Treeline Responses to Climate Change in High-Elevation Landscapes of Western Montana, U.S.A.

Author

David F. Mann, University of Tennessee - Knoxville

Date of Award

5-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geography

Major Professor

Henri D. Grissino-Mayer

Committee Members

Kenneth H. Orvis, Sally P. Horn, Timothy G. Rials

Abstract

The purpose of this research was to use whitebark pine trees at three major sites in western Montana to: (1) determine tree species response to climate, (2) reconstruct past climate conditions, (3) determine the effects of climate shifts on treeline, and (4) reconstruct fire history from fire-scar data. I collected samples from whitebark pine and subalpine fir and from remnant whitebark pine in the western Beaverhead-Deerlodge and Lolo National Forests.

In the climate response analysis, the Palmer Drought Severity Index (PDSI) had the highest correlations with whitebark pine growth. The strongest relationship occurred in the previous year’s June and July. Precipitation in the previous year’s May and June was also positively related to growth.

I reconstructed the previous year’s June and July PDSI at all sites using a transfer function with tree-ring indices as the independent variable. The most intense drought year since 728 occurred in 1468. The reconstruction showed no evidence of a shift in the intensity and duration of wet and drought periods between the Medieval Warm Period and Little Ice Age.

To evaluate possible effects of increased global temperatures since the beginning of the 19^th century on tree growth at high-elevation sites in western Montana, I established plots at 10 sites. I examined the establishment dates of all stems in each plot. Additionally, I examined the spatial relationships with establishment dates using a nearest neighbor statistic. Movement of treeline upslope was seen at the lower and upper elevation plots, while the mid-elevation plots remained stable. The greatest degree of movement (150 m) at treeline occurred in the 1980s.

Fire frequency and fire seasonality varied over time, reflecting the influence of climatic conditions. I collected 26 fire-scarred samples from a single site in the Gravely Range. The reconstruction of fire history revealed that fire was most frequent at the Gravely Range site during the Medieval Warm Period, but became less frequent during the Little Ice Age. Fire could possible play a role in the stand dynamics of the whitebark pine/subalpine fir ecosystem and limit the recruitment of whitebark pine.

Recommended Citation

Mann, David F., "Treeline Responses to Climate Change in High-Elevation Landscapes of Western Montana, U.S.A.. " PhD diss., University of Tennessee, 2008.
https://trace.tennessee.edu/utk_graddiss/361