Abstract
Fire exclusion has dramatically altered historically fire adapted forests across western North America. In response, forest managers reduce forest fuels with mechanical thinning and/or prescribed burning to alter fire behavior, with additional objectives of restoring forest composition, structure, and ecosystem processes. There has been extensive research on the effects of fuel reduction and restoration treatments on trees, fuels, regeneration, and fire behavior; but less is known about how these treatments influence understory vegetation, which contains the majority of vascular plant diversity in many dry conifer forests. Of particular interest is how understory vegetation may respond to the season and interval of prescribed burning. The season and interval of prescribed burning is often determined by operational constraints rather than historical fire regimes, potentially resulting in fire conditions and burn intervals to which native plants are poorly adapted. In this study, we examined how understory vegetation has responded to season and interval of prescribed burning in ponderosa pine (Pinus ponderosa) forests in the Blue Mountains of northeastern Oregon, USA. Using over a decade (2002–2015) of understory vegetation data collected in stands with different intervals (5 versus 15 year) and seasons (spring versus fall) of prescribed burning, we quantified how season and interval of prescribed burning has influenced understory vegetation compositional trajectories and indicator species over time. Season of prescribed burning resulted in different understory communities and distinct trajectories of understory composition over time, but interval of burning did not. Indicator species analysis suggests fall burning is facilitating early seral species, with native annual forbs displaying ephemeral responses to frequent burning, while invasive cheatgrass (Bromus tectorum) increased in abundance and frequency across all treatments over time. These findings indicate that understory vegetation in these ecosystems are sensitive to seasonality of burning, but the responses are subtle. Our findings suggest season and interval of prescribed burning used in this study do not result in large changes in understory vegetation community composition, a key consideration as land managers increase the pace and scale of prescribed fire in these forests.
Highlights
Fire plays a critical role in terrestrial ecosystems across the globe, shaping ecosystem composition, structure, and function [1,2,3,4]
This study focuses on how season and interval of prescribed burning influence the composition and compositional trajectories of understory vegetation, using a unique long-term (18 yr) experiment in ponderosa pine (Pinus ponderosa) dominated forests of northeastern Oregon, USA
In the context of thinned ponderosa pine forests, we found that the season of prescribed burn subtly altered the understory vegetation community in our study
Summary
Fire plays a critical role in terrestrial ecosystems across the globe, shaping ecosystem composition, structure, and function [1,2,3,4]. In western North America, fire exclusion has fundamentally altered the structure and function of fire adapted forests, forest types such as ponderosa pine and dry mixed conifer forests historically characterized by frequent low and mixed severity fire regimes [5,6,7]. Frequent fire in these forest types reduced overall stand densities and fuel loading [5,6,7], while maintaining heterogeneous spatial patterns of vegetation structure that promoted resilience and moderated fire behavior [8,9,10]. The pace and scale of fuel reduction treatments lags behind what is suggested to affect meaningful landscape change [14], while persistent operational and administrative constraints hinder more widespread application of fuel reduction treatments and restoration of fire as an ecological process [15,16,17]
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