A postglacial palaeoecological record from the San Juan Mountains of Colorado USA: Fire, climate and vegetation history

Continuous sediment, charcoal and pollen records were developed from a ∼4.5 m sediment core from Little Molas Lake (LML), 3370 m elevation, San Juan County, CO. LML formed by 11 200 cal. BP subsequent to glacial retreat. Turbated clay and gyttja was derived from in-lake productivity and outwash sediments from the drainage basin from ∼11 200 cal. BP until ∼10 200 cal. BP. Cessation of glacial input and replacement of tundra with Picea forest correlates with the termination of the Younger Dryas and indicates warming. An increase in diploxylon pollen (cf. P. ponderosa), probably from lower elevations, reflects the influence of the southwestern monsoon c. 10 160 cal. BP. Pollen ratios indicate that Picea and other conifers persisted near the lake for the remainer of the Holocene. The driest Holocene period occurs c. 6200 to 5900 cal. BP, when lake levels were the lowest as indicated by all the proxy records. Wetter conditions during the last c. 2600 cal. BP favoured the expansion of P. edulis and P. ponderosa. Lateglacial fire events occurred on average every 65 years with a doubling of the fire return interval in the early Holocene. The former may reflect an increase in biomass for burning during a period of rapid vegetation turnover. The lowest fire event frequency occurs during the Neoglacial (after c. 4100), during a period of moister and cooler climate. The most recent pronounced peak in charcoal coincides with the historically documented AD 1879 Lime Creek Burn.