Sclerotia as indicators of mid-Holocene tree-limit altitude, Colorado Front Range, USA

Studies in the Front Range of north-central Colorado have shown that sclerotia of Cenococcum geophilum, a common ectomycorrhizal fungus, are larger beneath spruce-fir-forest and tree-island vegetation than beneath tundra grasses and herbs. The presence, in alpine-tundra soils, of C. geophilum sclerotia with median diameters that approach or exceed 1.0 mm is evidence that tree limit formerly stood higher than it does today. A sediment sample collected 116 m above tree limit on Rowe Mountain, in Rocky Mountain National Park, contained charred sclerotia with a median diameter of 1.1 mm. Wood-charcoal particles too large to have been transported by wind were associated. Radiocarbon dates of 4770 ± 25 yr BP (charred sclerotia) and 4900 ± 15 yr BP (charred spruce twig) apply to the wildfire that destroyed forest or tree-island vegetation at this locality. Lack of evidence for post-fire regeneration suggests that growing-season temperatures had already begun to decline from elevated middle Holocene levels.