P-IID1. Effects of spatial and temporal environmental heterogeneity on stream fish assemblages
Summary:  We are measuring effects of spatial and temporal environmental heterogeneity on fish assemblage structure.

We have continued to monitor assemblage structure in the three 100m gradient sites established previously, and have extend our sampling downstream to at least one site with greater diversity (i.e. > 20 species). Fish samples have been taken once or twice yearly as per Freeman et al. (1988). Fish abundance, population structure, and physical measurements such as substratum composition, water temperature, and gage height   (see Freeman et al. 1988, and Grossman et al. 1995) have been quantified. These data are used to assess assemblage stability sensu Grossman et al. (1990) and determine the effects of temporal heterogeneity in the physical environment on assemblage stability and population structure of individual species. In addition, because several species (e. g. mottled sculpin, Cottus bairdii, longnose dace, Rhinichthys cataractae, and rainbow trout, Oncorhynchus mykiss) are present in at least two sites with differing physical characteristics (Grossman et al. 1995), we also are assessing the potential effects of spatial heterogeneity on population structure of these species. Finally, we are examining the relative importance of density-dependent and density-independent processes on population regulation in these species. This information is almost non-existent for stream fishes, especially non-game species (Grossman et al. 1990, Grossman et al. 1995).

A landscape approach is used to elucidate factors controlling the distribution and abundance of mottled sculpin in the Coweeta basin. Mottled sculpin numerically dominate many streams across northern North America. This species also has a small home range (<0.5 m2, Freeman & Stouder 1989). Although most abundant in riffles, mottled sculpin are microhabitat generalists (Grossman & Freeman 1987). In fact during and after the drought of 1985-1988, the microhabitat distribution of this species did not differ significantly from random (Grossman et al., unpublished data). Sculpin occupied patches that had prey abundances significantly higher than randomly selected patches (Petty and Grossman 1996), suggesting that prey distribution is driving microhabitat use by sculpin. We will expand this approach to broader spatial scales, to determine the relative importance of physical factors and prey abundance on the distribution and abundance of this species across reaches of Shope Fork. The availability of physical variables in reaches (as defined by Hill and Grossman 1987) are measured using the methods of Grossman & Freeman (1987) and a minimum of five benthic samples from riffles are taken. Correlations between sculpin abundances and physical parameters (e.g. depth, velocity, substratum composition, photosynthetically active radiation) across reaches, as well as prey abundance are then tested.

P-IID2. Disturbance and heterogeneity as determinants of species richness of animal assemblages
Summary:  The research  focuses on three aspects of the determinants of regional patterns of biodiversity of small mammal and salamander assemblages.

Small mammal (soricid and rodent) and amphibian communities at Coweeta and elsewhere in the southern Appalachians differ significantly in species richness, and evenness both within and between vegetational cover types (Ford et al. 1994, Laerm et al. 1996, Laerm et al., in press). Perturbation history may also influence richness and evenness.
The research  focuses on three aspects of the determinants of regional patterns of biodiversity of small mammal and salamander assemblages:

1) How do richness, and evenness vary within and between vegetational cover types ( spruce-fir, northern hardwood, cove hardwood, oak- hickory, oak-pine, and rhododendron riparian zones)?

2) How does spatial variation in habitat structure influence biodiversity? This is examined through correlation of habitat characteristics (elevation, aspect, coarse woody debris, soil moisture, soil-type, and vegetation diversity) with patterns of richness, and evenness.

3) Does perturbation history influence richness, and evenness? This is tested by comparisons among original growth, mature (80-I00 year old), mid-successional (40-60 year) and young (0-10 years) stands of several representative cover types described under 1) and 2) above. Using standardized Jollie-Siebert mark-release recapture methodology (Ford et al. 1994) we are estimating small mammal densities. Amphibians are estimated by direct observation, based on transects, and visual time-searches. Comparisons of relative abundances are based on drift-fences, using pitfall sampling.


P-IID3. Site factors, plant life-history traits, and gap studies
Summary:  This study has two goals: 1) Describe interactions between spatially-heterogeneous site factors and the life history traits of plants at different life stages that ultimately determine the ability of species to colonize and retain sites. 2) Through understanding how environmental variability and life history traits combine to determine forest assemblages, results should allow the prediction of how assemblages might change following modification in the environment by forces such global change.

Most recent explanations for coexistence of diverse forest assemblages invoke tradeoffs between the ability of plant species to colonize sites versus their ability to hold them (Tilman 1994, Pacala and Tilman 1994, Clark and Ji 1995). Both colonization and site retention will depend upon attributes of the species and physical factors at the site. Spatial variation in physical factors will interact with species attributes to determine species composition at any point in space and time.

The elevation gradient at Coweeta is an excellent resource for this study; by determining the life history stages that limit populations of key species at different locations along the gradient, using experimental approaches, such as creating canopy gaps, this study will show both where and how these species are sensitive to changes in the environment.

The three-year pretreatment phase of gap experiments was completed in 1993 and girdling of trees implemented in August/September 1994. Experimental gaps included three with and three without Rhododendron understories on both low- and high-elevation mixed oak stands, for a total of 12 gaps. Data collected on temperature, soil moisture, N mineralization, seedling censuses, seedling physiology, and tree growth rates since 1991 constitute the pretreatment baselines for experimental effects that began with the 1995 growing season. Most girdled trees did not leaf out in 1995, so responses of physical factors, N mineralization, and seedling physiology are expected to have begun in 1995/1996.  Following tree recruitment, successful colonization and the species composition that fill a gap may be determined by the species with the greatest resource use efficiency under that particular set of resource availabilities. Differences among species in resource use efficiency may play a significant role in their relative abilities to tolerate variation in the availability of key resources such as nitrogen, water, and light.

To investigate the importance of resource use efficiencies as an adaptive life history strategy, we examine the relationships among resource use efficiencies and availabilities of four understory tree species (Acer rubrum L., Quercus prinus L., Quercus coccinea Muenchh., and Quercus rubra L.) that naturally occur in these artificially created gaps. Because forest gaps can alter resource availability, the temporal and spatial patterns of gaps interact with species strategies for growth and survival. Species response to this change will vary with the magnitude, rate, and persistence of the resource change and with the life histories and resource requirements of the organisms that colonize the gaps. Pre-treatment measurements suggest that high elevation oaks have higher rates of net photosynthesis  than low elevation oaks and there is a difference among oaks within an elevation. However, if there is a difference in leaf duration between elevations, then the difference in total carbon gain over a growing season may not be as dramatic as suggested by PN data alone. In order to evaluate this total carbon gain for understory tree species it is important to understand the phenological development at both the high and low elevation sites. We  also have records important phenological development (i.e. bud swell, bud burst, leaf expansion, leaf color, and leaf abscission) of understory trees through the growing season at both elevations.

To investigate seed and seedling predation on tree recruitment, mammal exclosures were installed in gap plots in 1994. Twenty four 1 x 2 m exclosures and 12 control plots were established, in gap (3) and non-gap (3) areas of Rhododendron and non-Rhododendron locations. Exclosures were of two mesh sizes: 1) to exclude deer but not rodents (hogwire), and 2) impermeable to rodents and deer (hardware cloth). Preliminary experiments on seed predation show complete removal of Quercus in all treatments (to squirrels, because all exclosures were open at the top), heavy losses of Fagus above and below litter to insect damage, and no effects on Liriodendron. These preliminary results suggest recruitment might be strongly limited by seed predation for Quercus (squirrels) and Fagus, but not for Liriodendron. The fraction of removed Quercus that are planted elsewhere is not known, but we plan surveys to assess whether new first-year seedlings result from planted vs. surface germinants.

If canopy gaps and presence of Rhododendron are important controls of forest dynamics in the southern Appalachians, then we expect these variables to respond to canopy losses that began in 1995. Monitoring of changes in light, moisture, temperature, and mineralization provides the necessary environmental factors that contribute to those responses. Currently,  we are continuing censuses, measurements, and sampling at intervals used for the pretreatment phase. Pre- and post-treatment data are thus comparable, and comparisons with understory controls permit hypothesis tests of gap effects.

Interested in what some of these tree species look like? Visit University of Wisconsin-Madison's Botany Department Virtual Foliage web page for over 4,000 images organized taxonomically.

Investigators and Collaborators:
Jim Clark
Barry Clinton
Katherine Elliott
Gary Grossman
Joshua Laerm
Steven McNulty
Loyd Swift
Alan Yeakley

Previous (P-IIC)