Effects of Altitude on the Growth, Mortality, and Recruitment of Subalpine Tree Species in the West Elk Mountains of Colorado.

Abstract

Subalpine ecosystems in Colorado occur between 9,000-11,000ft in elevation and are composed of several species of conifers. The elevation range of each species is slightly different, but they tend to form established communities, such as the Engelmann spruce-subalpine fir community. It has been shown that many plant species adjust their range differently as the climate changes, leading to potential concerns about plant community assemblages changing. In this study, we are looking at the ways that different tree species in the subalpine habitat interact with altitude, specifically: Quaking aspen (Populus tremuloides), lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), and Rocky mountain douglas fir (Pseudotsuga menziesii var. glauca). We established five plots in the West Elk Mountains of Colorado. Each plot was 40 x 40m, and each tree was marked with a tagged number in every plot. A variety of measurements were taken for trees in 2018, and again in 2023. We used some of these measurements to calculate changes in growth, mortality, and recruitment for each of these five species at different elevations. We also used additional data from the Forest Service's Forest Inventory and Analysis database to supplement the data sample. The aim of the study is to see how these five species interact with elevation gradients. Our results did not indicate that there was a relationship between either mortality or recruitment and elevation, and there is most likely other factors more important in determining these things. However, there did seem to be a negative relationship between elevation and growth, in the form of DBH growth rate per year and basal area growth rate per year. Our FIA data analysis also confirmed this negative relationship between growth and elevation. Finally analysis of some of the FIA data shows an increase in mortality from 2012-2019 when compared to 2002-2011.

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