Posts Tagged: Kenneth Tate
Once perceived as a problem, conservation grazing by cattle a boon to vernal pools
Giving 1,200-pound cows access to one of California's most fragile and biologically rich ecosystems seems a strange way to protect its threatened and endangered species.
But a recently published study suggests that reintroducing low to moderate levels of cattle grazing around vernal pools – under certain conditions – leads to a greater number and greater variety of native plants.
Ecologists consider vernal pools – ephemeral ponds that form seasonally – “islands of native habitat” amid California's grasslands that are dominated by exotic grasses. These biodiversity hotspots harbor about 200 native species of animals and plants, such as the coyote thistle, which germinates under water and forms a snorkel-like straw to deliver oxygen to its roots – and then “fills in” its stem as the pool dries.
Specially adapted to survive in those stages of wet and dry, many of these species are found only in vernal pools scattered across California – making those pools an urgent priority for conservationists.
During the 1970s and 1980s, vernal pools were fenced off in parts of the state, in the hopes of protecting the flora and fauna from grazing cattle. In the early 2000s, however, UC Davis researcher Jaymee Marty found that grazing was actually crucial to vernal pool biodiversity: once livestock were removed from areas that had been grazed historically, the diversity of plants plummeted.
“Her research was critical to rethinking the best ways to protect the diversity in California's vernal pool ecosystems,” Eviner said.
The Michaels-led study, published in the Journal of Applied Biology, builds on Marty's work, by looking at scenarios where cattle had been blocked from vernal pools for decades, and then observes the rate at which biodiversity returns after reintroduction of the animals. Michaels said she wanted to provide some initial answers to the practical questions that ranchers and land managers have in potentially reintroducing cattle.
“A lot of them had these areas that had been fenced off from grazing for the last 20–30 years, and they were very concerned about what happens if we let cattle back onto these vernal pool grasslands – are there going to be negative impacts because that land had been at rest for a few decades?” Michaels explained.
They discovered that, after reintroducing cattle to areas that had been fenced off since the 1970s, there was a greater abundance of native flora (species like the vernal pool buttercup, bractless hedge-hyssop and bristled downingia), as well as increased diversity among the plants (both in number of species and in how evenly distributed they were).
“Encouragingly, diversity is rapidly restored,” Eviner said, “providing conservationists with strong data to show that rapid action can enhance plant diversity.”
And as for potential worries about cattle making a snack of vernal pool plants, Michaels and her colleagues observed that the cattle appear to be more interested in munching on grasses.
“Anecdotally, we saw very few signs of herbivory on the vernal pool species because the timing is such that [the plants] are underwater for a good part of the late winter and early spring, and then by the time they're blooming, there's plenty of good forage around for the cattle,” Michaels said.
In fact, the cattle seem to be performing a function filled for millennia by native grazers (namely, the once-abundant tule elk), helping to knock down vernal pool species' chief competitor in those transition zones: the grasses.
Instead, microdepressions created by the cattle appeared to encourage the proliferation of native plants. Each hoofprint became a miniature basin – “a vernal pool within a vernal pool.”
“Right in those transition zones, where they could be hosting either the vernal pool species or the upland grasses, just a couple centimeters of soil topography can make a big difference,” Michaels explained. “If a cow comes and steps in that transition zone, and that lowers the soil surface so it stays inundated a little longer, you end up seeing these pockets of vernal pool species that are able to persist.”
Michaels is currently conducting a follow-up study on the hoofprints to pinpoint their role in boosting native plant abundance and biodiversity. Because the prints can last for several years, they might be able to deliver some enduring benefits – and land managers might not have to bring cattle in to graze the pools as often.
“If it's really the hoofprints making the big difference, maybe we don't need to graze every year – only during certain times of year when we know the hoofprints will form well and harden, and then we're good for a few years,” Michaels said.
Aspen grove restoration doesn’t impact local streams
Besides their beauty, aspen groves are valued for providing myriad ecological services. The groves add species and landscape diversity to the Sierra Nevada. They offer higher water retention than the adjoining conifer forest, plus wildlife habitat and forage for livestock and wildlife.
Aspens are native to the Sierra Nevada and other areas with cool summer temperatures. Groves are typically clonal colonies that grew from a single seedling and spread when roots come to the surface and start a new tree. Each tree lives 40 to 150 years above the ground, but the root system can be much older; one colony in Utah is said to be 80,000 years old.
In the Sierra Nevada, aspen stands have been declining in vigor and sometimes died because of pressures from encroaching conifers, according to Malcolm North, research scientist with the U.S. Forest Service's Pacific Southwest Research Station in Davis.
Aspens thrive in bright sunlight. Without regular fires, conifers eventually shade them out. More than 90 percent of aspen stands in forested areas of California have some conifer shading. Removing conifers with mechanical equipment to increase sunlight in aspen groves is an effective technique to restore stands, but land managers have been reluctant to thin out conifers because of concern about impacts on nearby mountain streams.
A recent collaborative research project involving UC Cooperative Extension, UC Davis and the U.S. Forest Service confirmed that conifer removal to restore aspen stands can be conducted without degrading aquatic ecosystems.
Kenneth Tate, UCCE specialist in the Department of Plant Sciences at UC Davis. Research was underway from 2003 to 2010 and published in the December 2013 issue of the journal PLOS ONE.
“This project is an excellent example of the value of collaboration between the university, U.S. Forest Service, and stakeholders to integrate best management practices and best available science for the restoration of critical habitats in our national forests,” Tate said.
The scientists evaluated two aspen restoration projects next to mountain streams in the Lassen National Forest. Prior to conifer tree removal, they evaluated stream temperature, water quality, stream shade, overstory tree canopy cover, aquatic insects, soil compaction and soil moisture. Followup data collection took place two to seven years after the conifers were removed from the stands.
Water quality did not change following the timber harvest. Removing conifers reduced canopy cover, which resulted in an increase of sunlight on the stream, but there was no corresponding increase in water temperature. Soil moisture increased in stands where conifers were removed, compared to aspen stands that were untreated.
“It is a real win-win when we can use contemporary timber management strategies to restore aspen in the absence of a natural fire regime, and also safeguard the health of our aquatic ecosystems,” Tate said.
An aspen stand released from conifer encroachment. The fallen conifers are in the foreground.