Canopy gap size influences understory dynamics of shrubs and trees over 12 years

Background/Question/Methods

Canopy gaps are important to maintaining tree regeneration and diversity of forests.  The resource and microclimatic gradients created by gaps are necessary for canopy tree recruitment but these gradients can be altered by interactions with a variety of factors including shrubs and deer.  Over a 12 year period, we investigated variation in 1) naturally regenerated shrub and juvenile tree abundance and 2) planted seedling survival, across a gradient of gap sizes (6-46m in diameter) in a 60 year old, northern hardwood forest in northeast Wisconsin.  We hypothesized that small and intermediate gap sizes would increase juvenile density, size and survival of tree species by greater resource availability but that in large gaps, this effect would be eliminated by enhanced dominance by shrub species that out compete tree juveniles for resources.  In 1995, six gap treatments were created by timber harvest.  We surveyed abundance of natural shrubs and juvenile trees between 1994 and 2007.  In 1997, white pine (Pinus strobus), red oak (Quercus rubra) and yellow birch (Betula alleghaniensis) and hemlock (Tsuga canadensis) were planted in all gaps and surveyed for survival in 1998, 2000, and 2007. 

Results/Conclusions

Twelve growing seasons after harvest, woody structure varied strongly between across gap sizes.  Small gaps (<10 m diameter) were dominated by seedlings and saplings, particularly shade-tolerant species (e.g.Acer saccharum).  Large gaps (>10 m diameter) were dominated by shrubs such as raspberry (Rubus idaeus). White pine had the highest survival rate of any of the planted species with its maximum survival rate (25 ± 8 % (mean ± se)) in the largest gap size.  The largest gaps had the best survival for all species.  Hemlock initially showed survival trends inversely related to gap size, but, 12 years later, hemlock survival was the similar regardless of gap size (7-12% survival across all gap sizes). Yellow birch and red oak had lower survival overall than the conifers.  In large, but not small gaps, shrub layer competition appears to be depressing abundance of naturally regenerated shade tolerant tree seedlings, supporting our hypothesis.  However, mid-tolerant species that were planted had somewhat greater survival in larger gaps, suggesting competitive effects of shrubs are both gap-size and tree species dependent.