Phenology of Alpine Plants in Northern Colorado

During the summers of 1960 and 1961 ten primary phenology stations, representing variations in slope, exposure, elevation, snow cover, and vegetation cover, were established in a 300—acre study area, ranging from 10,600 to 11,200 ft elevation in the northern part of Rocky Mountain National Park, Colorado. At each station approximately 75 plants of various species were observed weekly from early June to late September. Measurements of the physical environment were recorded at least twice a week. June and July were characterized by moderate to strong winds and frequent thundershowers. Air temperature maxima were mostly between 50 degrees and 70 degrees F, and night minima were typically between 30 degrees and 50 degrees F. Soil temperatures at 15—cm depth were commonly 40 degrees—50 degrees F. Precipitation from June 10 to September 10 ranged from 3.5 to 8.9. in. in the 2 years. Initiation of new growth for many species at the relatively snow—free stations occurred in May, and as late as early August in snow—accumulation areas. Most species showed definite signs of dormancy by the middle of September. The duration of phenological phases varied considerably, but the following ranges were common: in bud, 2—5 weeks; in flower, 2—3 weeks; in fruit, 4—5 weeks. There was notable stability in phase duration for a given species, even at quite different sites. Snow cover is considered to be the primary factor influencing phenology. Possible correlations of penology with environmental data are suggested, including the relationship of moisture uptake and consequent development to soil temperature.