Ventenata Dubia Cover, forb diversity and forb richness

Questions: Grassland 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: 1) determine how prescribed fire and
livestock grazing influence forb community composition, cover, species
richness, and diversity over a 10-year span; 2) identify indicator species
for each treatment; and 3) quantify forb responses to increasing cover of
the invasive grass Ventenata dubia. Location: Pacific
Northwest Bunchgrass Prairie, Oregon, USA. Methods: We 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.
Results: Forb communities changed significantly over ten 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. V. dubia cover was negatively
correlated with forb cover and richness in 2016 and 2018 when cover of
this invasive species reached 20-37%. Conclusions: We 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 V. dubia invasions. This study provides
insights into management strategies aimed at conserving forb communities
and emphasizes the need for continued monitoring of the invasion
of V. dubia in this unique prairie.