Brush management of a whitethorn acacia-encroached grassland enhances resource-conserving ‘shrub islands’

Authors: JOHNSON, J. - University Of Arizona; Williams, Christopher - Jason; GUERTIN, D.P. - University Of Arizona; ARCHER, S.R. - (NCE, CECR)networks Of Centres Of Exellence Of Canada, Centres Of Excellence For Commercilization A; Pierson Jr, Frederick

Submitted to: Society for Range Management
Publication Type: Abstract Only
Publication Acceptance Date: 12/12/2019
Publication Date: 2/16/2020
Citation: Johnson, J., Williams, C.J., Guertin, D., Archer, S., Pierson Jr, F.B. 2020. Brush management of a whitethorn acacia-encroached grassland enhances resource-conserving ‘shrub islands’. Presented at the 73rd Annual SRM Meeting, Transformation and Translation, Society for Range Management, February, 16-20, 2020, Denver, CO, Poster Number 65.

Interpretive Summary: We conducted an integrated suite of measurements to quantify the ecohydrologic impacts of chemical brush management (tebuthiuron) at a shrub-encroached grassland in southeastern Arizona. We addressed two questions: Did tebuthiuron application 1) affect hillslope runoff following high-intensity rainfall?, and 2) alter bare ground connectivity and the magnitude of runoff and sediment transport? Vegetation and soil measurements were conducted in 2018 on rainfall-simulated, small- (0.5 m2, n=41) and overland flow-simulated, large- (13 m2, n=13) plots in a whitethorn acacia (Vachellia constricta) community treated with tebuthiuron (0.84 kg/ha a.i.) in 2013 and on an adjacent untreated area. The treatment successfully decreased live-shrub cover by > 99% from the untreated control to the treatment, and, consequently, increased grass cover by 383%. Importantly, vegetation and hydrologic treatment effects differed by microsites. Grass cover on skeletal-shrub microsites (75%) far exceeded that on live-shrub microsites (12%); interspace grass cover responded less markedly to herbicide treatment (25%) vs. 0% on control. Cumulative runoff from skeletal-shrub microsites during 100 and 120 mm/hr simulated rainfall were 79% and 70% lower, respectively, on the treated site, whereas cumulative runoff from the zones between shrubs were statistically comparable between treatments. Although the basal gap size distribution shifted to shorter gap lengths, rill widths, depths, and velocity during overland flow simulations were statistically comparable. We did, however, measure significantly lower runoff/sediment yields on the treated site at the highest overland flow rate (40 L/min). Our findings suggest mortality induced by tebuthiuron enhanced herbaceous cover on ‘shrub islands’ that in turn increased their infiltration capacity. The asymmetric runoff/sediment yield response of shrub patches vs. interspaces on treated vs. control sites leads us to hypothesize spatial heterogeneity of hydrologic properties persisted even after recruitment of grass cover. These results indicate resource-conserving patches may be more resilient to certain ecological transitions than previously suspected.

Technical Abstract: We conducted an integrated suite of measurements to quantify the ecohydrologic impacts of chemical brush management (tebuthiuron) at a shrub-encroached grassland in southeastern Arizona. We addressed two questions: Did tebuthiuron application 1) affect hillslope runoff following high-intensity rainfall?, and 2) alter bare ground connectivity and the magnitude of runoff and sediment transport? Vegetation and soil measurements were conducted in 2018 on rainfall-simulated, small- (0.5 m2, n=41) and overland flow-simulated, large- (13 m2, n=13) plots in a whitethorn acacia (Vachellia constricta) community treated with tebuthiuron (0.84 kg/ha a.i.) in 2013 and on an adjacent untreated area. The treatment successfully decreased live-shrub cover by > 99% from the untreated control to the treatment, and, consequently, increased grass cover by 383%. Importantly, vegetation and hydrologic treatment effects differed by microsites. Grass cover on skeletal-shrub microsites (75%) far exceeded that on live-shrub microsites (12%); interspace grass cover responded less markedly to herbicide treatment (25%) vs. 0% on control. Cumulative runoff from skeletal-shrub microsites during 100 and 120 mm/hr simulated rainfall were 79% and 70% lower, respectively, on the treated site, whereas cumulative runoff from the zones between shrubs were statistically comparable between treatments. Although the basal gap size distribution shifted to shorter gap lengths, rill widths, depths, and velocity during overland flow simulations were statistically comparable. We did, however, measure significantly lower runoff/sediment yields on the treated site at the highest overland flow rate (40 L/min). Our findings suggest mortality induced by tebuthiuron enhanced herbaceous cover on ‘shrub islands’ that in turn increased their infiltration capacity. The asymmetric runoff/sediment yield response of shrub patches vs. interspaces on treated vs. control sites leads us to hypothesize spatial heterogeneity of hydrologic properties persisted even after recruitment of grass cover. These results indicate resource-conserving patches may be more resilient to certain ecological transitions than previously suspected.