Assembly processes driving understory plant communities in a long-term study of fuel restoration treatments

Author

• Isabella G. Smith, University of Montana, MissoulaFollow

Year of Award

2026

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Committee Chair

Scott Ferrenberg

Commitee Members

Alina Cansler, Carl Seielstad

Keywords

Fire and Fire Surrogate, community assembly, understory vegetation, fuel reduction treatments

Subject Categories

Biodiversity | Forest Management | Other Ecology and Evolutionary Biology

Abstract

Understory plant communities play critical roles in forest biodiversity and ecosystem functioning and respond dynamically to environmental variation. In dry conifer forests of the western U.S., fire suppression and changing climatic conditions have altered disturbance regimes and forest structures, shifting resource availability and reshaping understory communities. Although fuel restoration treatments are implemented to mitigate these issues, their long-term effects on understory plant community dynamics, particularly in the context of their impacts on environmental conditions, remain poorly understood. This study addresses how understory plant richness, cover, and community structure respond to multiple fuel treatments, climate, topography, and forest stand structure, and the relative roles of deterministic and stochastic community assembly processes in driving these responses. Using over two decades of data from the Northern Rockies Fire and Fire Surrogate experiment, understory vegetation community dynamics were analyzed across time and in response to three fuel restoration treatments —thinning, prescribed fire, and their combination. Linear mixed-effects models were used to evaluate drivers of understory plant richness and cover, while community structure was assessed using permutational multivariate analysis of variance. Null models were used to compare observed levels of taxa turnover with random (null) expectations to infer the relative influences of deterministic and stochastic community assembly processes within treatments. Results show that understory richness and cover are driven primarily by climatic conditions, topographic factors, stand structure, and disturbance timing rather than treatment identity alone. Community structure varied significantly with disturbance history and environmental gradients, but most of the variation remained unexplained. Null modeling suggested that stochastic community assembly processes dominated at local scales, while deterministic processes are more influential at broader spatial scales. Overall, fuel treatments act as short-term disturbance pulses that enhance resource availability and diversity but have limited control over long-term compositional trajectories in understory plant communities. These findings suggest that effective understory management must account for broader environmental context and inherent ecological variability, reflecting a system in which stochastic assembly processes and dispersal dynamics can outweigh deterministic filtering by local conditions. As a result, treatment effects alone are unlikely to predict long-term community trajectories.

Recommended Citation

Smith, Isabella G., "Assembly processes driving understory plant communities in a long-term study of fuel restoration treatments" (2026). Graduate Student Theses, Dissertations, & Professional Papers. 12654.
https://scholarworks.umt.edu/etd/12654