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<br />o Weather stations
<br />used in preliminary
<br />analysis
<br />
<br />. Weather stations
<br />used in detailed
<br />analysis
<br />
<br />
<br />. Coring sites
<br />
<br />o
<br />Vallecito Dam
<br />
<br />.
<br />Durango
<br />
<br />.
<br />Pagosa
<br />o 10 Springs
<br />L...--..J Mil e s
<br />
<br />Figure 1. San Juan tree ring study sites. Map show-
<br />ing locations of sampling sites and
<br />weather recording stations.
<br />
<br />Vegetation: ground cover of grass excellent, mixed
<br />with composites; scattered shrubs in clearing; trees
<br />in clearing are aspen (mature, sapling, and seedling)
<br />and ponderosa pine (mature, sapling, and seedling);
<br />trees between clearing and ravine and trees on slope
<br />to low ridge are aspen, ponderosa pine, white fir,
<br />Ibuglas-fir, and a few Engelmann spruce.
<br />
<br />Species list of vegetation: herbaceous: Achillea
<br />lanulosa, Antennaria ~, Erigeron simplex, Geum
<br />triflorum, Helenium hoopesii, Helianthella parryi,
<br />Lupinus argenteus, Senecio sp., Stellaria jamesiana,
<br />Thermopsis divaricarpa, Vicia sp.; shrubs: Lonicera
<br />involucrata, Potentilla fruticosa, Quercus gambellii,
<br />Symphoricarpos utahensis, Rosa sp.; trees: Abies
<br />concolor, Picea engelmannii, Pinus ponderos~
<br />Populus tremuloides, Pseudotsuga menzeisii.
<br />
<br />SJS-2
<br />
<br />Geographic location: 19 miles NW of Pagosa Springs
<br />on Piedra Road (F.S. #2631) at junction with Turkey
<br />Springs Tra.il (F. s. 112629), San Juan National Forest,
<br />Archuleta County, Colorado; ele. 2725 m (8,380 ft.).
<br />
<br />Topography: level, broad ridge top; no apparent
<br />drainage patterns; soil moderate but sandy with
<br />abundant litter; no surface rocks or rock out-
<br />croppings.
<br />
<br />Vegetation: ground cover mainly composites and some
<br />herbaceous, little or no grass, shrub understory well
<br />developed and mostly gambell's oak; trees are
<br />primarily ponderosa pine (mature, sapling and
<br />seedling) with some Douglas-fir.
<br />
<br />Species list of vegetation: herbaceous: Antennaria
<br />rosea, Erigeron simplex, Helianthella parryi,
<br />Lupinus argenteus, Senecio sp.; shrubs: Potentilla
<br />fruticosa, Quercus gambellii, Rosa sp.; trees:
<br />Pinus ponderosa, Pseudotsuga m~isii.
<br />
<br />SJS-3
<br />
<br />Geographic location: 19 miles NW of Pagosa Springs
<br />on Piedra Road (F.S. #2631) at junction with Turkey
<br />Springs Trail (F.S. #2629), San Juan National
<br />Forest, Archuleta County, Colorado; ele. 2720 m
<br />(8,340 ft.).
<br />
<br />TopographYi base of wooded slope on floodplain of
<br />Roush Draw; drainage from Roush Draw and several
<br />rivulets from wQoded slope; soil good, silt deposi-
<br />tion and good humus layer; generally seepy due to
<br />drainage patterns.
<br />
<br />Vegetation: grass and other herbaceous cover very
<br />good; little or no shrubs; trees are ponderosa pine,
<br />Douglas-fir, Engelmann spruce, and aspen.
<br />
<br />Species list of vegetation: herbaceous: Achillea
<br />lartulosa, Antennaria rosea, Erigeron simplex,
<br />Geranium gunnisonii, Lupinus argenteus, Mertensia
<br />ciliata, Senecio sp., Vicia sp., Zygadenus elegans;
<br />shrubs: Quercus gambellii, Symphoricarpos utahensis,
<br />Ribes sp., Rosa sp.; trees: Picea engelmannii,
<br />Pinus ponderosa, Populus tremuloides, Pseudotsuga
<br />menzeis ii.
<br />
<br />Approximately forty cores from ten trees were
<br />composited into each site chronology. Only the last
<br />70 years of record from trees of more than 100 years
<br />of age were used so that problems due to the
<br />variation of growth layer width in young trees could
<br />be ignored. This permits the use of simple averages
<br />of annual increments rather than the more complicated
<br />standardization procedures of Fritts (1966) and
<br />Fritts et al. (1969). The master chronologies for
<br />each of the three study sites are shown in Figure 2.
<br />
<br />~. I
<br />
<br />Climatologic Data
<br />
<br />Temperature and precipitation data from stations at
<br />Durango, Lake City, Ouray, Pagosa Springs, Silverton,
<br />Telluride, and Vallecito Dam were available for use
<br />in the study (Figure 1). Because of relative
<br />proximity to the study area, the Durango, Pagosa
<br />Springs, and Vallecito Dam records were used for
<br />the majority of the climatic correlation analyses.
<br />These three climatic-stations are outside the target
<br />area. Although they reflect the regional climate of
<br />the area, effects of cloud seeding should not be
<br />expected in their weather records. Preliminary
<br />evaluation shows all three to be reliable data sets.
<br />Pagosa Springs and Vallecito Dam are about 245 m
<br />lower in elevation than the coring sites; Durango
<br />is about 490 m lower. Climatic station information
<br />is summarized in Table 1.
<br />
<br />Although the distances from weather stations to
<br />coring sites vary from 29 to 92 km, it is assumed
<br />that the trees and weather stations were in the
<br />same climatic regime prior to seeding. According
<br />to Bradley and Barry (1973), "the San Juan are~
<br />responds fairly uniformly as a region to seasonal
<br />variations in precipitation." Vallecito Dam and
<br />Pagosa Springs records exist for nearly the same
<br />period (1941-1974) and the changes in precipitation
<br />and temperature from month to month, although
<br />slightly different in magnitude for the two records,
<br />are always in the same direction. The Durango
<br />record exhibits slightly less agreement.
<br />
<br />To establish a meaningful growth function from
<br />climatic data, Fritts believes that at least 60
<br />years of data are needed (pers. corom.). Durango,
<br />although the farthest of the three climatological
<br />stations from the coring sites, has the longest
<br />record and was included for this reason.
<br />
<br />70
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