Subalpine forest mortality in the Colorado Rocky Mountains: assessing global change

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Cleveland, Jewel
Carpenter, Ansley
Davis, Ben
Edwards, Tom
Kennedy, Annie
Ramsey, Will
McDurmon, Luke
Powell, Mary Thomas
Worsham, Marshall
Kueppers, Laura
Issue Date
Scholarship Sewanee 2023 , University of the South , Forestry , Global Change , Mortality
The Colorado river provides water directly to >40 million Americans in the southwest, and indirectly to millions more, including Sewanee students, through winter agriculture. The Colorado River obtains as much as 75% of its water from snow melt, for which the upper elevation subalpine forests act as an important buffer to variation in precipitation. The summer of 2022 marked Sewanee’s first field campaign to the Rocky Mountain Biological Laboratory in Gothic, Colorado. RMBL is located at an elevation of 9,500 feet, making it an optimal location for cataloging the interannual dynamics and the environmental controls regulating the distribution of subalpine forests. We surveyed 12 40x40 m pre-existing forest plots to assess mortality and forest structure. These sites spanned across several gradients within the East River Watershed, Colorado, including aspect, slope, radiation load, topographic wetness index (TWI) and elevation. The primary tree species in this area are subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), and lodgepole pine (Pinus contorta). We recorded the species, height, diameter at breast height (dbh), crown illumination index (CII), and health status of 5876 trees across all plots. We constructed correlations between these data and the associated topographic variables of each plot. Tree mortality significantly (p<0.01) increased with TWI . Stocking density significantly decreased with an increase in hill slope angle. These findings suggest a relationship between TWI and mortality and between stocking density and hill slope angle, but further data must be collected in order to further support these conclusions. These results are crucial as we face future global climate change, since increased natural disasters and other pressures will affect future tree mortality. This ongoing work will continue to quantify the mortality of subalpine forest along different gradients in order to assess the long term stability of forests within the Colorado River Watershed.