Chemistry of Throughfall under Douglas Fir and Rocky Mountain Juniper

Abstract

Concentrations of sodium, calcium, magnesium, potas- sium, phosphorus and nitrate were measured in throughfall under isolated douglas fir (Pseudotsuga menziesii var. glauca) and Rocky Mountain juniper (Juniperus scopulorum) trees in northern Utah for 30 storms in 1970 and 1971. Concentrations were 3-16 times greater under the trees than in the open. Throughfall under douglas fir invariably had higher concentrations than that under juniper. Since most of the chemi- cal input occurs as dry fallout between storms, surface area and form of the canopy are believed to be the prime factors influencing throughfall chemistry in this region. The atmosphere is a source of chemical inputs to terrestrial eco- systems, as well as a source of water vapor for precipitation. Chemicals may become bonded with water vapor and delivered in a dissolved form with precipitation. Chlorine and sodium, for example, are com- monly dissolved in precipitation which falls along ocean coasts (Gambell and Fisher, 1966). Or chemicals may adhere to dust particles which become temporarily suspended in the atmosphere and return to the surface as dry fallout between storms. This is more com- mon in arid and semiarid interior areas where vegetation is sparse and soils are alkaline and dry. With large surface areas of foliage exposed to wind, forest vegetation serves as a trapping device for dust particles; the sticky exudates from some species may enhance their capacity to collect dust. Rainfall which drips from the foliage (throughfall) may be substantially enriched in some chemical elements. A recent study in northern Utah provides information on the influence of isolated trees in a semiarid environment on the chemical composition of throughfall (Parent, 1972). Study area.-The study was conducted as part of a larger project to examine the effects of vegetation and substrate on stream-water chemistry (Hart et al., 1973). This study was done in the Blacksmith Fork drainage in the Bear River range of the Wasatch Mountains, about 24 km SE of Logan, Utah. The area is open, with isolated douglas fir (Pseudotsuga menziesii var. glauca) and Rocky Mountain juniper trees (Juniperus scopulorum) as dominants of the vegetation, and with lower cover consisting of big sagebrush (Artemisia tridentata), various forbs and native wheat grass (Agropyron spp). At 1700 m elevation, this area is typical of lower elevation mountain rangeland in the Intermountain region. Blacksmith Fork drainage is virtually uninhabited; there is no industrial or agricultural usage in the drainage, and road development is minimal. We believe that almost all the dust originates from the Great Salt Lake and from Cache Valley, and that it is transported to the study area by SW winds in 1911