Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Philosophy

First Advisor

Eric Tilton

Second Advisor

Holly Barnard

Third Advisor

Noah Molotch

Fourth Advisor

Greg Tucker

Fifth Advisor

Thomas Veblen

Abstract

The mountain pine beetle is killing many trees in Colorado's high-elevation forests. The thinned canopies found in impacted forests intercept less snow and transmit more radiation than canopies in living forests, altering snow accumulation and melt processes. Employing field studies and hydrologic modeling, I compared snow, forest, and meteorological properties beneath living and pine beetle-killed tree stands. By monitoring eight pairs of living and dead tree stands for two years, I observed the different hydrologic processes at work during different stages of tree mortality. During year one, all eight dead stands were in the red phase of tree death -- the trees still retained needles. Snow accumulation was the same under living and red phase stands. However, snowmelt was more rapid in red phase stands, leading to advanced snowpack depletion. Solar transmission was not higher in red phase stands, suggesting faster melt and earlier depletion were caused by accelerated needle loss that lowered the albedo of the snow surface. By year two of the monitoring study, many of the dead trees had progressed to the needle-less grey phase of tree death. Snow accumulation in grey phase stands was 15% higher than in paired living stands. Snow in grey phase stands melted more rapidly than in living stands, likely as a result of increased canopy shortwave transmission. To further investigate the effects of naturally thinned canopies on canopy snow interception, I made daily snow depth and density measurements in three living and two grey phase adjacent lodgepole pine stands for 45 days the following year. Dead stand snowpacks were both denser and deeper than those in living stands, characteristics attributable to diminished canopy snow interception. Over 10 storms, living and dead stands intercepted 41% and 18% of snowfall, respectively. Finally, to better understand the interplay between increased subcanopy snow accumulation and decreased solar shading in grey phase stands, I parameterized a widely-used hydrologic model for living and dead pine conditions in Colorado. Results suggest melt timing in these dead conditions is highly sensitive to the length of time snow remains in a tree canopy, where snow is more likely to sublimate.

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