David Greenland

THE BIOCLIMATES OF THE COLORADO FRONT RANGE

This paper delineates bioclimatic zones of the Colorado Front Range. It develops a methodology which might be useful for making inventories of mountain bioclimates in other parts of the world. Following a description of the vegetation and the climate of the Front Range some insight is gained into the bioclimatic systems of the area by examining the applicability of established cli- matic classifications. The main part of the paper explains procedures for distinguishing the bioclimatic zones. The variables employed in determining the zones are the ratio of growing season thawing degree days to growing season precipitation, summer mean temperature, and growing season soil moisture deficit. Aspect is examined as a possible method of determining second order bioclimatic divisions. In attempting to determine the bioclimatic divisions, the study (1) establishes the feasibility of applying existing climatic classifica- tions to the area, (2) identifies important bioclimatic variables, (3) points out the limits of present understanding of the role of aspect in the area, and (4) provides information on the possibility of extrapolating the bioclimatic zones identified in Boulder County to other parts of the Front Range and beyond.

THE SUMMER WATER BUDGET AND ITS IMPORTANCE IN THE ALPINE TUNDRA OF COLORADO

Preliminary evidence is presented from plant phenology, plant productivity and enhanced snowpack, soil bacterial populations, and hydrology and stream water chemistry to demonstrate some ways in which water availability is a limiting factor in the alpine tundra of Colorado. We also consider the hypothesis that responses to perturbations will tend to facilitate ecosystem changes toward a state commensurate with the present climate. It is demonstrated that the water budget concept is effective, and has considerable potential, in aiding the understanding of ecological conditions of the alpine tundra.

THE “AIRSHED” CONCEPT AND ITS APPLICATION IN COMPLEX TERRAIN

A review of the term “airshed” suggests that it should be defined in a functional manner. Where the concept is applied to complex terrain, the degree and frequency of coupling between valley airflows and those at higher altitudes is a key to the delineation of airshed boundaries. In cases where decoupling is very frequent, observational data of vertical temperature structure or wind velocity and direction, or both, may be used to determine the upper boundary for valley airsheds. This is demonstrated for two valleys in N.W. Colorado.

WET ACID DEPOSITION IN COLORADO

Precipitation and wet atmospheric deposition patterns were investigated for Colorado for the hydrologic year 1985. Above average precipitation for the year probably gave rise to higher than average deposition of sulfates and nitrates but this potential varied by season across the state and was higher at higher elevations. Average weekly concentrations of nitrates and sulfates were greatest during spring and summer and the average weekly pH of precipitation was lowest during the fall. Usually, average nitrate and sulfate concentrations decreased with increasing elevation while precipitation pH became more acidic with elevation. There was no clear pattern between acid concentration and increasing precipitation and elevation but precipitation pH decreased with increasing precipitation. Trajectory analysis suggested that important sources of acid precursors existed to the southwest of the state in 1985, but that areas to the northwest, west, and south, and regions within the state itself, are also likely sources.