Legacies of plant invasion on ecosystem processes and microbial community structures

Background/Question/Methods

Millions of hectares of native grasslands have been invaded by cheatgrass (Bromus techtorum), leafy spurge (Euphorbia esula) and knapweed (Centaurea stoebe) in the mountainous West.  These invasions can drastically alter the structure and composition of communities as well as ecosystem processes, but differences among invasive plants that co-occur have seldom been documented and compared. We measured litter decomposition, soil nutrient availabilities, and soil respiration over a six-month period on three replicated areas that contained sites dominated by knapweed, spurge, cheatgrass and native grass communities.  Mass loss of root (buried at 5 cm depth) and shoot (placed on the surface) litter was used to calculate decomposition, and a reciprocal placement design allowed us to identify potential adaptations to litter type by the decomposer community.  Soil samples for nutrient analyses were taken in April, June and September, and soil respiration was measured monthly using a Li-Cor.      

Results/Conclusions

Knapweed and spurge roots decomposed three times faster than cheatgrass and native grass roots, a difference that showed a significant and positive correlation with litter P concentrations.  Higher P concentrations were also observed in knapweed and spurge shoot litter, and could at least partly be due to the much greater arbuscular mycorrhizal (AM) colonization in these roots relative to the grasses. Preliminary data also suggest shifts in AM fungal communities.  Shoot decomposition differed significantly among litter types, with spurge decomposing the fastest and native grasses the slowest.  Litter did not decompose faster when placed in a “home” environment, suggesting that decomposer communities are not adapted to litter type. Fungal and bacterial decomposer communities are currently being investigated and will be presented.  Soil nutrient availabilities mirrored decomposition rates and were significantly higher in spurge and knapweed invaded areas.  We observed a significantly lower soil  respiration in cheatgrass invaded sites, which could be due to its lower overall productivity and recalcitrant litter.  Knapweed and spurge invasions, on the other hand, did not appear to drastically change soil respiration.  Overall, our results indicate that plant invasions can have fundamental impacts on many ecosystem processes, but that the response depends on the invasive plant.  The consistent results across invaded areas also suggest that extrapolations to larger-scale impacts may be possible and could indicate regional shifts in carbon and nutrient cycles.  An increased knowledge regarding soil legacies from plant invasions is crucial to evaluate restoration efforts needed for the successful re-establishment of native plant communities.