Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology & Evolutionary Biology

First Advisor

Nichole N. Barger

Second Advisor

William D. Bowman

Third Advisor

Jeffry B. Mitton

Fourth Advisor

Tania Schoennagel

Fifth Advisor

Thomas T. Veblen

Abstract

Global climate change and altered disturbance regimes have already and are predicted to continue to cause significant shifts in vegetation distribution. Regional warming has increased tree mortality rates over the past several decades through increasing water deficits and insect outbreaks, which have dramatically changed forest and woodland structure and altered water and energy fluxes and carbon stocks. For the first part of my dissertation, I use field surveys and historical data to examine how changes in climate and recent piñon pine (Pinus edulis) mortality events may affect tree regeneration dynamics in the widely distribution piñon-juniper (Juniperus osteosperma and J. monosperma) woodlands of the southwestern USA. First, I show that piñon seed cone production declined by 40% from the 1974 decade (1969-1978) to the 2008 decade (2003-2012) in revisited stands throughout New Mexico and northwestern Oklahoma. Seed cone production was highly correlated with late summer temperatures at the time of cone initiation. Further, declines in seed cone production were greatest among populations that experienced the greatest increases in growing season temperatures, which were the populations located at the cooler, upper elevations. Second, I examine the effects of increasing temperatures and recent piñon mortality on tree recruitment and growth across the southwestern USA and determine how these effects are moderated by local climate, biotic interactions, and soil properties. In addition to changing climate and recent mortality events, large tracts of land across the western U.S. have been managed over the last century in an effort to increase forage production and timber harvesting yields, reduce the risk of wildland fires, and/or restore ecosystem structure, and these human disturbances can also dramatically alter these woodland ecosystems. The second part of my dissertation research documents the spatial extent and cost of past management treatments done within piñon-juniper woodlands of the Colorado Plateau. In addition, I determine the legacy effects of these past treatments on understory vegetation and woodland structure as well as the efficacy of current management practices at accomplishing management goals. Overall, my findings have provided important insights into how woodlands recover following disturbances under a warmer climate.

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