Young seedling dynamics depend on harvest gap size and surface disturbance

Background/Question/Methods

Increasing tree regeneration diversity may become an important management objective in decreasingly diverse northern hardwood forests.   Land managers may have to rely on natural regeneration to increase seedling diversity as economics often preclude planting seedlings. As such, a greater understanding of the ecological constraints on tree species regeneration is needed. In rich-mesic northern hardwood forest in Michigan, USA, we are conducting stand-scale experiments aimed at identifying the multiple factors constraining the transition from seed to mature seedling for multiple species.  Here we report on the impacts of harvest gap size, competing vegetation and scarification (exposure of mineral soil) on young seedling dynamics. In plots located in each of 40 harvest gaps (0-2500m²) and four unharvested areas, four sub-plots (2m²) were located per plot and assigned a 2x2 factorial of surface scarified/not scarified and non-tree competing vegetation removed (i.e. clipped)/not removed.  Five-hundred seeds of Betula alleghaniensis (hereafter Ba with similar acronyms for other species), B. papyrifera, Tsuga canadensis, and Pinus strobus were added to each sub-plot to augment the natural seed rain/seed-bank. We examined the effect of the treatments and light availability through GLM mixed models using poisson logarithmic linear regression for germination, and a binomial logistic linear regression for survival.

Results/Conclusions

First-year germination was unaffected by competing vegetation, for which % cover averaged 29% in unclipped plots. Scarification had strong positive impacts on the germination of light seeded (Bp, Yb, and Tc) but not heavier seeded species (Fraxinus americana, Acer saccharum). For Fa and As, germination was negatively affected by high light availability. First-year survival increased with increasing light for Ps, Ba, Fa and Tc, whereas Bp survival decreased with increasing light and competing vegetation density. In contrast, competing vegetation increased sugar maple survival. For second year survival, low light availability continued to negatively impact Tc and Fa, while competing vegetation continued to negatively affect Bp. Collectively, these results suggest that for this group of species, scarified medium sized harvest gaps (900m²) support relatively high seedling densities for the greatest number of species. Scarification greatly enhances light seeded species germination (Bp-16x, Ba-94x, Tc-12x greater p<.0001). Medium sized gaps provide a combination of high enough light to ameliorate low light mortality (Ps-(34), Ba-(30), Fa-(29), and Tc-(34) % increase)in survival in medium compared to small gaps (p <.05), but not so high that germination of heavier seeded species or the survival of As or Pb is strongly adversely affected.