This research addresses two questions:
(1) How do riparian zone nutrient cycling and hydrologic processes vary with elevation, topography, stream order, and soils? See IIIBa
(2) What is the impact of Rhododendron removal on carbon, nutrient, and water dynamics? See IIIBb

P-IIIB1. Riparian Zone Delineation and Nutrient Cycling
Summary:  Objective criteria for delineating the hydrologic basis of riparian zones in mountainous watersheds from terrain and soil features have yet to be determined.

Riparian zones in streams in the Coweeta basin grade from narrow areas confined by steep hillslopes in headwater catchments to shallow sloped areas in the broader valley floor beyond the basin. Expansion and contraction of near-stream saturated areas has long been depicted as the primary mechanism of stream flow generation at Coweeta (Hewlett and Hibbert 1963). Bi weekly measurements along 4 piezometer transects on the headwater riparian experimental site, however, have shown a surprisingly stable phreatic surface, with all 4 upslope piezometers (1-3 m from stream) having free water surface levels varying no more than 10 cm each during 1995 (Figure 15). Soilwater zones on those steep sites are apparently uncoupled from stream water and unlikely to experience saturated conditions on time scales longer than the duration of an infiltration front from a high intensity storm. We are testing if this result applies generally to headwater catchments at Coweeta. We further hypothesize that phreatic surfaces in the riparian zone expand with stream order, becoming broader, more responsive to seasonal precipitation variation (i.e. to timescales longer than an isolated storm event), and more interactive with soil water in 5th order streams such as Coweeta Creek.

Shallow soils and steep slopes at higher elevations have a profound effect on nutrient processing. Prior work at Coweeta has shown that the dominant anion in stream water varies from SO₄ at high elevations to HCO₃ at lower elevations (Swank and Waide 1988). Recent storm studies have shown that SO₄ and SiO₂ can be partitioned to determine relative contributions of rainwater, soilwater, and groundwater within streams during storms at Coweeta (Figure 16) (Webster and Yeakley, unpublished data). We have established piezometer transect pairs from hillslope to stream in each of four 1st order watersheds (high elevation WS27 and WS36; low elevation WS2 and WS18), as well as in two 3rd order streams (Ball Creek and Shope Fork), and in two contrasting channel morphologies in the 5th order stream (Coweeta Creek) beyond the basin (16 transects total). Measurements are conducted at two frequencies, monthly and during individual storms, and include water levels and nutrient concentrations. Terrain and soil features would be analyzed using terrain analysis programs used previously (Yeakley et al. 1994). Nutrients measured will include those expected to vary either with elevation or during storms (i.e., NO₃, SO₄, HCO₃, SiO₂, Ca, K). Products from this work include quantification of near-stream saturated zone variation at Coweeta with respect to gradients in topography, soils and stream power. Results would augment an ongoing effort to parameterize models of soil moisture dynamics across the Coweeta basin (Yeakley et al., in preparation). Further this work will provide information on how near-stream saturated zone nutrient delivery dynamics vary with precipitation frequency and duration, topographic, soils and stream size in the southern Appalachians.

We also are determining the fate of precipitation borne sulfate in samples collected from riparian zones located within and outside the Coweeta basin. The potential negative impact of excess sulfate loading on streams in the surrounding forests is believed to increase if this anion becomes mobile and thus, causes leaching of divalent cations. Although mechanisms for organic S formation and sulfate adsorption are well established for forest soils in the basin, until recently, little was known of these processes in near-stream soils, or active channel and stream sediments. Samples from three transects that bisect a first order stream in Watershed 55 will be collected for the six-year grant period on a quarterly basis, and these will be assayed for the capacity to form organic S and adsorb added sulfate. This work will be carried out in conjunction with assays for organic S content, total S, and C (Table 5).

P-IIIB2. The Role of Rhododendron in Riparian Zone Function
Summary:  In both riparian zones and upper slope topographic positions, rhododendron forms dense thickets which intercept most incoming light (Clinton 1995), and inhibit regeneration of other species (Clinton et al. I994, Clinton and Vose I996), resulting in a thick litter layer.

In the early 1990's, we initiated research to test the role of Rhododendron maximum in riparian zone function. We hypothesized that riparian rhododendron thickets act as organic filters that impact riparian and stream ecosystems by:
1) Reducing light transmission.
2) Preventing terrestrial debris and soluble nutrients from reaching the stream.
3) Altering carbon and nutrient cycling processes in the forest floor.

In the previous years, we initiated a removal experiment to determine the role of rhododendron in riparian zone function (i.e. carbon, nutrient, and water cycling). We are now continuing most components of that experiment in our research today. After collecting two years of pre-treatment data (Table 5), rhododendron on the treatment hillslope (10 x 30 m section) was removed (cut and moved off-site) during August 1995. Shortly after the cutting, the control site experienced several treefalls due to Hurricane Opal. Upper slope lysimeters on the control hillslope were destroyed, but we retained operation of near-stream lysimeters and wells on the control hiIIslope, as well as lysimeters on the uncut portion of the treatment hillslope. Pre-treatment data include seasonal and annual dynamics of microbial C, N, and microbivorous nematodes (Maxwell and Coleman 1995), and we are now measuring microbial P as well. Litter mass loss, litter inputs, litter movement, in situ N mineralization (Knoepp and Swank 1995), sulfur dynamics, site hydrology, stream periphyton biomass, dissolved nutrient and organic carbon fluxes, root growth dynamics, and vegetation re-growth (treated site only) are also being measured on control and treated sites. We will continue observations on these hillslopes for three years, until the fall of 1998, to determine organic matter, nutrient and water flux responses on the treatment hillslope.

For more current information and photos, visit our slide show.

Investigators and Collaborators:
Dave Coleman
Bruce Haines 
Jennifer Knoepp
Judy Meyers
J. Alan Yeakley
Fizgerald

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