Aspen recovery following livestock removal: A landscape-scale natural experiment on Hart Mountain National Antelope Refuge

Background/Question/Methods

Across much of the western United States (US), quaking aspen (Populus tremuloides) woodlands have been experiencing long-term decline. We evaluated the riparian aspen age structure (i.e., number of aspen/ha vs. year of establishment) in the high desert of southeastern Oregon (i.e., Hart Mountain National Antelope Refuge) to better understand the potential influences of (a) livestock herbivory and (b) climate on long-term aspen demography. We found the onset of livestock grazing in the late 1800s began to significantly limit (p<0.05) aspen recruitment. 

Results/Conclusions

We found the onset of livestock grazing in the late 1800s began to significantly limit (p<0.05) aspen recruitment.  Although livestock grazing on the refuge was regulated after 1936, only low levels of aspen recruitment continued to occur. However, following the termination of livestock grazing in 1990 aspen recruitment significantly increased (p<0.05). Climate variables (i.e., Palmer Drought Severity Index, annual precipitation, and annual temperature) had little influence on long-term patterns of aspen recruitment. Overall, results indicate that top-down forcing by livestock herbivory was the major factor associated with over a century of reduced aspen recruitment. This study, in combination with other recent research, indicates that where aspen declines are currently underway in the western US, land managers need to carefully consider the potential effects of ungulate herbivory to ensuring retention of aspen woodlands and associated ecosystem services.