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Nature’s Fire Fuel Reducers

Climate change in recent years has extended the length of the fire season and increased the intensity and frequency of wildfires in our area, as is the case through much of the American West. Recent analyses commissioned by the Midpeninsula Open Space District and the Santa Cruz Mountains Stewardship Network suggest these trends will continue. In 20 to 40 years from now, at Jasper Ridge we can expect summer, winter, and annual temperatures to be about 3oF to 6oF hotter than was the case near the end of the 20th century, annual precipitation to decrease, and soils and vegetation to become drier.  As a result, wildfire risk is predicted to increase by perhaps 1-2% locally, and by as much as 3% in nearby parts of the Santa Cruz Mountains.

Maps produced by the the Santa Cruz Mountains Climate Adaptation Project. Red-bordered dots mark the location of Jasper Ridge Biological Preserve.

In response, Jasper Ridge Biological Preserve is embarking on a long-term experiment to understand how to accomplish two goals that at face value are contradictory: reducing the risk of wildfire while at the same time maintaining its naturally functioning ecosystem and biodiversity, so critical for sustaining the research, education, and conservation missions for which the preserve was created. 

We are not alone in grappling with how to achieve both goals at the same time. What we learn here will be important far beyond our borders, for example, in helping California succeed in two of its major environmental initiatives, the Community Wildfire Prevention and Mitigation Program, and the 30x30 Conservation Plan.

Usually fire-fuels reduction occurs through mechanical means. Heavy equipment such as masticators are sometimes used, which disturb soil and ground cover, can accelerate erosion, and open the way for invasive species. More often, hand-crews wielding chain-saws limb trees, remove all vegetation up to a height of several feet, and haul woody debris to chippers, which grind it to small pieces that are either spread locally or disposed of off-site. The result is a landscape with no ladder fuels (good for reducing fire risk), but also no habitat for small ground-dwelling animals and reduced habitat for large ones, pathways for non-native and noxious plant species to invade, and altered soil-moisture and soil-quality regimes (bad for biodiversity and ecosystem function.)

A masticator at work clearing ladder fuels along one of Jasper Ridge’s borders in 2021.

Nature invented different solutions for reducing fire fuels, solutions that maintain biodiversity and ecosystem function at the same time.  One of those is herbivores, which in prehistoric times at Jasper Ridge included black-tail deer, Tule elk, and pronghorn antelope. Deer are browsers, which means their main diet consists of tender leaves and fresh shoots of bushes and low-hanging tree branches. Pronghorn are grazers, which means they concentrate on grasses.  And Tule elk are in between, utilizing both grasses and shrubs, with their large size having the added effect of breaking through and opening up dense stands of vegetation. The combined effect of diverse and abundant herbivores kept ladder fuels much reduced relative to the present condition by converting vegetation to animal biomass. Animal waste products kept the soil fertile and the churning of many hooves increased soil permeability. All of which led to landscapes that were more productive, but with fewer fire fuels. 

The other solution that nature has come up with is fire itself: California grasslands, woodlands, and chaparral have in a very real sense evolved to burn.  At Jasper Ridge and surrounding areas, prehistoric fires burned through about every 10 to 16 years as recorded by fire scars in redwood stumps, and chaparral like that found in Jasper Ridge typically burned every 30 to 90 years. Many of these fires probably resulted from intentional cultural burning by native Ohlone peoples. Lightning too probably ignited some fires, although lightning is relatively rare in our area. These naturally-occurring fire intervals had the net effect of clearing out ladder fuels and opening up patches of the landscape on a fairly regular basis.

Neither of nature’s solutions have been operative through much of California over the past century or so. At Jasper Ridge, fire has not cleared understory in well over a hundred years, Tule elk and pronghorn have been absent since Ohlone times, and the last regular grazing was by cattle in the 1960s.

Now, as part of our ongoing efforts at reducing fire risk in ecologically sustainable ways at Jasper Ridge, we are beginning to reintroduce nature’s herbivore solution for fire-fuel reduction. This effort is in conjunction with Stanford’s Wildfire Management Plan, which was developed to reduce fire risk throughout Stanford lands in a sustainable way, and is part of our Anthropocene Biodiversity and Science for Land Stewardship Initiatives highlighted in our strategic plan.

An initial foray into this experiment launched in 2018, when goats were first used to effectively reduce fire-fuels in the primarily grassland habitat of the Boething area of Jasper Ridge. Now we are expanding the experiment by bringing three kinds of large and medium-size herbivores into areas bordering our Westridge fence-line. Beginning April 18, Boer goats, Dorper sheep, and Rambouillet sheep will be browsing and grazing near Mapache Gate. The goats are known to be effective at reducing a wide variety of ladder fuels, ranging from grasses to low-hanging tree limbs, while the two sheep varieties tend to concentrate on many different kinds of low-growing vegetation.  

This year’s trial will essentially be a learn-as-we go experiment.  The goal is to assess the effectiveness of the three kinds of herbivores, both singly and together, in reducing ladder fuels in blue oak woodlands, canyon oak woodlands, poison oak scrub, and chamise chaparral.  One part of the area to be grazed and browsed was cleared of ladder fuels last year by using a remote-controlled masticator (pictured above). In this area we will assess how well the various species do in maintaining the previously-treated vegetation in fire-safe conditions, particularly their control of invasive species such as thistles that are beginning to colonize the masticated area. Other parts of the area where animals will be introduced have vegetation that has not seen browsing, grazing, or mechanical treatments in decades, probably in more than a century.

In all of the trial areas, we will be collecting information on how vegetation, soil, and small-animal habitat responds over the coming seasons and years. The animals will be confined to study areas by moveable electric fencing; some areas will employ only goats, some only sheep, and some both sheep and goats. Also in the mix will be sheep dogs on guard to deter mountain lions.

The herders will be camped inside Jasper Ridge near the end of Road E. During the duration of the experiment, Trail 12 and access through Mapache Gate will be closed to general use.

The work offers a unique opportunity to blend basic science with on-the-ground adaptive management. As work progresses and information accumulates over the next few years, we anticipate not only decreasing fire-risk in our part of the wildland-urban interface, but also providing critical information about how fire-risk can be reduced in ways compatible with maintaining healthy ecological function and biodiversity.