The lush beauty of a meadow inspires a natural simplicity, yet meadow ecology is one of the most complex sciences. John Muir described the Sierra Nevada's intricate meadows as "so complete that you cannot see the ground and, at the same time, so brightly enameled with flowers and butterflies that it may well be called a garden-meadow, or meadow-garden." Ranger Carl Sharsmith peeked underneath the vegetation, where, upon one occasion, he found a shiny black Mount Lyell salamander—rarely seen since its earlier 1915 discovery—beneath a monkeyflower along a cold stream in a moist alpine meadow.
Meadows are sponges, absorbing water as snowpack melts and holding that water like an underground water tower. At just 3% of Yosemite National Park's area, meadows contain a much larger proportion of Yosemite's species. Recently, in 2003, an entirely new species of orchid, called the Yosemite bog-orchid, was discovered in Yosemite's meadows. It's possible, in fact, that one-third of all of the plant species found in Yosemite occur in meadows. In terms of wildlife, a large portion of the park's animal populations, such as willow flycatchers, mule deer, and small mammals such as the Belding's ground squirrels and pocket gophers, depend on meadow habitat to feed or breed. And, people benefit, too: Most of San Francisco's water is filtered by Yosemite's meadows. Meadows, as biologically rich sponges, have absorbed the attention of California's scientists, therefore. Ecological studies of Yosemite's meadows began in the mid-1960s and many others continue today.
In the simplest terms, what defines a meadow? Hydrology, vegetation, and soil characteristics. Note that "meadow" is not a technical term as much as a cultural term. Classification systems, set by state and federal regulation, often set detailed guidelines that differ as to what constitutes a meadow. Sierra Nevada scientists who study meadows, therefore, have come to a consensus:
A meadow is an ecosystem type composed of one or more plant communities dominated by herbaceous species.
It supports plants that use surface water and/or shallow ground water (generally at depths of less than one meter).
Woody vegetation, like trees or shrubs, may occur and be dense but not dominant.
Wetland types, namely wet meadows and fens, typically form what are referred to as meadows in the Sierra Nevada.
Although 3% sounds like a small percent of meadow habitat today within Yosemite, this tiny fraction mirrors the amount the park had more than a century ago as well as the amount of meadow habitat present in the broader Sierra Nevada range. Especially known for its meadow complexes in Wawona, Tuolumne and Yosemite Valley, Yosemite National Park supports approximately 3,000 meadows that range in elevation from 3,300 feet to more than 12,000 feet. Visualize meadows, perhaps, as "islands" of high diversity—in a network of habitat patches. Meadows in Yosemite are believed to have been stable for approximately the past 10,000 years, since close to the time of the last ice age.
History of Yosemite's Meadows
When Euro-American settlers first set their eyes on Yosemite's meadows in the 1850s, large meadow complexes were surrounded by open conifer forests and oak woodlands. Deer and small mammals probably lightly grazed the meadows. Conditions rapidly changed as settlers transported stock, both cattle and sheep, onto Yosemite Valley meadows and often planted non-native forage species for grazing purposes. Summer grazing at higher elevations by "hoofed locusts," as Muir referred to domestic sheep, could denude a landscape. In the 1860s, grazing in the mountains increased due to droughts in the Central Valley—during the peak of the wool industry after the decline of the Gold Rush. Joseph LeConte, in 1870, observed: "Tuolumne Meadows are celebrated for their fine pasturage. Some twelve to fifteen thousand sheep are now pastured here. They are divided into flocks of about twenty-five hundred to three thousand." Use of meadows by sheep declined shortly after 1900. Settlers also influenced meadow ecology by stopping anthropogenic burning of the meadows by the American Indians. From an ecological viewpoint, it is likely that fire historically promoted meadow stability by reducing the encroachment of surrounding forests; however, based on the observed geologic stability of many of Yosemite's meadows, it is unlikely that those Indian fires were key to maintaining all meadows. Today, more than a century later, it remains a huge scientific (and management) challenge to decipher the relative impacts to Tuolumne Meadows of a variety of factors—including historic sheep grazing and burning, more "modern" infrastructure developments, current recreational use, and documented climatic changes like less snowpack and earlier snowmelt.
How Meadows Develop Until recently, typical academic theory taught botanists that meadow development resulted from succession—a glacial terrain that evolves into a lake, a meadow and then a forest. We know now that not all meadows are part of this successional path: not all will change at some future point into a forest. Meadow location, rather, is determined by topography. On the western slope of the Sierra Nevada, for instance, meadows occur in situations where a relatively flat landform is surrounded by steep terrain with a large watershed that offers a shallow water table and fine textured soils.
Identifying Zones of Wetness During spring, rising groundwater levels and streams inundate meadows, bringing nutrients that sustain the landscape. Water table and soil moisture gradients strongly influence vegetation composition and structure in these wetlands. Most meadows contain a complex mosaic of wet, moist, and dry areas that support distinctly different plant and animal communities. Wet areas (or wetlands, including fens and riparian wetlands) are flooded or saturated with water for much of the growing season with sufficient moisture during the late-growing season to nurture plants through the critical reproductive stage. Fens, which make up the wettest portions of wet meadows, allow for a slowed decomposition of dead plant litter promoting at least 40 centimeters of peat accumulation in the top 80 centimeters of soil. Fens can be found in Yosemite at various sites, including Happy Isles in the Yosemite Valley, Crane Flat, Badger Pass, and Virginia Canyon. Moist areas may be flooded soon after snowmelt but do not always stay saturated through the growing season due to a lowering of the water table or a lack of precipitation. Drier areas with "xeric" meadow plants are often on the meadow margin but may be scattered throughout on high spots with a deeper water table. Meadow elevation also strongly determines the types of vegetation that occur due to differences in growing season length, climate, soil development, and glacial history. "Montane" meadows are low enough to be surrounded by forest (a 3,000- to 8,000-foot elevational band). "Subalpine" meadows are near the upper limit of trees (8,000 to 9,500 feet). "Alpine" meadows are, by definition, above tree-line and are generally surrounded by rock and boulders (above 9,500 feet). Yosemite's wet meadows support plants called hydrophytes that have structural, morphological and life-cycle adaptations allowing them to survive in an oxygen-deprived conditions. A true wetland incorporates a high water table that causes waterlogged and anaerobic (or oxygen lacking) conditions that wet meadow vegetation can tolerate but is inhospitable to competing upland vegetation communities. Some slow-growing wetland plants then adapt, in part, through a pressurized gas flow: air that moves into the aerial leaves (positioned above the water) and is forced down through the stem into the roots by a slight pressure generated by temperature and water vapor pressure.
Specific to Sierra Nevada's Meadows
How do Sierra Nevada meadows differ from meadows in other parts of the country? The Sierra Nevada's deep snowpack, relative to other mountain systems, is a critically important source of water for high-elevation meadows. California has a "Mediterranean" climate with very little summer precipitation compared to other parts of the country. Therefore, while meadows elsewhere may depend on year-round precipitation, Sierra Nevada meadows rely on shallow underground water supplied by snowmelt. Finally, the Sierra Nevada is dominated by granitic bedrock, which supports wet meadows with a neutral to slightly acidic pH runoff rather than an alkaline runoff.
Meadow's Many Functions Besides supporting species not found elsewhere, meadows offer many functions. Meadows, for instance, filter sediment from water flowing from surrounding slopes—providing clean water for wildlife and healthy habitat for aquatic animals that live in lakes and streams. Meadows provide an important breeding ground for invertebrates that form one base of the food web; meadow invertebrates are a key food source for many birds, amphibians, and reptiles. Meadow plants also provide food and habitat structure for small mammals that, in turn, provide an important prey base for raptors, coyotes, and other predators. Individual wetlands can provide different functions—for instance, one can provide excellent flood or pollution control while another offers groundwater recharge capabilities.
Challenging Meadow Health
Though Yosemite’s meadows exhibit long-term stability, they are potentially very sensitive to human-caused changes. High-elevation meadows have a short growing season with relatively shallow soil and may be very sensitive to even small changes in water availability. Even in protected wilderness areas, human-induced factors of change include altered precipitation (especially the timing and amount of snowmelt), trampling, grazing, roads, non-native plants, air pollutants, and altered fire regimes. Any factor that alters the underlying hydrology of meadow systems has the potential to shift species composition of these mountain wetlands.
Within Yosemite, large declines in meadow area have been witnessed within Yosemite Valley. In the late 1800s, narrative accounts indicate that Yosemite Valley’s meadows were wetter and more vast than they are now. Scientists have determined the Valley’s meadow acreage has decreased by about half today—from more than 700 acres to just over 300—due to farming activities like plowing and seeding and to fire suppression.
G.G. Goucher, a Yosemite state commissioner, in 1889: “There is not any lady who visits Yosemite Valley that is apt to go on to those marshy meadows. I know that the meadow between Barnard’s and Cook’s, which is fenced, but which has not been plowed, a great deal of it is under water, which is backed up from overflows.”
Galen Clark in 1894, reflecting on four decades: “My first visit to Yosemite was the summer of 1855. At that time, there was no undergrowth of young trees to obstruct clear open views in any part of the valley from one side of the Merced River across to the base of the opposite wall … the area was clear open meadow ground, with an abundance of luxuriant native grasses and flowering plants, at least four times as large as at the present time.”
An increase of conifers into a meadow is referred to as conifer encroachment. Within Yosemite Valley, incense-cedar and ponderosa pine are among the conifers that have been spreading into meadows. Conifer encroachment has taken hold also in the higher-elevation meadows with lodgepole pines. Scientists are trying to understand how widespread conifer encroachment is within Yosemite's 3,000 meadows and whether it is caused by a big-scale regional change, such as earlier annual snowmelt, more local and site specific disturbances, such as trampling, or natural plant succession from meadow to forest. Lodgepole pines, interestingly, appear to grow well in conditions that other conifers would find unfavorable such as inundated soils. For close to a century, the National Park Service has cleared young lodgepole pines in meadows to mitigate the perceived tree encroachment—even placing CCC crews there in the 1930s to do the job.
While Yosemite's high-elevation meadows have a low incidence of non-native plants, research strongly suggests they are no way immune to invasion. Lower-elevation meadows, for instance, have been invaded by velvet grass, oxeye daisy and bull thistle. Nitrogen pollution from atmospheric deposition has the potential to affect productivity and species composition of wetland vegetation, and, depending on seasonal timing, may affect aquatic organisms such as algae, microbes, invertebrates, and amphibians. Impacts on the primary producers of the aquatic food chain have the potential to significantly alter all wetland biota.
Raising the Restoration Question
Any meadow restoration effort must involve restoring the hydrologic processes that allow for a shallow water table throughout the summer. Scientists have restored the Happy Isles fen, which had a parking lot placed over a portion in 1928. In the 1980s, the pavement was removed, and, in 1993, more work included "decompaction" of the soil, removal of non-native species and planting of native species, such as Carex amplifolia and Scirpus microcarpus, to restore the original hydrologic regime and conserve the biological integrity of the Happy Isles plant community. Climatic change may be a threat to late-season water supply as it has the potential to affect groundwater levels and vegetation across Yosemite's meadow. The effects of earlier snowmelt, earlier drying out of ephemeral streams, and lower groundwater levels on meadow ecosystems should be studied.
So, What Can You Do? It's important to protect the meadows before they are impacted. One person or animal crossing a meadow is insignificant, but 100 people or 1,000 people can cause stress—resulting in a lasting change. Stay on designated trails because social trails, often found in El Cap Meadow and Tuolumne Meadows, can result in the "closure" of a meadow in order to restore the damage done. High-elevation meadow vegetation can be very slow to recover from trampling disturbance. Repeated trampling kills plants, cuts the sod to expose soil below the rooting zone soil, and increases erosion. Too much soil compaction can also reduce the ability of meadow soils to hold the shallow watertable that is vital for the health of meadow vegetation. Wherever you go in Yosemite, tread lightly and avoid decorating nature's meadows with footprints and tire tracks.
"Now that the annual May floods have transformed a large part of the meadows into a clear calm lake, we understand what at first seemed an inexplicable mystery—namely, why a raised wooden pathway has been built right across the meadows, between the two hotels. If the waters go on rising, it will soon be a necessary bridge." – Valley resident F. Gordon Cummings in 1884