Miller et al.  studied fossil evidence of forest levels in 9 locations in the western U.S. over the past 3500 years, including Whitewing Mountain and San Joaquin Ridge, Inyo Craters Chain in the eastern Sierra Nevadas, near the bristlecones of the White Mountains (about which I’m going to post an interesting graphic on their altitude changes), reporting that:
A volcanic eruption of the Glass Creek vent (Inyo Craters chain, eastern Sierra Nevada) during medieval times 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 300-500m lower, suggesting an effectively warmer climate during medieval times
Figure 1. Subfossil log on Whitewing Mountain.
The above is from a poster Miller et al. . It discusses some other interesting sites, which I’ll mention separately in the future. The location of the sites is shown in the maps below (I apologize for the scale – I’ve blown up the original pdf, but it’s still not easy. Inyo County is on the Nevada border in eastern California. The AD1350 Glass Vent eruption looks like it was a big deal in regional geology and is reported on by geologists. You can find informaiton by googling and I’ll try to post up some info some time. (It’s nice to get a little geological hook every now and again.)
Figure 2. Location Maps of Whitewing Mtn and San Joaquin Ridge, eastern California
Miller et al. go on to say:
“⠠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. Alternating zonal decline and increase may reflect response to Little Ice Age cool/dry climates and then warming of the 20th century.
“⠠Best estimate Glass Creek Vent eruption: AD 1350 (~650 years ago)
“⠠Vegetation trajectories in this area likely influenced by combined climate change and volcanic effects
They provide a couple of interesting graphics showing the ages of trees at these sites:
Figure 3. Left: Species identified from summit dead wood (currently krummholz P. albicaulis, rarely P. flexilis): Pinus albicaulis (37), P. monticola (20), P. flexilis (7), P. contorta (7), P. jeffreyi (6), P. lambertiana (2) (spp currently exotic east of crest), Tsuga mertensiana (1). Right: Date Ranges of Whitebark Pine Downslope of Whitewing Mtn: 1. Upper forest border (2900m) live whitebark pine invading current barren uplands. 2. Mid-slope (2800m) live whitebark pine in current western white pine, whitebark pine, and red fir forests. 3. Lower-elevation (2700m) dead whitebark pine stems in current lodgepole pine and red fir (Abies magnifica) forests.
They conclude that:
Climate variability as a primary driver of ecological shifts at these scales has not been fully recognized as a significant natural force of change, nor have implications been integrated into conservation analysis and planning.
http://www.x-cd.com/mcss04/papers/P47.pdf Miller, Constance et al. , Climate as an ecosystem: decadal scale variability and century scale variability.