Examination of combined plant community restoration techniques 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. Broad scale control of invasives that also promotes natives is difficult and may require an integrated approach. Our objectives were 1) to determine community composition and environmental characteristics of mounds most resistant to invasion and 2) 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.  The experiments were: 1) test effectiveness of Imazapic, sucrose soil amendment and native seed addition (Pseudoroegnia spicata and Festuca idahoensis), for invasive annual grass reduction on two geologic substrates (basalt and alluvium), 2) test effectiveness of a bio-control insect (Mecinus janthinus) for invasive Linaria dalmatica reduction, and 3) test effectiveness of an experimental strain of rhizobacteria, Pseudomonas fluorescens D7, for invasive annual grass reduction. We surveyed native and exotic species composition and cover in 1 m² vegetation plots in June-July 2009, applied treatments in summer and fall 2009, and resurveyed plots in June-July 2010. Mean change in abundance over time was compared among treatments using ANOVA.  We used ordination analysis to characterize community composition and environmental variables associated with resistance to invasion.

Results/Conclusions

High cover of the native species, Amsinckia retrorsa and Eriogonum heracleoides seemed to lead to invasion resistance on the mounds.  The substrate underlying the mounds (alluvium or solid basalt) affected community composition and invasion.  Four invasive grass species (Bromus tectorum, B. commutatus, B. arvensis and Ventenata dubia) were abundant over both substrates. Herbicide decreased abundance of total invasive species, invasive grasses and L. dalmatica (p < 0.001). Herbicide had a greater effect on alluvial mounds (p = 0.007). Sucrose soil amendments reduced the abundance of native species on both substrates (p = 0.031) and the abundance of invasive species on alluvium (p = 0.023).Addition of native seed , M. janthinus, and P.f. D7 had no significant effects to date (effects of perennial grass seed addition and P. f. D7 may take more than 1 year to develop). This study will serve as baseline data for monitoring and will inform land managers attempting to restore native species in channeled scabland habitats.