Effects of grazing and climate on Greene’s mariposa lily in the Cascade-Siskiyou National Monument

Calochortus greenei S. Wats., Greene’s mariposa lily, is listed by the United States Fish and Wildlife Service (USFWS) as a federal species of concern, and is proposed for listing as a threatened species in Oregon. It is also a Bureau of Land Management (BLM) special status species. This project aims to monitor  C. greenei populations and associated plant communities within the Cascade-Siskiyou National Monument to observe population trends for a 10 year period. The objectives of this report are to: summarize population trends and plant community data, evaluate influences of major herbivores on C. greenei and associated plant communities, as well as evaluate the potential effects of climate change on the species, and project future trajectory of the C. greenei populations on the Cascade-Siskiyou National Monument based on forecasts from climate change models.

Herbivores were observed to negatively affected plant size and population viability of C. greenei. Fencing improved conditions for the species, and may be warranted to enhance some patches or populations of plants. Even protecting plants from just large herbivores provided a substantial benefit to the plants. However, Removing herbivores from plots generally did not result in improvements in native plant abundance, even after 10 years. Grassland vegetation on the Cascade-Siskiyou National Monument that has been degraded due to long-term grazing by livestock is unlikely to improve without additional restoration practices, such as removal of non-native plants and seeding with native vegetation.

Our results also showed that climate change may improve conditions of C. greenei, at least at sites similar to those examined in this study. This conclusion is preliminary but suggests that some aspects of climate change could benefit the species.

Further research to better measure seed germination and seedling establishment would improve C. greenei population modeling. Such studies could include a combination of greenhouse and field experiments in which the fate of individual seeds is tracked at multiple points in the wet and dry growing season for multiple years. Experiments across study areas might not be essential, since C. greenei plant performance appears generally similar across the plant communities of the Cascade-Siskiyou National Monument.