To learn more about insects and their roles in the ecosystem, please see US Department of Agriculture's Agriculture Research Service web site on the chemical ecology of insects.

P-IIC1. Herbivory and plant quality
Summary:  Two factors determine the densities of defoliating insects on oak trees worldwide: the timing of budburst and leaf-fall and foliar phenolics.

The phenology of oak budburst and leaf-fall influence herbivore densities among individual trees. Trees that leaf out early and drop foliage late often support the highest densities of defoliating insects (Hunter 1992). Second, concentrations of foliar phenolics influence herbivore densities among trees. High tannin amounts result in low densities of leaf-chewing insects (Hunter 1996). Tree phenology and tannin concentrations for oak insects are unstudied along elevational gradients.

Coweeta Basin is ideal for this because:
1) Oak foliage remains for four weeks longer at lower elevations than at higher elevations at Coweeta.
2) Nutrient availability (which affects foliar phenolic concentrations) is known at five elevation gradient plots, and varies among plots (Griffith 1993).
3) Canopy walkways facilitate estimating herbivore population densities, herbivory levels, and foliage chemistry (Reynolds and Crossley 1995).

Using a photographic method (Hunter, Reynolds),  to estimate budburst dates from individual trees of each important canopy tree species (Quercus rubra (red oak), Q. prinus ( Oak) and Acer rubrum (red maple). Photographs of expanding buds were taken weekly and scanned from slides onto a computer to measure bud and leaf expansion and to estimate the date of 50% leaf expansion. Budburst estimates are made at each of the five gradient plots. Measures of phenolic chemistry are made from the same trees once each month from full leaf expansion to leaf-fall. Branches are collected by a combination of shotgun sampling and collection from canopy walkways. Small disks are punched from leaves and placed in  methanol for high performance liquid chromatography (HPLC) analysis. HPLC analysis is used for simple phenolics (for tannins), depsides, and flavonoids. Sequential extraction provides an estimate of total phenolics, including tannins (Waterman and Mole 1994). Herbivore densities and herbivory are estimated monthly that are accessible from canopy walkways (methods in Hunter 1992, 1994).

P-IIC2. Herbivory and soil processes
Summary:  Although herbivory may have a dramatic effect on nutrient availability and decomposition in soils, the relationships between canopy herbivory and soil processes are poorly known in forest systems.

Canopy defoliation results in a variety of inputs into soils via insect frass, modified stem- and through-fall, and green-fall (portions of leaves dropped during defoliation by herbivores). For example, Crossley et al. (1988) reported large inputs of ammonium and phosphate to forest floors, and nitrates to a stream following an insect outbreak (Alsophila pometaria) at Coweeta. Litter arthropod diversity and abundance may increase following defoliation events (Schowalter and Sabin 1991). The elevation gradient at Coweeta provides an opportunity to study the effects of spatial heterogeneity in defoliation levels on soil microarthropod abundance and decomposition. We will collect frass in funnel traps by opening 12 traps at each gradient plot for 4 hours of daylight and 4 hours of darkness once each month from leaf expansion through leaf-fall (12 traps x 5 plots = 60 samples per month). These will be used for estimates of frass fall, and related to herbivore densities (above). An additional 12 through-fall traps, adjacent to the frass traps, will be used to assess the effects of herbivory on ammonium and phosphate concentrations in through-fall (again, correlated with herbivore densities and defoliation). The abundance and diversity of oribatid mites will be measured monthly in 12 individual 1m2 quadrats directly adjacent to the frass and through-fall traps in each gradient plot. Litter-fall traps already established at each site will be used to estimate green-fall. From 1996-1998, we gathered estimates of frass-fall, green-fall, and through-fall for each plot, as described. From 1999-2001, frass-fall, green-fall, and throughfall are manipulated experimentally into quadrats (using data from 1996-98 to establish appropriate quantities for manipulation). Quadrats receive either half or double the average input of frass (group 1), green-fall (group 2), or through-fall (group 3) and compared with controls (group 4). Each treatment (and controls) are replicated 6 times per plot. The response of oribatid mite density and diversity to experimental manipulation of herbivore-derived inputs are measured monthly in each experimental quadrat from 1999-2001 of the project. Overall, we will use correlation and analysis of variance techniques to establish the effects of natural (sampling) and experimental (manipulated) additions of herbivore-derived inputs for soil arthropod abundance and diversity.

Investigators and Collaborators:
Mark Hunter
D.A. Crossley

Previous (P-IIB) | Next (P-IID)