Recent Climatic Change and Development of the Bristlecone Pine (P. Longaeva Bailey) Krummholz Zone, Mt. Washington, Nevada

Abstract

Wood remnants above the present upper tree line are the basis for reconstruction of past changes in tree form and distribution. Living bristlecone pines (Pinus longaeva Bailey) on Mt. Washington, in the Snake Range of east-central Nevada, show progressive gradation in form from tall, erect trees in the upper forest zone to dwarfed, prostrate krummholz at the highest elevations. A similar gradient existed from 4,000 to at least 2,000 radiocarbon years ago, but the boundaries between vegetational zones based on tree stature were at least 100 m higher than today. The altitude of the krummholz zone is probably controlled by environmental factors affecting hardiness of the tree foliage. The downward shift of the krummholz zone since late Altithermal time can be attributed either to a decrease in summer temperature, an increase in summer precipitation, or both. The krummholz forms of bristlecone pine do not appear to be genetically distinct from the erect trees growing at lower elevations.