L-IB1. Forest disturbance and stress along an environmental gradient
Summary:  The study of forested ecosystems over a complex environmental gradient was initiated in 1991 and has continued to generate many interesting results as well as several new studies that are currently underway. The gradient has five intensive plots, established from a relatively dry oak ecosystem to a mesic high elevation northern hardwoods ecosystem, as well as 20 extensive plots providing greater spatial coverage of these ecosystems across the Coweeta basin.

Several new studies established on the gradient plots include a 15-year small log (bolt) study established by James Vose and D.A. Crossley. During the course of the study, including nine commonly transplanted species on all sites, periodic biomass sampling along with gas flux measurements were conducted. Two year results indicate surprisingly high decomposition at the high elevation northern hardwoods site, the site expected to exhibit the lowest decomposition rates. This same site exhibits unexpectedly high soil nitrogen mineralization. In a second set of studies, the area of the gradient plots was enlarged from 20 x 40 m to 80 x 80 m in an effort to map and model single and multiple tree gap dynamics. Seed rain, seed bank dynamics, seedling dynamics, and overstory survival and growth have been quantified. This last component allows for a complete analysis of all life stages of the vegetation across the complex gradient. The larger plots have also been used to map and quantify coarse woody debris on the gradient plots.

Macro and micro climatic gradients
Large differences exist in macro-climatic regimes: upper elevation plots received 24% more precipitation (Figure 4), and average annual air temperature was 5oC lower. Climate was mediated by site factors such as topography, soil water holding capacity, and vegetation characteristics (leaf area index and water use efficiency) that regulate plant water use (see growing season soil moisture, Figure 4).    
Hence, at the plot level, micro-climatic gradients did not necessarily follow macro-climatic gradients. Macro scale climatic data indicate a strong gradient in precipitation and temperature among the plots, with high elevation plots receiving more precipitation and experiencing lower air temperatures. Topography, soils, and vegetation mediate these macro-climatic factors such that plot-level micro-climatic gradients differ from macro-climatic gradients. Watershed-scale nutrient budgets and plot-level responses do not necessarily agree: e.g., at the watershed scale, N budgets indicate very similar N fluxes on high versus low elevation watersheds; however, N mineralization rates are much greater at high elevation northern hardwoods site, but this elevated N availability is not being stored in the vegetation. It appears that at the watershed scale, plot-level responses are being dampened by subsurface processes occurring in other vegetation types on the watershed.
 

Vegetation
Biomass ranges from 150 to 236 Mg ha-1yr-1 with lowest biomass in the xeric oak-pine site and highest in the cove site. Low biomass appears to reflect moisture and nutrient limitations. We have begun to identify the factors limiting recruitment of trees through analysis of seed rain, models of seed dispersal, and seedling demography. Short dispersal distances limit the availability of Quercus, Cornus, Carya, Fraxinus, Pinus rigida, and Nyssa, but not Acer, Liriodendron, Tsuga, or Betula . Seed production and dispersal appears much less limiting than are germination success due to seed predators and other factors.

A recently completed vegetation map based on over 400 permanent plots combined with continued basin-wide climate modeling enabled us to apply our plot-level data to the larger landscape. Vegetation diversity decreases as elevation increases, but is neutral to changes in landform. Productivity responds to both landform (increasing from ridge to cove) and elevation (increasing from high to low). Species composition changes predictably in response to landform and elevation-mediated environmental variation.

We identified significant differences in the physiological responses of dominant overstory tree species, including responses to light, temperature, air vapor pressure deficit, and soil moisture. Light-saturated photosynthetic rates (Pnet) ranged over two-fold (Sullivan et al., in press), indicating that species composition as determined by environmental gradients can have a considerable effect on forest productivity. Much of the variation in Pnet among species was related to leaf N (r2 = 0.6), which increases with increasing elevation within a species (Griffith 1993). This emphasizes the importance of linking C with nutrient cycles.

Litterfall along the elevational gradient ranged from 2653 to 3823 kg ha-1 yr-1. This variation was not related to elevation, basal area or productivity. Maximum leaf area index (LAI, m2/m2) generally followed litterfall mass patterns and ranged from 3.3 (xeric oak-pine) to 9.0 (low elevation mixed oak), and vertical distribution and light attenuation varied considerably among community types (Vose et al. 1995). The high LAI value reflects the contribution of Rhododendron maximum -- a major component of both mixed oak stands.

Root biomass was determined for all plots with root coring, and phenology (growth and turnover) was determined with root observation boxes. Total summer root biomass ranged from 13 to 21 Mg ha-1 with the lowest value on the cove site and highest on the high elevation mixed oak site. Root:shoot ratios generally declined with increasing average annual soil moisture.

Canopy herbivory
Herbivory was measured from 3 canopy walkways coincident with the gradient plots. Percent leaf area removed by herbivory decreased significantly as elevation increased (Reynolds 1995, Reynolds and Crossley 1995). Arthropod guild structure was significantly different for Acer rubrum at all sites, but was similar for Quercus rubra. For more research on this topic, please visit Coweeta herbivory research.

 Forest floor
Forest floor mass ranged from 9 Mg/ha in the cove site to 33 Mg/ha in the high elevation mixed oak site. Although decomposition rate varied along the gradient, the variation was unrelated to macro-climatic variation (Hoover and Crossley 1995). The lower elevation cove site had the lowest decomposition rate despite the fact that it was warmer and more moist. Microarthropod abundance was generally lowest on the northern hardwood site and highest on the high elevation mixed oak site. Microarthropod diversity is extremely high at Coweeta with over 45 oribatid mite genera and 160 species collected in a 1 m2 plot.

Soils
Soils were analyzed for most cations and anions. Variation in extractable Ca is large (29 mg/kg to 441 mg/kg); soil C ranges from 3.33 to 9.87% ; soil N ranges from 0.09 to 0.7%; and soil C:N ratios ranges from 39 to 14. Soil pH is less variable and ranges from 3.9 to 4.2. Soil N cycling processes (especially N mineralization) varied greatly among years and plots. Most notably, the northern hardwood site had extremely high N mineralization rates. In a dry year (1993), N mineralization rates decreased substantially. Soil S cycling analyses show greater S immobilization in the northern hardwood site (Stanko-Golden et al. 1992). For more on this research please see our Coweeta research on soils and insects.

Mammals
We determined patterns of distribution, diversity, and abundance of small mammals along altitudinal and vegetational gradients at Coweeta using traps (Laerm et al., 1996). Soricid (shrew) abundance is correlated with coarse woody debris, soil type, and past watershed disturbance. Two rare shrew species (Sorex palustris and S. dispar) occur at Coweeta. Post note: Co-PI Joshua Laerm suffered an untimely death in September 1997. Though Josh can never be replaced, his position in the Coweeta LTER program was filled by Ronald Pulliam, terrestrial ecologist, from the University of Georgia. For more on this research please see our Coweeta research with terrestrial systems.

L-IB2. Succession in canopy gaps
In the southern Appalachians, formation of small (<300 m2) canopy gaps is the primary mechanism by which forest structure, composition, and diversity are maintained. A recent region-wide severe drought led to overstory tree mortality and provided an opportunity to characterize canopy gap formation (Clinton et al. 1993) and to monitor long-term successional dynamics within permanent canopy gap plots (Clinton et al. 1994). In addition, we have experimentally created canopy gaps with and without evergreen understory at both high and low elevations (Figure 2) by girdling trees after gathering 2 years of pre-treatment data. Changes in microclimate following overstory mortality have been characterized using automated instrumentation (Clinton 1995) and combined with results from seed predator exclusion fences to form the basis for understanding seedling recruitment and success. An extension of this research can be found at environmental heterogenity-tree gaps research.

L-IB3. Riparian zone
Summary:  Our research in the riparian zones is investigating the role of riparian zones as regulators of terrestrial-aquatic linkages.

Riparian zones from critical linkages between forest and stream ecosystems are dominated by Rhododendron maximum in the southern Appalachians. Evergreen understory distribution shows little relation to elevation, slope or soils, but is correlated with aspect, distance from stream, and precipitation. All variables in the rhododendron removal project have had two years of post-treatment measurement since removal of the rhododendron understory in August 1995. In October 1995 Hurricane Opal destroyed the overstory of one of our monitoring slopes. Though our initial reaction was of frustration, project scientists were soon relieved to know that areas upslope from both the treatment and hurricane impacted slope were intact and functioning as control locations. Soil lysimeter, microclimate, decomposition, sulfur dynamics, and other variable collections were continued and provided a rare situation of both a hurricane and treatment comparison complete with two years of pre-disturbance data. In reaction to the disturbances, biomass felled by hurricane Opal was quantified and seedling monitoring studies were initiated in April 1997.

Initial results suggest that the uprooting of vegetation by hurricane Opal in comparison to the chain saw removal of the rhododendron understory resulted in differing seed bed conditions favoring tulip poplar (Liriodendron tulipifera) seedlings in the rhododendron removal and sweet birch (Betula lenta) in the hurricane impacted site.

Several investigators established forest gap plots resulting from hurricane Opal. This progression to more and widespread plots will allow for the investigation of gap dynamics across a larger geographic area and elevational gradient of the Coweeta Basin.  Results also showed that in the two years following the hurricane and Rhododendron removal treatments, soilwater nutrient concentrations on the vegetation cut hillslope generally did not vary significantly, although a small increase in NO₃-N was seen in one plot on the vegetation cut slope. In contrast, nutrient concentrations on the storm impact hillslope showed marked changes. NO₃-N concentrations showed consistent increases of at least two orders in magnitude in all lysimeters on the storm impact hillslope. Marked and persistent changes were also seen in SO₄ (decrease), Ca (increase) and Mg (increase) in the soilwater. In groundwater, SO₄ showed no differential response following the vegetation removal and hurricane events. For other nutrients (NO₃-N, Ca and Mg), however, responses in groundwater were similar, although of lesser magnitude, to soilwater. Nutrient concentrations varied seasonally, with major changes occurring in summer and early autumn in both soilwater and groundwater.

Experimental rhododendron removal
Current analyses of aerial photographs have shown that rhododendron coverage has increased over the past 20 years. Over a 17-year period (1976-93) area of rhododendron increased by nearly 15% (66 ha) (Dobbs 1995).  We hypothesize that extensive near-stream thickens of rhododendron exert control on hill-slope export of carbon and nutrients to streams. By manipulating vegetation  we determined the effect of removal of the streamside rhododendron on the export of hill-slope nutrients (K, Na, Ca, Mg, N, P, S) and organic matter. Experimental hill-slope transects that follow topographical flowpaths from a local highpoint to the stream are instrumented with lysimeters at two depths. We developed a model of hill-slope water movement that is linked with the lysimeter data to determine nutrient and organic matter flux throughout the riparian zone (Yeakley et al. 1994).

No significant differences have been observed in processing of precipitation-borne sulfate in riparian soils with vs. without rhododendron. Added sulfate was largely immobilized by physicochemical adsorption, although longer term storage was as organic sulfer. During most seasons, ester sulfate was the major organic form synthesized, but immediately after leaf fall, sulfonate-S formation dominated as a consequence of increased fungal activity.

We are also monitoring soil microbial communities on reference and treatment hillslopes. In both areas microbial biomass peaked in December and in April prior to experimental removal (Maxwell and Coleman 1995). Omnivore-predator populations of nematodes were maximal in December and June and were positively correlated with microbial biomass N.

In addition, a study by Barton Clinton and Erik Nilsen looked at the mechanisms for rhododendron impact besides shade. Preliminary results show that the effect of litter decomposition and mychorhizal associations favor rhododendron rather than hardwood tree seedlings. An array of seedlings were established in rhododendron thickets and physiological measurements were conducted in the summer of 1997. Results showed that the impact of rhododendron was highly detrimental to seedling establishment and growth.

For more information on this research check out riparian research.

L-IB4. Streams
Summary:  We examined the role of macrobiota in structuring the benthic communities of two low-order southern Appalachian streams, one draining intact forest (Ball Creek) and one draining pasture (Jones Creek).

Fishes and crayfishes were excluded from areas of both streams using an electric exclusion technique; chlorophyll a, ash free dry mass (AFDM), and invertebrates were sampled over a 40-day period. In both streams, chlorophyll a and AFDM were higher in exclusion than control areas, although these trends were not consistently significant across all sampling dates. In Jones Creek, significantly more large (> 4 mm) aquatic insect larvae were found in exclusion than control areas, most likely due to exclusion treatments providing a refuge from macrobiotic predators. This refuge effect was also evident in Ball Creek, where exclusion treatments contained significantly more filterers. Results indicate that macrobiota influence the structure of southern Appalachian benthic communities by decreasing the amount of organic matter (algal and detrital) available for other consumers and by preferentially preying on certain sizes and taxa of invertebrates. Compared to some low-order tropical streams, however, macrobiotic influences are low. Weaker effects may be attributed to decreased abundance of macrobiota and increased influence of benthic insects in southern Appalachian streams.

Our Coweeta Basin stream ecology research is highlighted by two major publications. The first is a publication in Science by Wallace et al. 1997 regarding a litter exclusion project. The leaf litter was excluded from the first 200 m of a headwater stream which led to a reduction in standing stock of benthic organic matter, a shift in the resource base of the food web, less secondary production of benthic invertebrates, and increased periphyton biomass. The periphyton increase was probably a result of decreased nutrient competition with heterotrophic microbes and less shading by leaf litter on the stream bottom.

The second major publication was in Ecological Monographs by Grossman et al. (in press). They examined a variety of assemblage-level characteristics in Coweeta Creek between 1984-1992. This period encompassed both a major drought (1985-1988) as well as three years with extremely high flows (1989, 1990, 1992).  Species richness (total = 16) was significantly higher in drought periods. Assemblage structure samples clustered on the basis of hydrologic period rather than season or year. Variations in the abundance of potential competitors or stonefly larvae predators did not produce strong shifts in microhabitat use by assemblage member. In conclusion, results indicate that variability in flow had a much stronger effect on the structure, stability and use of spatial resources than either interspecific competition for space or predation. 

For more research on this research check out aquatic communities.

To view all citations from past research web page, please see our past references page.

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