Floristic Analysis of the Distribution of Ephemeral Plants in Treeline Areas of the Western United States

Abstract

Annuals and ephemeral perennials (leaves present for about 2 mon) are rare plant life forms in all treeline areas. The results of this floristic study show that they are much more abundant in the Sierra Nevada, California than in other western North American mountain ranges. Approximately one-half of the Sierran treeline annuals and ephemeral perennials occur in the montane and subalpine zones of other ranges but are absent at treeline. Information from published vegetation studies, climatic data, floras and herbarium specimens was used to hypothesize reasons for the high life-form diversity in the Sierra Nevada. Compared to the Cascade, Olympic, or Rocky mountain ranges, the summer environmental regime in the Sierra Nevada is warmer and drier, factors that favor life forms with short growth periods. Another reason for the high numbers of species possessing these life forms at treeline in the Sierra Nevada is the proximity to an additional source area. Almost one-half of the treeline annuals and ephemeral perennials are Sierran endemics. In the Sierra Nevada, species of these life forms that occur above treeline have broad altitudinal distributions, mainly extending into the montane coniferous forest zone. Most are not species from the desert as suggested by their floristic origins and by the low abundance of these high-altitude species in montane sagebrush communities as demonstrated by vegetation sampling along an altitudinal gradient.

Geographical and altitudinal distributions of annuals and ephemeral perennials therefore suggest that their relative abundance above treeline in the Sierra Nevada is due partially to an additional source of pre-adapted species with Pacific coastal affinities. It can also be attributed to unusually favorable summer conditions in the Sierran alpine zone, suggesting that the failure of montane species to survive under the more stressful environmental regime at treeline, rather than failure to arrive in the alpine zone, may explain the rareness of these life forms in most alpine areas.