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Precipitation and Throughfall Chemistry in Pinus Contorta ssp. Latifolia Ecosystems, Southeastern Wyoming

“Precipitation and Throughfall Chemistry in Pinus Contorta ssp. Latifolia Ecosystems, Southeastern Wyoming” Canadian Journal of Forest Research 18 (1988): 337-345.

Fahey, Timothy J., Joseph B. Yavitt, and Greg Joyce

STUDY AREA

Precipitation and trough fall quantity and chemistry were measured at four sites in the lodgepole pine ecosystem of the Medicine Bow Mountains, southeastern Wyoming. Information about the location and biotic features of the sites is provided in Table 1, and more complete descriptions are available in Knight et al. (1985) and Fahey and Yavitt (1988). The climate in the study area is cold temperate, with about two thirds of the average annual precipitation coming in the form of snow from October to May. Most of the midwinter snowstorms in the study area follow the zonal flow of westerly winds, but wet spring storms may arise from “upslope” conditions from the east and southeast. Summer rainstorms usually are convective and local, but air masses usually arrive from the west; large, semi-arid basins are found to the west of the study area, and several large, coal-fired power plants also are located to the west, within 200 km of the site.

METHODS

Precipitation and throughfall quantity

Rainfall magnitude was measured with static rain gauges positioned in large forest openings adjacent to four lodgepole pine stands from 1979 to 1982 and adjacent to two stands in 1983 – 1984. Measurements usually were made within 24 h of the end of each rainfall event. Throughfall quantity (net rainfall) was measured with 30 funnel collectors (15 cm diameter) at the Nash Fork stand (Table 1) for a total of 86 rainstorms during the study period.

Precipitation and throughfall chemistry

Bulk deposition was collected for several years using polypropylene funnel collectors (20 cm diameter) positioned about 1 m above the ground in large foresta openings adjacent to several of the sites. From 1979 to 1982, the Nash For, French Creek, Dry Park, and Chimney Park sites were monitored, and in 1983 – 1984, collections were made at Nash Fork and Fox Park (Table 1). Samples usually were collected within 24h of the end of a rainfall event, and the funnels and polyethylene collection bottles were rinsed thoroughly with deionized water. These collectors remained open to the atmosphere between rain events. In addition, in 1983 and 1984 shielded precipitation collectors (Aerochem Metrics Inc.) were used to sample wetfall at the Nash Fork and Fox Park sites.

Throughfall was collected with networks of ten polypropylene funnel (20 cm diametrer) collectors placed randomly in fixed positions beneath the forest canopy at the Nash Fork, Dry Park, and French Creek stands from 1979 to 1982 and the Nash Fork and Fox Park stands in 1983 – 1984. Samples usually were collected within 24h of the end of a rainfall event, and funnels and polyethylene bottles were rinsed thoroughly deionized water. For small rainfall events (less than about 5mm) samples were pooled in the field or subsequent chemical analysis.

Atmospheric chemistry

Atmospheric particulate were sampled during the summer of 1984 at the Nash Fork site using a Hi-Vol air sampler positioned about 15m above ground level in a large opening surrounded by a 20-m tall forest. A constant flow rate of 0.84 m3/ min was maintained during six 1-week runs between 28 June and 17 September, and particulates were collected on pre-weighed and pre-ashed glass-fiber filters (effective pore size, 0.45 µm). Total particulate load was measured as increase in mass following drying to constant mass at 55˚C. Each sample filter was quartered for measurement of chemical concentrations. Metal cations were extracted from the filters by shaking for 8 h in 0.1 M HCl and NH4+, NO3-, Pi, and SO42-, by shaking in 0.1 M HCl/1.0 M KCl.

Sample handling and chemical analysis

Water samples were returned to the laboratory the same day as collected and analyzed for pH by glass electrode. From 1979 to 1981, total alkalinity was estimated by titration to pH 4.5 with carefully standardized HCl (Golterman et al. 1978). From 1982 to 1984, a Gran titration to pH 3.5 was used to estimate total alkalinity (Stumn and Morgan 1981), and “nonvolatile alkalinity” was estimated by back-titration under N2 to the original pH with NaOH after purging samples of CO2. Purged samples were titrated under N2 to pH 8.3 with NaOH to provide an estimate of “nonvolatile acidity.”

Samples were then filtered through a prerinsed, glass-fiber filter on the day of collection and stored at 4 ± 2 ˚C for up to 72 h before being analyzed for NH4+ (phenol hyphochlorite method; Solorzano 1969) and NO3- (cadmium reduction; Rand 1976) on a continuos-flow analyzer (CFA, Scientific Instruments) and for orthophosphate (molybdate blue method; Murphy and Riley 1962) on a Beckman model 25 UV/VIS spectrophotometer. In 1982 and 1983 a variety of organic fractions was analyzed. Dissolved organic carbon was measured by CO2 coulometry following sealed ampule oxidation (Huffman 1976). Background leaching tests of funnels and collection bottles indicated no contamination at our levels of detection (0.2 mg/L). Colorimetric methods were used to estimate dissolved carbohydrates (Handa 1966), polyphenolics (Rand 1976), free amino acids (Satake et al. 1960), and proteins (Bradford 1976). For selected samples, an organic fractionation was performed using a liquid chromatography procedure (Leenheer and Huffman 1976). Subsamples for this analysis were stored in glass containers with minimal head space at 4˚C prior to analysis. Low molecular weight organic acids were separated by gas-liquid chromatography of both nonderivatized samples (C2 – C5 acids; DiCorcia and Samperi 1974) and trimethylsilyl derivatives (phenolic acids; Kaminsky and Muller 1977).

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