Graduate Thesis Or Dissertation
 

Long-term Fire Effects on Plant Succession and Exotic Weeds in Protected Area Sagebrush Steppe, John Day Fossil Beds National Monument, Oregon

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  • The historical function of fire in sagebrush steppe has been altered by 19th century overgrazing, active fire suppression, annual grass invasion and encroachment of woody species. The significance of fire to these systems is known, but research on long-term fire effects are limited. The Sheep Rock management unit of the John Day Fossil Beds National Monument, Oregon offers an opportunity to study long-term (15 year) responses of Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) communities to fire. Using generalized linear mixed models, this study examined responses of A. tridentata ssp. wyomingensis, Bromus tectorum (cheatgrass), Gutierrezia sarothrae (broom snakeweed), Poa secunda (Sandberg bluegrass), Pseudoroegneria spicata (bluebunch wheatgrass) and Taeniatherum caput-medusae (medusahead), along gradients of community type and topography through time post-fire. Community types were differentiated between A. tridentata ssp. wyomingensis dominant plots (brush plots) and Juniperus occidentalis (western juniper) dominant plots (forest plots). Cover of B. tectorum was greatest in brush plots. Bromus tectorum cover increased dramatically 2 years post-burn. At 5 years post-burn cover of B. tectorum was 135% of pre-burn cover in brush plots and 301% of pre-burn cover in forest plots. Analysis determined abundance of T. caput-medusae was not enough pre-burn to analyze fire effects. Examination of mean T. caput-medusae cover, in plots through time, suggests it is expanding within the Sheep Rock management unit. Pseudoroegneria spicata is more abundant in forest plots than in brush plots. In forest plots, P. spicata cover decreased by 49% 1 year post-burn, but recovered by 5 years post-burn. In brush plots, cover of P. spicata declined, but not significantly through time post-fire. On northern exposures recovery of P. spicata cover occurred between 1 and 2 years post-burn, whereas on southern exposures recovery occurred between 2 and 5 years post-burn. The cover of P. secunda did not show a dramatic response to fire through time post-burn. Fire reduced A. tridentata ssp. wyomingensis cover in brush plots by 99% and 100% on northern and southern exposures, respectively. In forest plots, fire reduced A. tridentata ssp. wyomingensis cover by 86% and 47% on northern and southern exposures, respectively. Similarly, A. tridentata ssp. wyomingensis density in brush plots declined by 96% on both northern and southern exposures 1 year post-fire. In forest plots fire reduced A. tridentata ssp. wyomingensis density by 71% on northern exposures and 0% on southern exposures. The cover and density of A. tridentata ssp. wyomingensis did not change significantly through time after an initial decrease 1 year post-burn, except for density in forest plots, which didn’t show significant change through time. Artemisia tridentata is a long-lived species and therefore continuation of this study is required to gain a fuller perspective on its long term response to fire. Cover of G. sarothrae declined by 92% in brush plots and by 73% in forest plots. Gutierrezia sarothrae cover returned to pre-burn cover by 5 years post burn in brush plots, but did not recover in forest plots. Topography was not identified as a significant parameter for G. sarothrae cover in response to fire. Similarly to G. sarothrae cover, G. sarothrae density decreased 1 year post-burn and recovered by 5 years post burn in brush plots, but it did not recover in forest plots. In contrast to G. sarothrae cover, topography was identified as a significant parameter for the response of G. sarothrae density. We hypothesize south slopes favor G. sarothrae establishment, while north slopes favor increased G. sarothrae size, which explains topographic significance for density, but not for cover. Results of this study can be used by land managers to consider species specific responses to fire in both the short and long-term. This information is useful when developing plans for reintroducing fire into A. tridentata ssp. wyomingensis systems or when developing post-wildfire management strategies.
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