Year of Award

2013

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Forestry

Other Degree Name/Area of Focus

Ecosystem and Conservation Science

Department or School/College

College of Forestry and Conservation

Committee Chair

Ron Wakimoto

Commitee Members

Andrew Larson, Heiko Langner, Peter R. Robichaud, Stefan H. Doerr

Publisher

University of Montana

Subject Categories

Forest Sciences

Abstract

Within the Rocky Mountain region of the Western U.S.A the average number of wildfires over 40 hectares have quadrupled and the frequency of large wildfires (> 4,000 hectares) have increased seven times since the 1970’s. The effect of this increase on resource management and ecosystem function is of increased interest to agencies and land managers within the region. The research presented in this dissertation aimed to contribute to the growing knowledge of post-fire hydrology, specifically with regards to the role of wildfire ash within recently burned ecosystems. Ash should not be considered a generic term, as it is an important element of post fire landscapes, and should be categorized and taken into consideration when assessing post-fire ecosystems and hazards.

The main findings of this research were that saturated hydraulic conductivity of ash spans three orders of magnitude with some ash capable of decreasing an order of magnitude following initial hydration. More specifically, the hydrologic response of low- and high-combustion ash can prompt the formation of surface seals in post-fire systems by either creating a low conductive ash layer or a chemical ash crust layer. Mid-combustion ash, on the other hand, acts as a capillary barrier storing water thus explaining reported hydrologic buffering effects of ash layers. While numerous authors have previously reported the presence of an ash crust within post-fire ecosystems, this work is the first to document the formation of an in-situ ash crust within a month after wildfire activity. The formation of an ash crust decreased ash hydraulic conductivity by an order of magnitude, as well as significantly decrease ash layer bulk density and porosity. While raindrop impact increased the strength of an ash crust, raindrop impact alone is not sufficient to form an ash crust, instead mineralogical transformations must occur to produce a hydrologically important ash crust. Therefore, initial ash composition, the presence of oxides and a hydrating rainfall event are all necessary precursors for crust formation. The variations in initial ash characteristics as well as temporal alterations indicate that ash layers should be considered in modeling systems aimed at predicting post-fire infiltration response.

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© Copyright 2013 Victoria Balfour