Millar, Constance, Robert Westfall, Diane Delany, John King and Harry Alden, Climate As An Ecosystem Architect; Responses Of High-Elevation Conifers To Past Climate Variability.
Millar et al. reports on higher medieval treeline for 1) limber pine in the eastern Sierras, near the bristlecones in the White Mountains and 2) in the Wassuk Range as follows:
Whitewing Mtn and San Joaquin Ridge
A volcanic eruption of the Glass Creek vent (Inyo Craters chain, eastern Sierra Nevada) during medieval times [best estimate AD1350] buried forests in the adjacent region under several meters of tephra. Large, mostly downed, dead trees on nearby Whitewing Mtn (3052m) and San Joaquin Ridge (3105m) appear to have been killed by the eruption and preserved in arid, cold environments; tentative radiocarbon analysis (Univ AZ, Lab of Tree Ring Research, 1980) gave dates in the eruption era. The presence of these stems suggests a tall forest existed at the time the trees were alive. Conditions at this time were warm with two 150-200 year dry periods between AD 900-1350 in the Sierra Nevada (Stine 1994). The dead stems contrast with current conditions of these habitats, which are treeless or have only occasional krummholz whitebark pine. Live forests of whitebark pine downslope suggest cycles of shifting treeline in more recent centuries, during the cool centuries known as the Little Ice Age. No previous studies have examined the paleoecological, climatic, or eruptive-sequence implications of the downed logs or downslope contemporary forests”⤮
· Medieval-age (AD 900-1350) forest composition, structure, and growth on summits were typical of forests currently 300-500 m lower, suggesting an effectively warmer climate during medieval times
· Relative to present, sugar pine has been regionally extirpated and species diversity declined from 7 to 1; crown form altered from upright, straight stems to stunted krummholz, growth rates declined significantly, conditions went from forest to mostly barren
· 500 years ago (AD 1500, Little Ice Age), whitebark pine grew 100m below its current optimum; this zone has been moving upslope since, and at present whitebark is colonizing barren slopes above 2900m.
We report here a substudy on watershed-scale response of limber pine in the Wassuk Range to climate variability over the last 3500 years. Limber pine now grows only in remnant groves on mostly north-facing slopes of Mt. Grant (3415m) and scattered locations on other peaks. Dead wood, however, is abundant in all major drainages and aspects of Mt. Grant where no live trees now exist, and at elevations extending above current treeline.