Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology & Evolutionary Biology

First Advisor

Russell K. Monson

Second Advisor

Steven K. Schmidt

Third Advisor

William D. Bowman

Fourth Advisor

Alan R. Townsend

Fifth Advisor

David J. P. Moore

Abstract

Lodgepole pine (Pinus contorta) subalpine forests are an important regional carbon sink dependent on winter precipitation. Understanding how climate variability and disturbance impact the carbon cycle and biogeochemical pools is critical for predicting future carbon sequestration. In this thesis, I investigated the impacts of mountain pine beetle (MPB) induced tree mortality and changes in spring snow depth on forest carbon balance and soil biogeochemical pools. Using parallel disturbance chronosequences of natural and simulated beetle-kill, as well as 8-year records of gross primary productivity (GPP) and respiration fluxes, I found that soil respiration sharply declines with GPP after tree mortality, reflecting the loss of autotrophic respiration and rhizodeposition. During this time, the forest soil also lost a significant fraction of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). Five to six years after tree mortality, soil respiration rates, DOC, and DON recovered before declining again, co-incident with an increase in decomposing needle litter. Generally, tree mortality caused an increase in soil moisture and a decrease in soil temperature, although these effects were confounded by stand density and time since disturbance. MPB disturbance lead to an increase in extractable soil ammonium, but not nitrate concentrations, and this accumulation was correlated with increased soil moisture availability and decreases in microbial biomass C:N. Ultimately, consideration of the dynamics of labile C supply suggest that beetle-killed forests lose less carbon to the atmosphere than previously estimated. Finally, I investigated the effects of spring snow pack depth on the autotrophic and heterotrophic components of subalpine forest soil respiration over two summer growing seasons. I found that snow additions can transiently increase growing season soil respiration rates likely through enhanced rhizodeposition, and that the autotrophic component of soil respiration may be sensitive to the effect of snow moisture availability on the photosynthetic water-use efficiency of trees. These results collectively demonstrate the significant effects of climate variability and disturbance on the mechanistic linkages between gross primary productivity and ecosystem respiration.

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