Climate change and invasion: Does a loss of ecological integrity affect the cultural expression of an indigenous culture?

Background/Question/Methods

The Bighorn and Little Bighorn Rivers of Montana are highly regulated, and are now heavily dominated by the invasive Russian olive, which is slowly replacing native plains cottonwood (Populus deltoidesL.). Historically to present day, the Crow Tribe in south-central Montana use cottonwoods for socio-cultural and ritual purposes, yet Tribal Elders have expressed concern over a decline in specific size classes of cottonwood and a continued spread of Russian olive. Predicted climate changes, such as increases in mean temperature and precipitation in the upper northwest, may serve to exacerbate the spread of this invasive species.  We used Maximum Entropy Modeling (MaxEnt) to predict suitable habitat and future spread of Russian olive along riparian corridors of the Little Bighorn and Bighorn Rivers within Crow Reservation boundaries.  To determine cottonwoods available for harvest to and from ceremonial sites, we established riparian forest plots and used semi-structured open interviews with respected Elders in the Crow community between November 2010 and April 2011.

Results/Conclusions

Climate variables (n = 22) contributed significantly to the model, suggesting that climate change may contribute to the spread of Russian olive, particularly downstream from dams. Perceptions of cottonwood availability by the Elders matched the field plot data.  Elders overwhelmingly agreed (~80%) that sub-adult trees were the most difficult to find now compared to 25 years ago. Maximum distances to travel to obtain sub-adult size classes used in ceremony, increased in present-day by 30 km as opposed to recent history. Cottonwood stem heights (from 7.32 m plots where n=10) were significantly shorter (< 150m) near ceremony sites compared to sites farther away (> 2.4 km away; p < 0.01). There were more cottonwood trees with diameter at breast height (dbh) < 5cm in plots located far  from ceremony sites than near (p < 0.05).    Near-term (10 years) climate change predictions, based upon the MaxEnt model, suggest that Russian olive will continue to spread. Coupled with local perceptions of reduced cottonwood availability, we suggest that mid-sized cottonwoods may become increasingly difficult to locate for ceremonies.  Containment of Russian olive and management plans to increase cottonwood availability will improve the cultural as well as the ecological integrity of these important riparian systems.