Forb community response to prescribed fire, livestock grazing, and an invasive annual grass in the Pacific Northwest Bunchgrass Prairie

Abstract

AbstractQuestionsGrassland degradation due to agriculture, changing fire regimes, and invasive species negatively affects forb communities. Conserving forbs and the services they provide requires a better understanding of their responses to interacting disturbances. Although fire and livestock grazing are important disturbances, their effect on forb communities in the Pacific Northwest Bunchgrass Prairies is not fully understood. Our objectives were to: (a) determine how prescribed fire and livestock grazing influence forb community composition, cover, species richness, and diversity over a 10‐year span; (b) identify indicator species for each treatment; and (c) quantify forb responses to increasing cover of the invasive grass Ventenata dubia.LocationPacific Northwest Bunchgrass Prairie, Oregon, USA.MethodsWe monitored plant responses in a manipulative study established in 2004 with four treatments (prescribed fire, livestock grazing, both fire and grazing, and a control with no fire or grazing). Fall burns occurred in 2006 and 2016.ResultsForb communities changed significantly over 10 years and were influenced more by fire than cattle grazing. Treatments involving fire were more associated with annual forbs and fewer perennial species than treatments not burned. Although forb cover was lower in grazed compared to ungrazed treatments, richness and diversity did not differ among treatments. Several fire‐adapted forb species were associated with burned treatments. Forb cover and richness was negatively correlated with Ventenata dubia cover in 2016 and 2018 when cover of this invasive species reached 20%–37%.ConclusionsWe found fall prescribed burns at a ten‐year return interval affected forb community composition and cover but not richness or diversity. Fire may promote annual and fire‐tolerant perennial forbs, but this relationship may be compromised by Ventenata dubia invasions. This study provides insights into management strategies aimed at conserving forb communities and emphasizes the need for continued monitoring of the Ventenata dubia invasion in this unique prairie.