Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Department or School/College

College of Forestry and Conservation

Committee Chair

Philip Higuera

Commitee Members

Kimberley Davis, Solomon Dobrowski, Anna Sala

Keywords

Pinus ponderosa, Pseudotsuga menziesii, Rocky Mountains, tree regeneration, growth-climate relationships, climate change

Abstract

Widespread changes in climate and disturbance regimes, including prolonged drought and increases in the size and frequency of wildfires, have raised concerns regarding forest resilience to environmental change. Dry mixed-conifer forests have persisted for centuries under mixed-severity fire regimes; however, climatically driven increases in the frequency of large wildfires in recent decades may lead to increased tree mortality and declines in post-fire tree regeneration. Climatic warming and increased drought may also impact tree growth, with implications for the carbon cycle. Lower-treeline forests near the edge of their climatic tolerance may be particularly vulnerable to these impacts of future climate warming and increased fire activity.

This thesis includes two studies focused on quantifying the impacts of climate change, climate variability, and wildfires on forest dynamics. In Chapter 1, I compared the accuracy of field-based methods to precise dendrochronological techniques to age ponderosa pine and Douglas-fir seedlings sampled from three study regions across the western U.S. The use of precise dendrochronological tree aging was well justified, as node counts systematically underestimated ring counts, with bias increasing with tree age.

In Chapter 2, I studied the impacts of climate variability on lower-treeline forests in the northern Rocky Mountains of the U.S., by quantifying how post-fire tree establishment and radial growth varied with seasonal climate over the 20th and early-21st centuries. Climatic conditions favoring regeneration differed between ponderosa pine and Douglas-fir, suggesting species-specific responses to future increases in temperature and drought. Radial growth was also sensitive to moisture availability and temperature, but this sensitivity varied over the past century and between life stages. While adult growth was consistently sensitive to moisture availability, juvenile growth, particularly for ponderosa pine, was sensitive to moisture availability during the warmest and driest decades, suggesting that directional shifts in temperature, accompanied by increasing moisture stress, may be changing climate limitations on growth. This research demonstrates the increased vulnerability of post-fire tree regeneration and decreased growth in dry mixed-conifer forests given increases in temperature and drought. Shifts towards conditions unfavorable for regeneration and growth will likely result in shifts in species composition of lower-treeline forests or transitions to non-forested states.

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© Copyright 2018 Lacey Hankin