Control methods for invasive grasses on Mima Mounds at Turnbull National Wildlife Refuge, WA

Background/Question/Methods

Invasive plants are abundant on Mima Mounds in the Channeled Scablands of eastern Washington. Mima Mounds, ~1 m high aggregations of fine particle soil over alluvium or basalt bedrock, provide valuable forage and habitat for native plants. On Mima Mounds, broad scale control of invasives, that also maintains natives, is difficult and may require a combination approach. Our objective was to test the effect of combining invasive plant control techniques on native and exotic species on Mima Mounds at Turnbull National Wildlife Refuge using  three experiments; 1) we tested the effect of herbicide, sucrose soil amendment, native seed addition (Pseudoroegnia spicata and Festuca idahoensis), and geology (basalt or alluvial underlain mounds) on invasive annual grasses (16 treatments), 2) we added a biocontrol insect (Mecinus janthinus) for Linaria dalmatica to a subset of the above treatments on mounds over alluvium (10 treatments), and 3) we tested the effect of an experimental strain of rhizobacteria, Pseudomonas fluorescens D7, on invasive annual grasses on alluvium (2 treatments). We surveyed native and exotic species composition and cover in1 m² vegetation plots in June-July 2009, applied treatments in fall 2009, and resurveyed plots in June-July 2010. Mean change in abundance was compared using ANOVA.  

Results/Conclusions

We identified four invasive grass species that were abundant over both substrates (Bromus tectorum, B.commutatus, B.arvensis and Ventenata dubia) Herbicide decreased both measures of abundance for the four invasive grasses and Linaria dalmatica (p < 0.001) and had a greater effect on alluvial mounds (p = 0.01). Sucrose soil amendments, native seed addition, M. janthinus addition, and P.f. D7 addition had no significant effects to date (effects of perennial grass seed addition and P. f. D7 may take more than 1 year to develop).  The greater effect of herbicide on alluvium underlain mounds than basalt may be due to generally deeper soils and greater moisture availability in those sites.  The results of this study will be used to inform land managers attempting to restore native species and control invasive species in channeled scabland habitats.