Abundance of Green Tree Frogs and Insects in Artificial Canopy Gaps in a Bottomland Hardwood Forest
Horn, Scott, Hanula, James L., Ulyshen, Michael D., Kilgo, John C., The American Midland Naturalist
We found more green tree frogs (Hyla cinerea) in canopy gaps than in closed canopy forest. Of the 331 green tree frogs observed, 88% were in canopy gaps. Likewise, higher numbers and biomasses of insects were captured in the open gap habitat. Flies were the most commonly collected insect group accounting for 54% of the total capture. These data suggest that one reason green tree frogs were more abundant in canopy gaps was the increased availability of prey and that small canopy gaps provide early successional habitats that are beneficial to green tree frog populations.
Southeastern bottomland forests serve as an important source of hardwood timber, as well as critical habitat for many wildlife species. Land-clearing for agriculture since the colonization of North America has resulted in a large reduction of these wetland habitats (Dahl, 1990) and current projections estimate that, by the year 2030, hardwood timber removal will increase by 64% over 1984 levels (USDA Forest Service, 1988). Therefore, it is imperative to understand these ecosystems and to develop sustainable practices that retain the full complement of species associated with bottomland hardwoods while still generating timber revenue. Currently, clearcutting is the most common method of harvesting and regenerating bottomland forests (Clatterbuck and Meadows, 1993). It remains a valuable silvicultural practice for efficient removal of trees and provides habitat for many early successional wildlife species (Costello et al, 2000). Despite the benefits of this even-aged method, public concerns have risen about the use of clearcntting (Guldin, 1996), so alternatives are needed.
Group-selection harvesting is an uneven-aged method used to mimic small-scale disturbances within forests. This practice may be advantageous in an ecosystem management approach (Guldin, 1996). These smaller disturbances are thought to encourage species diversity by increasing sunlight, minimizing competitive exclusion and creating habitat heterogeneity (Connell, 1978; Hartshorn, 1978; Denslow, 1980; Uhl and Murphy, 1981; Sousa, 1984; Brokaw, 1985; Shure and Phillips, 1991). Changes in plant diversity and abundance are followed by corresponding changes in insect communities (Gorham et al., 1996; Waltz and Whitham, 1997). For example, Walters and Stiles (1996) found pollinator visitation rates were greater in forest canopy gaps compared to surrounding closed canopy forests. Similarly, studies have shown carabid beetle diversity and abundance increased in disturbed areas (Thompson and Alien, 1993; Beaudry et al., 1997; Heliola et al., 2001; Koivula et al., 2002; Warriner et al., 2002).
Like arthropods, many vertebrates are known to take advantage of the resources available in newly created canopy gaps. Several studies have noted certain species of birds use canopy gaps, both natural (Wilson el al., 1982; Blake and Hoppes, 1986; Martin and Karr, 1986; Schemske and Brokaw, 1981; Wunderle el al., 1987; Levey, 1988) and artificial (Kilgo et al., 1999; Moorman and Guynn, 2001), more than closed canopy forest. It is likely that populations of reptiles and amphibians are affected by group-selection harvests as well. Amphibians are an important part of most southeastern communities (Bennett et al, 1980) and are widely considered to be good indicators of environmental health and function (Phelps and Lancia, 1995). Because of their importance within ecosystems, several studies have sought to determine how forest management affects their populations. Clearcutting is a major factor altering herpetofaunal communities (Enge and Marion, 1986; Pough et al, 1987; Petranka et al, 1993; Phelps and Lancia, 1995), but Greenberg (2001) found that gaps created by wind disturbance in the southern Appalachians had little effect on terrestrial amphibian abundance relative to the surrounding intact forest.
Few studies have examined how forest management affects species such as the green tree frog (Hyla cinered) that spend little time on the ground. …