Use of uprooted invasive buffelgrass (Pennisetum ciliare) parent plants as thatch to reduce progeny seedling emergence

Background/Question/Methods

Buffelgrass (Pennisetum ciliare) is a perennial bunchgrass native to Africa that has invaded pristine areas of the Sonoran Desert in southern Arizona. It threatens many native plant species by means of competitive exclusion as well as increased fire frequency and intensity. Since the 1990s, efforts have been underway in southern Arizona to control buffelgrass by means of manual removal. A problem with manual removal is that resulting bare, disturbed soil provides a favorable environment for buffelgrass seed germination. This study examined whether a thatch composed of uprooted buffelgrass parent plants spread over disturbed areas reduces the density of progeny seedlings. We established 2 X 2 m plots in dense stands of buffelgrass at a field site in the mountains north of Tucson. Thirty plots were randomly chosen for thatch treatment, and another 30 plots for unthatched control. Seven to ten months after removal and thatch placement, buffelgrass seedlings were counted in each plot. We expected sunlight attenuation by thatch to be a principal factor affecting seedling emergence. This factor was tested using seeds in containers in the field and the greenhouse. Shade cloth was used to provide standardized levels of light attenuation to be compared with bare ground controls. 

Results/Conclusions

Field plots with thatch had significantly fewer buffelgrass seedlings than plots without thatch. These results suggest that the laying down of thatch over areas disturbed during manual treatment of dense stands of buffelgrass will increase the efficiency of follow-up treatment in these areas. Because each individual plant that emerges following manual removal must be pulled to achieve long-term control, a lower seedling density will allow an often limited work force to treat more areas and protect a larger portion of valuable Sonoran Desert habitat. Percent buffelgrass seed germination did not differ significantly between any of the groups in the field container study suggesting that light attenuation does not play a significant role in the limitation of seedling density by thatch. In the greenhouse, however, a significantly higher percentage of buffelgrass seeds germinated in bare soil containers than in containers in which soil was covered by shade cloth or thatch. The mixed results that we obtained suggest that light attenuation could be a major mechanism by which thatch reduces buffelgrass seedling emergence, but that other factors such as autoallelopathy, seed predators, seedling herbivores or pathogens may also be involved.