on opening longevity of these larger gaps.  Full-crown release of legacy trees resulted in minimal mortality (1.17% year ^-1) and nearly a 3-fold increase in diameter increment.  Results of a nonmetric multidimensional scaling ordination indicated that tree species composition across the wide range of opening sizes examined (200-1460 m²) was surprisingly similar, with most openings dominated by shade-tolerant maples (Acer spp.).  Yellow birch densities, however, did increase with opening size.  A plot by plot comparison of 2005 and 2012 yellow birch occurrences indicated that only 32% of the plots where yellow birch occurred in 2005 still contained yellow birch regeneration, while the number of plots with yellow birch remained virtually unchanged (14 vs. 12%).  Since inception the region has been plagued by growing season droughts which are likely responsible for the high initial mortality of yellow birch seedlings.  The complete census and mapping of yellow birch and eastern hemlock in 2012 suggests that cool southerly gap edges and microsites in the shadow of the legacy tree crown are the most conducive to the growth and survival of these species given current climatic conditions.  Our results highlight the contingent role of abiotic conditions and initial species composition on the success or failure of silvicultural treatments designed to promote tree species diversity in managed uneven-aged forests.