Pollen Analyses of Surface Materials from the Southern San Juan Mountains, Colorado

Abstract

Information about modern pollen deposition is needed to interpret fluctuations in the postglacial pollen record in mountain regions. Thirty-five samples of surficial water-laid sediment and moss were collected in the vicinity of Durango and Silverton, Colorado, from sites ranging between 6850 and 12,600 feet above sea level. The regional differences in pollen concentration are not clearly shown when, as is the common practice, the pollen taxa are expressed as percentages of the total pollen at each site. Various site factors can obscure existing trends. Pollen concentrations in surface samples are most easily compared by examining ratios determined by the frequencies of only two pollen types in the sample. The effect of other pollen is thus excluded. Three ratios are examined utilizing combinations of pine (Pinus), spruce (Picea), oak (Quercus), and sage-brush or wormwood (Artemisia) pollen. These taxa are easily identified and comprise the bulk of the pollen in the samples. The ratios Picea/Pinus, Artemisia/Quercus, and Pinus/Quercus exhibit linear trends when plotted against elevation on semilogarithmic paper; their relations to elevation can be approximated by exponential functions. Several factors affect the value of a ratio. For example, the Picea/Pinus ratio depends upon: (1) elevation (and the control that the moisture and temperature regime exerts on vegetation); (2) density of the surrounding forests; (3) size of the basin in which the pollen accumulates; and (4) statistical variation associated with random counting errors. The highest values of Artemisia pollen, released late in the flowering season, were found in sediments of cirque lakes above timber line. The cirque lakes remain frozen until midsummer, overflow with meltwater until late in the pollination season, and then become standing bodies of water until the fall freeze. The cirque lake sediments are deficient in early-season pollen. The logarithm of certain pollen ratios may be correlated with elevation because environmental factors associated with elevation control the distribution of the plants in mountain regions. If the elevations of the vegetation zones varied in the past because of different climatic conditions, the ratios of the taxa at any one site would be expected to change and indicate an “apparent elevation” different from the present. Study of the apparent changes in elevation of a site in the Animas Valley region, based on variations in the pollen ratios of its post-glacial sediments, may provide evidence of past climate fluctuation.