Year of Award
2026
Document Type
Dissertation
Degree Type
Doctor of Philosophy (PhD)
Degree Name
Systems Ecology
Department or School/College
W.A. Franke College of Forestry and Conservation
Committee Chair
Andrew J. Larson
Commitee Members
C. Alina Cansler, Sean A. Parks, David L.R. Affleck, Ashley P. Ballantyne
Abstract
Density-dependent interactions between trees (e.g., competition and facilitation) play a fundamental role in shaping the composition, structure, and function of forest ecosystems. Yet, our understanding of how these interactions mediate or are themselves altered by climate change and disturbance remain limited. This dissertation investigates density-dependent interactions across gradients of environmental stress and disturbance to clarify how they may magnify or buffer forest responses to global change. Chapters 1 and 2 examine these dynamics in highelevation subalpine forests using a 2 × 2 factorial design (burned vs. unburned; upper vs. lower elevation). In these systems, short growing seasons and physiologically stressful winters have historically been the primary constraints on tree growth, and high levels of abiotic stress near elevational treeline are expected to promote facilitative interactions. I show that ongoing climate change has ameliorated these conditions, producing net-competitive dynamics near treeline and reorganizing growth–climate relationships independent of elevation or disturbance history. Analysis of regenerating cohorts at these sites did not reveal major fire-catalyzed shifts in juvenile tree populations but did indicate that fire may no longer reliably promote natural regeneration of the threatened whitebark pine (Pinus albicaulis) in the way it has previously. Chapter 2 also presents a spatially explicit, landscape-scale model of whitebark pine juvenile density across the Bitterroot Range to help guide conservation and restoration of this important species. Chapters 3 and 4 explore similar fundamental questions in competition-dominated montane forests by leveraging a replicated, long-term thinning experiment. Here, I demonstrate that density-dependent competition amplifies growth reductions associated with extreme climate events in northern temperate forests and document a previously unquantified density-dependent mechanical damage pathway with measurable consequences for individual tree growth and mortality as well as long-term stand yield. As a whole, this work provides new insights into the role of density-dependent interactions in forest ecosystems and illustrates how knowledge of these relationships can inform adaptive forest management in a changing world.
Recommended Citation
Beisel, Joshua James, "DENSITY-DEPENDENT INTERACTIONS AND THEIR ROLE IN MEDIATING RESPONSES TO DISTURBANCE AND CLIMATE ACROSS FOREST ECOSYSTEMS" (2026). Graduate Student Theses, Dissertations, & Professional Papers. 12637.
https://scholarworks.umt.edu/etd/12637
© Copyright 2026 Joshua James Beisel