129-36 A Geomorphic and Stratigraphic Study of Holocene Forest Dynamics in Durango, Colorado

Abstract:

Soils and paleoenvironmental research in the San Juan Mountains of Durango, Colorado reveal spatial and temporal patterns of Holocene forest change on a geomorphic template dominated by glacial end moraines and 150m-high glaciofluvial terraces. The treeless landscapes depicted in late 19th century photographs stand in stark contrast to the city’s rich, modern canopy. We use a multi-proxy approach that combines soil stratigraphy, pollen analysis, radiocarbon dating, historical research, and repeat photography to ask the following question: Do early photographs accurately reveal a semi-arid and treeless late-Holocene Durango, or was the Durango area naturally forested, clear-cut by 19th century settlers, and later reforested? Wetland cores from within city limits produced early Holocene ages of ~9570 to 8700 14C years BP with dominant herbaceous pollen species such as Artemisia, Asteraceae, and Amaranthaceae and limited forest pollen. These results suggest early-Holocene Durango was likely semi-arid and open, as opposed to forest. A buried paleowetland at the base of an Animas River terrace, also within city limits, produced ages of ~920 14C years BP with pollen showing 63% pine and spruce suggesting dominant conifer forest cover in the late Holocene. Soil and sediment profiles from Animas River cutbanks show multiple stacked AC profiles and two continuous charcoal lenses providing evidence of general instability and basin-wide forest fires in the late Holocene. Durango's 2024 tree inventory shows the modern canopy includes many introduced and ornamental species. These lines of evidence suggest that Durango's tree canopy is not simply a remnant of a lost forest, but rather a living archive shaped by Holocene climate change, disturbance, settlement, planting, and urban management. Further data are being collected to fill in the gaps, specifically between ~150 and 900 years BP. This novel study opens a new window into Durango's paleoenviroment and offers useful context for future urban forest planning in a changing climate.

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