Ground layer competition and herbivory effects on cherrybark oak (Quercus pagoda Raf.) regeneration in experimental canopy gaps1

Journal of the Torrey Botanical Society, Jul-Sep 2003 by Collins, Beverly

COLLINS, BEVERLY (Savannah River Ecology Laboratory, Aiken, SC 29802). Ground layer eompetition and hcrbivory effects on chcrrybark oak (Quercus pagoda Ruf.) regeneration in experimental canopy gaps. J. Torrey Bot. Soe. 130:147-157. 2003.-Windthrows and small-scale forest harvests generate environmental heterogeneity and dense regrowth. In such sites, interactions among environmental factors, competition with ground layer vegetation, and herbivores attracted to the disturbed area all potentially influence recruitment and early establishment of canopy tree species. I examined the interactive effects of overstory gap size, canopy openness, herb layer competition, and mammalian (deer, swamp rabbits) herbivory on emergence and seedling bank formation of cherrybark oak, Quercus pagoda Raf., in experimental gaps created by mechanized logging in a bottomland hardwood forest. Acorns were planted in exclosures (deer, swamp rabbits, deer + swamp rabbits, neither) in the center of three gaps of each of six gap sizes (7, 10, 14, 20, 29, 40 m radius) and outside the exclosures in center, edge, and forest positions. Mortality was greatest during recruitment (within the first season), and was due primarily to damage to acorns by seed predators and shade effects on seedling establishment. After three seasons, seedlings from damaged acorns in the exclosure plots were shorter than those from undamaged acorns; however, these seedlings had higher relative growth rate, which suggests that acorn damage effects decline over time. Herb layer competition and shade clue to position within a gap or gap size were associated with reduced survival or height of seedlings, both in the exclosures and in center, edge, and forest positions. Deer or rabbit herbivory had no measurable effect on seedling survival or growth over the first three seasons. Natural or created canopy gaps > 14 in radius can be foci for Q. pagoda regeneration if acorns are protected from predators; however, the dense vegetation in regenerating gaps could limit seedling height growth and multiple openings may be necessary for ascent out of the seedling bank.

Key words: canopy gaps, Quercus pagoda, bottomland hardwoods, oak regeneration.

Windthrows and small-scale harvests generate environmental heterogeneity that can mediate forest regeneration (Grubb 1977). Most obviously, these disturbances create canopy gaps and generate light variation both from the open to closed overstory and among gap sizes. Shade tolerance can mediate canopy tree regeneration over gap sizes and within gaps (Marquis 1975; Runkle 1981; Hibbs 1982; Canham 1989; Lorimer 1989), although the strength of this relationship varies among species and forests (Ashton and Larson 1996; Coates 2002; Roberts and Zhu 2002). At the ground level, heterogeneity in the soil environment or features such as fallen logs or pit-and-mound topography can influence recruitment and seedling survival (e.g., Collins and Battaglia 2002; Harrington and Blulini 2001).

Windfalls and forest harvests often generate dense regrowth (Barrett et al. 1962; Buckley et al. 1998; Perison et al. 1997), and biotic interactions in the understory could override or exacerbate plant-environment interactions. Aboveand below-ground competition with herbaceous and woody ground layer vegetation can limit plant survival or growth (Roy et al. 2000), especially for slower growing canopy species such ax oaks. Interactions with seed predators and herbivores attracted to disturbed areas can influence regeneration. seed predation and herbivory can be lower where regrowth is dense because seeds, plants, and foraging animals are less apparent (Fuchs et al. 2000; Rousset and Lepart 2000). Alternatively, seed predation and herbivory can be greater where dense understory vegetation provides browse and protection lor foraging (Schupp and Frost 1989; Johnson et al. 1995; Tilghmun 1989; Castleberry et al. 2000; Schreiner et al. 2000). A number of studies have found greater seed predation in gaps or disturbed areas than in undisturbed forest (Boman and Casper 1995; Schupp 1988a,b; Schupp and Frost 1989). Herbivores can affect regeneration directly by browsing shoots (Tilghman 1989; Holl and Quiros-Nietzen 1999; Buckley et al. 1998; Fuchs et al. 2000) or indirectly by changing vegetation composition (Tilghman 1989; Buckley et al. 1998; Meiners and Handel 2000; Liang and Seagle 2002).

Potentially complex interactions among environmental and biotic influences argue for multifactor approaches to studies of canopy tree regeneration in windfalls and forest harvests. The objective of my research was to investigate effects of ground layer competition and mammalian (deer [Odocoileus virginianus], swamp rabbits [Sylvilagus aquaticus]) herbivory on regeneration (from seeds) of a common canopy species of bottomland hardwood forests, cherrybark oak (Quercus pagoda Raf.), over a range of experimental canopy gap sizes from 7 m radius through 40 m radius.

Quercus pagoda is a later-successional or slower-growing species (Hodges, 1997) in bottomlands. Regeneration of such species involves recruitment (emergence and establishment), early survival and growth in a seedling (sensu Marks and Gardescu 1998) or sprout bank, and further growth into sapling (midstory) or subcanopy layers (Clark et al. 1999). Microenvironments, such as those in canopy gaps, that promote emergence and early establishment may not be favorable for seedling growth (Fuchs et al. 2000; Coates 2002); i.e., niches may be discordant (Schupp 1995) over the regeneration process, and multiple canopy openings, which provide pulses of resources, may be required for ascent into canopy.

First-year recruitment (1995) of Q. pagoda was compared among mammalian herbivore exclosure types (deer, rabbits, deer + rabbits, neither) and gap sizes (7 m, 10m, 14 m, 20 m, 29 m, and 40 m radius) in a bottomland hardwood forest (Pauley et al. 1996). Emergence and initial seedling establishment were strongly influenced by seed predation, which ranged from excavation with little damage to the acorn to complete destruction. Herb layer removal/addition treatments did not affect seedling establishment. By August of the first season, seedlings in gap centers were tallest in midsize gaps (14 m radius) and seedlings from undamaged acorns were taller than those from damaged acorns. The research reported here extends through the third season following gap creation to include the early seedling growth phase of Q. pagoda regeneration; the objective was to determine if the influences on recruitment in the first season persist through lime to affect seedling growth and seedling bank composition.

Methods. SPECIES DESCRIPTION. Quercus pagoda Raf., cherrybark oak, is a highly prized timber species of bottomland hardwood forests (Almquist et al. 2002; Gardiner and Hodges 1998; Kellison and Young 1997; Krinard 1990). It is a pioneer oak of the later-successional oakhickory association on better-drained or higher elevation sites in bottomlands (Hodges 1997). As with other forest species (Roberts and Zhu 2002), some shade may be optimal for Q. pagoda establishment and early seedling growth (Gardiner and Hodges 1998; Guo et al. 2001). Although it is weakly flood-tolerant (Hook 1984; Townsend 2001); and seedlings may not survive one year of continuous root flooding (Angelov et al. 1996), open-grown seedlings in drier microsites may be water stressed (Gardiner and Hodges 1998). Greater growth response to light in the second year after seedling establishment (Gardiner and Hodges 1998, Guo et al. 2001) suggests that Q. pagoda recruitment and seedling survival niches could differ. Suboptimal height growth in open environments and positive response to midstory removal (Janzen and Hodges 1985) suggests multiple canopy opening disturbances may be required for ascent into the canopy.