A team of Forest Service scientists working near La Grande, Oregon, evaluated how the herbivory, or eating habits, of elk, cattle, and pocket gophers affected nutritious shrubbery growing in dry, conifer forests in the western U.S.
These research results provide robust new knowledge about how mammalian eating habits affect understory vegetation in dry coniferous forests following fuels reduction and prescribed burning. The scientists found that cattle and elk herbivory substantially reduced height growth for Scouler’s willow and black cottonwood while pocket gophers dramatically reduced both shrub species’ survival rates. This is the first known study of the effects of above- and below-ground mammalian eating habits on forest shrubbery.
“Mammalian herbivory effects were thought to be benign to forest health, so the results surprised us,” said Michael Wisdom, a research wildlife specialist with the Forest Service’s Pacific Northwest Research Station.
Black cottonwood and Scouler’s willow grow initially as shrubbery, out-competing young pines, after a combination of treatments to thin overly crowded stands of trees disturbs the landscape. (Due to decades of fire suppression in western forests, fuel treatments are conducted to improve forest health and reduce the fire danger.)
“Cattle like to focus on grasses, while elk prefer the shrubs with their high nitrogen content, especially in late summer when they are lactating,” Wisdom said. “But plant height for both cottonwood and willow shrubbery lowers significantly when either ungulate species is grazing in the area.”
Add root-munching pocket gophers into the mix, and the shrubs have a hard time even surviving. “Gophers are the primary factor in willow and cottonwood understory decline,” Wisdom said.
The scientists monitored the effects of the elk, cattle, and gopher eating habits on forest shrubbery over the course of seven years by establishing a collection of large paddocks on the Starkey Experimental Forest and Range near La Grande, Oregon. The paddocks enclosed the cattle and elk together, separately, or fenced them out altogether. Some paddock areas included populations of pocket gophers that pioneer into recently disturbed landscapes. This serendipitous situation gave the scientists an ideal opportunity to study how the gophers’ below-ground eating habits, in conjunction with the ungulates above-ground grazing and foraging, contributed to the shrubbery’s growth and survival. With the large mammals fenced out, a flush of willow and cottonwood shrubbery sprang up to form a nutritious understory. The densities of willow and cottonwood plants were five times higher in enclosures where elk and cattle were excluded from grazing and no pocket gophers lived.
While nutritious, this same type of flush understory may also create a fuel ladder for fire, allowing a blaze to burn up into the tree tops where it can grow into a dangerous crown fire. More research is needed to determine the fire danger of an ungrazed willow-cottonwood understory over time.
The scientists plan to continue monitoring the enclosures for another 15 to 20 years to see if the effects of the mammalian eating habits change over time.
Current and future research results will help forest supervisors and other land managers achieve their landscape objectives. Those objectives will vary depending on a forest’s location and its age, the desired biodiversity for the area, and the current fire danger. In most cases, managers will want to use an integrated approach that sustains a diverse plant community and prevents accumulation of coniferous ladder fuels associated with long-term fire suppression, and consider the potential additive effects of high herbivory on increased conifer fuel loading.