Annual bromes decrease with increasing fall defoliation intensity

Authors: Waterman, Richard; Vermeire, Lance

Submitted to: Global Ecology and Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2021
Publication Date: 5/29/2021
Citation: Waterman, R.C., Vermeire, L.T. 2021. Annual bromes decrease with increasing fall defoliation intensity. Global Ecology and Conservation. 28. Article e01652. https://doi.org/10.1016/j.gecco.2021.e01652.
DOI: https://doi.org/10.1016/j.gecco.2021.e01652

Interpretive Summary: A greater understanding of effects of dormant or fall defoliation on subsequent year net primary production and relevance of targeted fall defoliation pressure on unwanted non-native graminoids, such as Japanese brome, is crucial to implementing management practices that improve rangelands in the Northern Great Plains. Reported results supported our hypothesis that brome abundance is inversely related to fall defoliation intensity, particularly during the final study year when regional brome abundance was greatest. Our hypothesis that fall defoliation intensity does not affect perennial grass cover was partially supported in that no differences were detected between non-mowed sites and any defoliation level. However, cover trended down from the non-mowed treatment with mowing to 1 cm and trended up for other mowing heights, resulting in reduced perennial grass cover with mowing to 1 cm compared to all other mowing heights. Old dead mass decreased with increasing mowing intensity for all but the year following severe drought, yet litter cover only reflected the treatments during the third and fourth years. Rangelands in the Northern Great Plains are dynamic and respond well to grazing, however, without proper management are susceptible to non-native plants, such as bromes, which change plant community structure and make rangelands less beneficial to livestock production goals.

Technical Abstract: Annual brome invasion into rangelands diminishes native plant communities and hinders livestock performance. Objectives for this study were to utilize fall defoliation with four different mowing heights (10, 6, 4, or 1 cm) compared to a non-mowed control on two study sites (a silty and claypan ecological site) with each site consisting of 25 plots (5.5 x 1.6-m). This study was conducted over 4 yr (2013-2017) and fall mowing reduced old dead mass every year except 2016, which followed a severe drought. Spring soil water was not reduced by fall defoliation. Mowing to 1 cm reduced brome biomass by 40% compared to non-mowed plots and brome production was highly dependent on Sep-Oct precipitation. Brome biomass was least in 2015 during a drought and greatest in 2017 following double the median Sep-Oct precipitation. Non-brome biomass was similar across treatments. An 11% reduction in current-year biomass was observed for mowing heights of 10, 6, and 4 cm whereas a 22% reduction was measured for the 1 cm mowing height compared to non-mowed plots. Perennial grass basal cover was similar for 10, 6, and 4 cm mowing heights and greater than that for mowing at 1 cm whereas, no differences were detected between non-mowed plots and any mowing height. Increasing fall defoliation intensity reduced annual bromes while allowing perennial grasses to persist, indicating fall mowing and possibly grazing are viable options for annual brome management.