Bulgaria's forest ecosystems (31 percent of the country's area) are considered vulnerable to dry and wet pollution deposition. Coniferous forests that cover one-third of the total forest land are particularly sensitive to pollution loads. The USDA Forest Service, Sofia University, and the Bulgarian Forest Research Institute (FRI) established a cooperative program to study a Rila Mountain forest ecosystem site, its climatology, air quality, surrounding watershed, hydrology, and soil characteristics. The cooperative program was initiated in September 1991 by upgrading an existing climatology site: Ovnarsko no.3 in the Govedartsi Valley on the north slope of the Rila Mountains. The dominant tree species surrounding the site include Scotch pine (Pinus sylvestris L.), black pine (Pinus nigra Arn.), Norway spruce (Picea abies Karst.), white fir (Abies alba Mill.), beech (Fagus sylvatica L.), and several oak species (i.e., Quercus petraea Liebl, Q. pubescens Willd., Q. conferta Kit., Q. cerris L., and Q. robur). Weekly concentrations of nitrate, sulfate, sulfur dioxide, calcium, sodium, and potassium were measured using a filter-pack technique. Data for 18 weeks between October 1991 and November 1993 were collected.
