Differential effects of understory and overstory gaps on tree regeneration1

Journal of the Torrey Botanical Society, Jan/Feb 2008 by Beckage, Brian, Kloeppel, Brian D, Yeakley, J Alan, Taylor, Sharon F, Coleman, David C

BECKAGE, B. (Department of Plant Biology, University of Vermont, Marsh Life Science Building, Burlington, VT, USA 05405), B. D. KLOEPPEL (Odum School of Ecology, University of Georgia, Coweeta Hydrologic Laboratory, 3160 Coweeta Lab Road, Otto, North Carolina USA 28763), J. A. YEAKLEY (Environmental Science and Management, Portland State University, Portland, Oregon, USA 97207), S. F. TAYLOR (Odum School of Ecology, University of Georgia, Coweeta Hydrologic Laboratory, 3160 Coweeta Lab Road, Otto, North Carolina USA 28763), AND D. C. COLEMAN (Odum School of Ecology, University of Georgia, Athens, Georgia, USA 30602). Differential effects of understory and overstory gaps on tree regeneration. J. Torrey Bot. Soc. 135: 1-11. 2008.-Gaps in the forest canopy can increase the diversity of tree regeneration. Understory shrubs also compete with tree seedlings for limited resources and may depress tree recruitment. We compared effects of shrub removal and canopy windthrow gaps on seedling recruitment and understory resource levels. Shrub removal, with the canopy left intact, was associated with increased levels of understory light and soil moisture and coincided with increased species richness and diversity of tree regeneration compared to both control plots and canopy gaps. Canopy windthrow gaps, however, resulted in a more than 500 fold increase in soil nitrate concentrations, and seedling growth rates that were twice as high as that observed with shrub removal. Our results suggest that gaps in the understory shrub layer and the overstory canopy may have complementary effects on resource availability with corresponding benefits to seedling establishment and growth.

Key words: canopy gaps, Rhododendron, shrubs, species richness, tree seedling, understory gaps.

Low light levels beneath the forest canopy can limit tree regeneration to species that are tolerant of deep shade (e.g., Canham 1988, Pacala et al. 1994). The diversity of tree species observed in the forest overstory, which includes shade intolerant species, may result from elevated levels of light and other resources that are found beneath gaps in the forest canopy (e.g., Pickett and White 1985, Whitmore 1989). Canopy gaps result from disturbances such as windstorms, drought, insect outbreaks, or pathogens that remove overstory trees, creating openings in the forest canopy. Canopy gaps can provide recruitment opportunities for tree seedlings, increasing the diversity of tree regeneration, and have figured prominently in empirical and theoretical investigations of mechanisms that promote forest diversity (e.g., Shugart 1984, Platt and Strong 1989, Busing and White 1997). The empirical evidence supporting the role of gaps in forest regeneration, however, has been equivocal with studies both supporting (Barden 1980, Runkle 1981, Kneeshaw and Bergeron 1998, Lusk and Smith 1998) and questioning the importance of canopy gaps in promoting the diversity of the forest overstory (Brewer and Merritt 1978, Hibbs 1982, Cho and Boerner 1991, Hubbell et al. 1999, Webb and Scanga 2001). Ambiguous evidence in support of the role of gaps in forest regeneration may result from differences in the density of understory vegetation as well as from variability in gap characteristics such as their size and mode of creation (Putz et al. 1983, Canham et al. 1990, Beckage et al. 2000).

Understory vegetation can mediate tree regeneration in forests through their influence on resource levels at the forest floor (Connell et al. 1997). Dense forest understories compete with tree seedlings for resources, limiting tree recruitment (Lorimer et al. 1994, Baker and Van Lear 1998, George and Bazzaz 1999, Beckage et al. 2000). Understory shrubs also usurp resources made available by canopy gaps, reducing tree regeneration (Phillips and Murdy 1985, Nakashizuka 1989, Clinton et al. 1994, Beckage et al. 2000). Understory gaps may be functionally similar to canopy gaps, elevating resource levels at the forest floor and increasing tree regeneration (Connell et al. 1997), but we are aware of few studies that compare canopy and understory gap effects on both abiotic conditions and tree regeneration (e.g., Pecot et al. 2007). We used the serendipitous occurrence of windthrown canopy trees within an existing shrub removal experiment to compare the effects of understory and canopy gaps on the species richness and diversity of tree regeneration in a temperate deciduous forest. The windthrow occurred within a control plot adjacent to a treatment plot where the understory shrub Rhododendron maximum L. had just been removed. Soil moisture and nutrient levels had been monitored for the prior year in both plots providing a unique opportunity to relate seedling recruitment to environmental responses. We subsequently established two additional control plots and monitored seedling recruitment over four years while continuing to monitor nutrients and soil moisture and also measuring light levels. While the unplanned nature of this experiment limited us to an unreplicated study, our results are suggestive of differential responses to these two modes of disturbance (overstory windthrow vs. shrub removal) and are reported in order to be subject to further exploration and testing.