Responses of Chaparral and Oak Woodland Plant Communities to Fuel-Reduction Thinning in Southwestern Oregon

Abstract

Fire suppression has led to large fuel accumulations in many regions of the United States. In response to concerns about associated wildfire hazards, land managers in the western United States are carrying out extensive fuel-reduction thinning programs. Although reductions in cover by woody vegetation seem likely to cause changes in herbaceous communities, few published studies have reported on consequences of such treatments for native or exotic plant species. We compared vegetation and abiotic characteristics between paired thinned and unthinned chaparral and oak woodland communities of southwestern Oregon 4-7 yr posttreatment and contrasted impacts of manual vs. mechanical treatments. Herbaceous cover increased on thinned sites, but species richness did not change. Herbaceous communities at thinned sites had an early postdisturbance type of composition dominated by native annual forbs and exotic annual grasses; cover by annuals was nearly twice as high on treated as on untreated sites. Absolute and proportional cover of native annual forbs increased more than any other trait group, whereas exotic annual forbs and native perennial forbs declined. Exotic annual grass cover (absolute and proportional) increased, whereas cover by native perennial grasses did not. Shrub reestablishment was sparse after thinning, probably because of a lack of fire-stimulated germination. Manual and mechanical treatment impacts on abiotic site conditions differed, but differences in vegetation impacts were not statistically significant. Fuel-reduction thinning may have some unintended negative impacts, including expansion of exotic grasses, reductions in native perennial species cover, persistent domination by annuals, and increased surface fuels. Coupled with sparse tree or shrub regeneration, these alterations suggest that ecological-state changes may occur in treated communities. Such changes might be mitigated by retaining more woody cover than is currently retained, seeding with native perennials after treatment, or other practices; further research is needed to inform management in these ecosystems. Resumen En muchas regiones de Estados Unidos, la supresión del fuego ha provocado grandes acumulaciones de combustible. En repuesta a las preocupaciones respecto a los peligros asociados a los fuegos no controlados, los manejadores de tierras del oeste de Estados Unidos están conduciendo programas extensivos de aclareo para reducir el combustible. Mientras que la reducción de cobertura de la vegetación leñ osa parece que probablemente cause cambios en las comunidades herbáceas, pocos estudios publicados han reportado las consecuencias de tales tratamientos en las especies nativas o exóticas. Comparamos características de vegetación y abióticas entre comunidades apareadas de ''Chaparral'' y ''Oak'' con y sin aclareo en el sudoeste de Oregon 4 y 7 añ os después de aplicar los tratamientos, y contrastamos los impactos de tratamientos manuales contra mecánicos. La cobertura herbácea se incrementó en los sitios con aclareo, pero la rique...

Decade-Long Plant Community Responses to Shrubland Fuel Hazard Reduction

“…Local climates and plant varieties may cause the seemingly similar dominant plants across California's chaparral to respond to treatments differently. Despite regional differences, our findings expand upon results from other studies in California's chaparral (Beyers and Wakeman 2000;Keeley 2000Keeley , 2002Keeley , 2004Merriam et al 2006;Perchemlides et al 2008;Brennan and Keeley 2015) and can be used in combination with local studies to aid in fuel hazard reduction planning.…”

Development of Vegetation and Surface Fuels Following Fire Hazard Reduction Treatment

“…Although the high variability in post-treatment shrub cover precluded clear findings with respect to development over time, the hypothesis that thinning and prescribed fire treatments would promote shrub growth, exacerbating potential fire hazards, is not supported by these data, as total shrub cover was nearly always low (<20%) in control and treatment sites alike. Similarly low levels of shrub cover have been observed in other dry western forests following treatment [16,67].…”

Long-term consequences of mechanical fuel management for the conservation of Mediterranean forest herb communities

“…Although some community attributes recovered over long time frames, species richness appeared essentially constant through the chronosequence, while herbaceous cover reached a peak in just about 2 years. In common with other studies, patterns derived from these relatively coarse attributes masked complex responses that could only be uncovered and interpreted by using finer attributes, such as, for instance, the variation in richness and cover within functional groups (Bonet and Pausas 2004;Decocq et al 2004a;Perchemlides et al 2008;von Oheimb and Härdtle 2009;this study).…”

“…However, effects were still noticeable after about 70 years and would probably be felt for longer, as variation of some community attributes did not appear to converge to a plateau within the time frame of the chronosequence, thereby suggesting that they will still change for at least some time in the future. Understanding the ecological consequences of fuel management, even when the studied biological group has short life cycles, may thus require study periods much longer than that normally analysed (e.g., Calvo et al 2005;Perchemlides et al 2008;Pérez-Ramos et al 2008), which is in line with the long time frames usually required for appreciating the dynamics of forest ecosystems (Bellemare et al 2002;Frelich 2002;Thomas and Packham 2007).…”