A Landscape Renewed by Fire

by on July 01, 2005

 

A fire-charred log is surrounded by rampant Lupinus propiquus, a species of bush lupine that appeared for several springs following the fire, then vanished.

Photo by Richard Blair, www.richardblair.com.

 

 

Enter the woods on Inverness Ridge and pause for a moment to listen. Natural history weaves itself into stories for those willing to hear—whether teased from the patterns in stone, distilled from the rings of a tree, or gathered from the melody of birdsong. Here in the forest, bishop pine trees whisper in the cool rush of an onshore breeze, their voices spinning a million-year-old tale.

In October 1995, several teenagers camping illegally on nearby Mount Vision lit a campfire. They drenched and buried the embers, but the fire apparently smoldered underground in old pine duff. Three days later, fed by 40-mph winds, the fire erupted. This combination of specific conditions—gusty weather, dry landscape, the meeting of spark and fuel-created the firestorm that followed.

The resinous, oily sap of a bishop pine tree burns hot, and the flames spread swiftly. A stiff northwesterly blew the blaze into the Paradise Ranch Estates, a housing development on the ridge above Inverness Park, torching 45 houses. On the second day the winds turned erratic, and the fire increased its area fivefold. The third day saw the winds shift to the northeast, and so began the fire’s march from the ridge to the sea, where it met what firefighters call “the great Pacific firebreak” at Limantour Beach.

In its course, the wildfire burned through a number of distinct habitats including bishop pine and Douglas fir forest, coastal scrub, grasslands, and riparian zones. All these ecosystems have been shaped over time by fire, yet each responds differently to the burn.

Bishop Pine Forest

The liquid whistle of an osprey pierces the rising shadows of the forest, where new trees now stubble the face of Mount Vision like stout whiskers on the jaw of an oft-shaven giant. Bishop pine (Pinus muricata) grows contorted by the contrary forces of high wind and thin, granitic soil. Once widespread (as suggested by the fossil record), it now occurs only in scattered stands along the California coast from Humboldt to Santa Barbara counties, with isolated populations south to central Baja.

A relict from the Pliocene epoch (two to five million years ago), the bishop pine belongs to the category of “closed cone pines,” which require intense heat to reproduce. These so-called pyrophytes evolved at a time when lightning-caused wildfires were more common. The bishop’s cones have scales sealed closed with pitch, and they open to release their seeds only under high temperature. Direct sunshine on a late summer or autumn day can pop open the occasional cone with a bang (the sound of which, overheard in the depths of the woods, can be startling), but most cones remain closed until touched by the heat of fire, thereupon “flowering” like grotesque tan blossoms and dropping their seeds onto the freshly charred earth in which they germinate best. Most every tree in a bishop pine forest is the same age, born from a single high-intensity blaze.

Such fire-adapted species are commonly found in landscapes with a Mediterranean climate. In these temperate zones of winter rain and summer drought, the prolific sprouts of spring dry up and leave plenty of tinder for the conflagrations of fall. Wildland fire is part of the life cycle.

A bishop pine forest untouched by fire grows decrepit, unable to regenerate itself. Bishop pines have a normal life span of 60 to 80 years, so any given forest prospers with one major fire on a regular basis within that time frame.

Here on Inverness Ridge, where the Vision Fire burned 1,000 acres of bishop pine forest, the “new” forest demonstrates fire’s regenerative power. Witnesses described the bishop pine seedlings coming up “like a carpet.” These trees, now ten years old, cover the ridge in dense thickets around the blackened snags of their cremated forebears. Muscular branches of the rare Marin manzanita reach up through the tangle, competing with ceanothus and huckleberry for the bright sunlight that penetrates the diminishing spaces between the 20-foot treetops.

In addition to renewing the trees, fire sets in motion a series of changes in the forest community, removing certain species from the stage and cueing the conditions for others to flourish. Ecologists call this “succession,” the natural sequence by which certain groups of plants and animals are replaced over time by others.

It begins in the soil. Soon after the Vision Fire, UC Berkeley microbiologist Tom Bruns documented changes in the subsurface populations of mycorrhizal fungi in the bishop pine forest. These symbiotic organisms colonize the fine roots of trees and plants and perform the specialized job of collecting water and nutrients from the soil; in exchange for these goods brokered by their fungal partners, the roots pay in sugar, which the fungi could not otherwise obtain.

Bruns, who collected root fungus species in the bishop pine forest both before and after the fire, discovered that the mycorrhizal community had completely changed. Different species colonize the roots of seedlings rather than those of mature trees, and he found that the fire shifted the underground population radically from the latter to the former. This indicates that a substantial “spore bank” of the fungi that colonize young trees had lain dormant in the soil for decades (probably since a previous such fire, as the one in 1927), waiting for the return of conditions under which they could flourish. By helping the bishop pine seedlings grow and survive, these early-successional fungi prepare the eventual fuel for future forest fires that will lead to their next generation.

The common bolete mushroom, Suillus pungens, allowed Bruns to track the spread of that mycorrhizal fungus through pre- and post-fire forest. Before the fire, he found huge swaths (more than 3,000 square feet) of genetically identical fungus, which means it had spread like a clone, vegetatively, from a single individual’s DNA. This was a tired population in need of renewal. But after the fire, the genotype of every Suillus pungens mushroom collected was different—the result of spore colonization, the fungal analog of sexual reproduction. Like a phoenix, healthy diversity had arisen from the ashes of monoculture.

Fire has other effects on the soil. It burns off the upper layers of duff, the decomposing leaves and twigs that fall and accumulate over the years, exposing the raw mineral soil underneath. Ash contains nutrients such as potassium, calcium, and phosphorus, which are returned to the soil wherever ash accumulates (dispersed in patterns by the flow of water and the currents of wind); but combustion vaporizes nitrogen, which becomes scarce in the ecosystem after a burn.

Enter the legumes, or members of the family Fabaceae, whose roots contain nodules that fix nitrogen in the soil. A “big bang” of clover, lotus, and lupine followed the fire, awakened from a dormant seedbank deep underground and no doubt energized by the revitalized mycorrhizae. Roughly 85 percent of vegetative cover by the second year was composed of these plants. They bloomed and set seed profusely, replenishing their own seedbank in the soil, then died off as the taller trees and shrubs overwhelmed them. Notable was Lupinus propinquus, the blue-flowered cousin of the yellow bush lupine (L. arboreus), which grew into five-foot shrubs by the end of the first year and painted the hills violet with their blossoms in the second spring. Today, L. propinquus is no longer found in the forested regions at all, but we can assume it will spring up again after the next fire.

Plants and insects ride the roller coaster together. Transitional communities of legumes, for example, attract a transient population of butterflies and moths, documented by UC Berkeley professor Jerry Powell. In the first year, native butterflies appeared abundantly in association with the lotus, especially the orange sulphur (Colias eurytheme) and the acmon blue (Icaricia acmon). Abundant milkmaids (Cardamine californica) attracted masses of the exquisite mustard white butterfly (Pieris napi), which flourished through three or four generations flying until October (highly unusual for this species). All declined in numbers as the low-growing plants yielded to manzanita and pine.

For the best example of life lurking on the margins, consider the rare yellow blossoms of rush-rose (Helianthemum scoparium), listed in 1990 as being “of doubtful occurrence in the national seashore.” This plant sprouted by the thousands after the fire, and within two years Powell discovered an association with a tiny black moth of the species Mompha, whose larvae fed on the rush-rose seeds. Enormous populations of the Mompha did not diminish by year five, and a specialist has declared this moth “previously undescribed,” i.e., a species unknown to science. “Based on past behavior of Helianthemum following large fires,” wrote Powell, “in time this plant and its predator will become rare again.” Sure enough, most of the Helianthemum were gone by year eight, and Powell has not seen any of the mystery Mompha in the last few years.

Another nitrogen fixer, the blue blossom (Ceanothus thyrsiflorus), proliferated from dormant seed on Inverness Ridge, stimulated to germinate by the intense heat and emboldened by sunlight, in the absence of the burned-away understory. A survey by San Francisco State biogeographer Barbara Holzman, who counted plants at 50 points along 30 transects in the post-fire forest, found this blue blossom had spread significantly, with an observed increase of almost 200 percent in the number of plants in the study area between years one and two. They reached ten feet tall in year five, codominant with bishop pine, and by year ten they have maintained a position of relative supremacy. Those growing in dense stands of pine will eventually fade, but the strong population along the forest margins and in the chaparral most likely will continue to thrive.

Bishop pine seedling
Bishop pines require intense heat to reproduce. The VisionFire gave the conditions for the renewal of the bishop pine forest ofthe Inverness Ridge. A new bishop pine seedling, with seed cap still inplace (Feb 1996). Photo by Charles Kennard.

Years three through seven saw the heyday of the Marin manzanita (Arctostaphylos virgata), another rare endemic. The heat of the Vision Fire, and the removal of the competing
undergrowth, triggered thousands of these seedlings in areas where no plants had been known to occur in our generation. Peeking through forest openings, they can reach treelike heights of 15 to 20 feet with gnarled red trunks and a gorgeous January bloom. But even now many ten-year-old manzanitas are suffering in the deepening shade of the rising bishop pine canopy, and don’t have long to live. Survivors will be rare. Yet plenty of new seed now waits underground, brooding on the future.

All these organisms compete with each other for limited resources of water, nutrients, and sunlight in the seemingly random patterns of the forest. Dense stands of bishop pine will slowly thin out as the stronger trees survive and the weaker die off; scientists estimate that only three of 100 trees in an even-aged stand like this will survive to maturity. But even dead trees or “snags” play a role-they are room and board for wood-boring insects and the birds that like to eat them, and they provide nesting and perching sites for raptors. Falling snags knock over young trees and create openings in the forest, allowing shrubs like coffeeberry, huckleberry, wax myrtle, thimbleberry, poison oak, and others to reach the sunlight. Some snags roll down slope and into streams, damming the flow and creating habitat for fish and other aquatic creatures.

Douglas Fir Forest

The northern end of Inverness Ridge remains the stronghold of bishop pine, while the shale-derived southern end is dominated by Douglas fir (Pseudotsuga menziesii). Mixed with oaks, bays, and madrones, and with a substory of hazelnut, red elderberry, and an occasional big-leaf maple, the Douglas fir community is more of a melting pot. This forest was heavily logged well into the 1950s, but many old-growth firs still remain, especially on the steeper slopes; some of these trees are 400 years old, rivaling redwoods in height and girth.

As Douglas fir ages, it develops a thick and spongy bark that insulates it from fire; over time the tree tends to shed its lower limbs, which might otherwise act as “ladder fuels” and carry fire into the crown. Thus adapted to the periodic burn, older Douglas firs survive most fires very well, and they regenerate readily in the aftermath. Forest fires not only clear duff from the forest floor, exposing the mineral soil in which the tree’s seeds prefer to grow; they also eliminate competition for the seedlings from the burned-away understory of more shade-tolerant species. As a result, Douglas fir occurs in mixed-age stands, with young saplings growing beside old granddaddies and every age in between—a marked contrast with the stand-replacing self-immolation of bishop pine.

The Vision Fire ran through 1,500 acres of Douglas fir forest, but most of this burned at a low to medium intensity, far less hot than the firestorm in the bishop pines. Thus many of the older Douglas firs were merely scorched on the outside and otherwise undamaged. The mortality rate of Douglas fir in this fire was 28 to 46 percent (vs. 42 to 82 percent mortality for bishop pine). Oaks, bays, and madrones here enthusiastically stump-sprouted after the blaze. The forest is also now peppered with a fresh new helping of young Douglas firs.

Coastal Scrub

As the ridge descends toward the Pacific, the forest gives way to coastal scrub, a “soft chaparral” that occurs in patches at middle elevations on the western side of the ridge and on the northern point. Here the songs of white-crowned sparrows and wrentits serenade the sunrise, and trails of gray fox and mule deer weave among the coyote brush. Other common plants like sword fern, lizard tail, cow parsnip, coffeeberry, bracken fern, salal, huckleberry, blackberry, and poison oak compose the thick tangle of a northern coastal scrub plant community, especially on the north-facing slopes that conserve more moisture. Certain dry south-facing slopes contain California sagebrush (Artemisia californica), an aromatic species with feathered gray-green foliage more common in Southern California; here at Point Reyes it reaches the northernmost limit of its range.

All these scrub species respond well to fire, and regrowth after the Vision Fire was rampant. Today the casual observer can hardly tell that a fire happened here at all. But the scrub usually burns in a patchwork pattern, forming a checkerboard of early- and late-stage growth; this maintains the equilibrium of the habitat, with food and shelter for animals redistributed according to the vagaries of the fire. The Vision Fire disproportionately “zeroed the odometer” on thousands of acres of this habitat.

This spells difficult times ahead for the Point Reyes mountain beaver (Aplodontia rufa phaea), an unusual and prehistoric species found nowhere else in the world. Roughly the size of a muskrat, the mountain beaver digs tunnels in cool, north-facing slopes under moderately dense coastal scrub; its primitive kidneys require it to live in moist areas and to drink one-third of its body weight in water every day. It feeds on coyote brush, sword fern, cow parsnip, and other scrub vegetation.

The Vision Fire burned 40 percent of the mountain beaver’s known range, including the majority of its prime habitat. A vast network of holes was revealed underneath the burned-away scrub. Roughly 98 percent of the mountain beaver population in the burn area died in their burrows—roasted, asphyxiated, or parched.

But all was not lost. Monitoring eight sites that showed post-fire activity, U.S. Geological Survey biologist Gary Fellers found signs of recovery at all but one site. The presence or absence of cow parsnip (Heracleum lanatum) appeared to play a factor in the survival of the rare rodents—presumably because this robust shrub grows quickly and thus provides more immediate food and shelter. But to thrive,

the mountain beavers require the thicker protection and taller architecture of a late-stage scrub community—especially the taller woody shrubs like coyote brush and coffeeberry, which can reach ten feet in height and usually shed their lower limbs as they grow, opening up the understory beneath them. Fellers says we must wait at least five more years, probably more, before this biome achieves the maturity necessary to support the estimated pre-fire population of 5,000 animals.

Riparian Corridor

Rivers, streams, and creeks-riparian corridors-trace serpentine lines of living green upon the landscape. Because they channel fresh water, these zones grow more lush trees and vegetation, and host more wildlife. In a fire, the presence of water means that plants are better hydrated, and thus more resistant to burn. Indeed, the 500 acres of riparian woodland in the Vision Fire perimeter burned at low (and occasionally medium) intensity, with a tree mortality rate of only 5 percent.

Wherever the riparian zone does burn, turnover happens fast-blackberry, thimbleberry, rush, nettle, man-root, poison oak, and cow parsnip swiftly recolonize the burned areas, then yield to fast-returning willow and alder, bay and buckeye. Sediment and ash washed downhill from the denuded forest leave their fertilizing deposits in rich alluvial fans, spurring even greater growth. Many animals displaced from other burn zones take refuge in the riparian corridors; here the fast-growing shoots make good forage for mammals, while the tangle of old and new growth provides excellent shelter for rodents, reptiles, and birds. According to Powell, who monitored Lepidoptera larvae, plants on the ridge in the bishop pine forest accumulated their caterpillar faunas slowly, while the lower canyon’s riparian woods reestablished Lepidoptera sooner and more quickly.

PRBO Conservation Science biologists Geoff Geupel and Tom Gardali discovered interesting fire-driven dynamics in the riparian bird community. Monitoring song sparrows (Melospiza melodia) in burned vs. unburned sites after the fire, they found significantly more birds in the burned area. As the song sparrow builds its nests in low shrubs, it likely benefited from the swift and dense regeneration of the riparian understory, and moved there en masse. An analogous boost in seed production, small insects, and other food sources likely drove the return as well.

Other bird species, such as the American goldfinch and the Allen’s hummingbird, show more flexibility in their nest-height placement—yet they were also more abundant at the burned sites than in the unburned. The proliferation of early food sources (seeds for the goldfinches, nectar for the hummingbirds) may have drawn these species back to the burn zone. Not surprisingly, most birds that live primarily in trees (e.g., the chestnut-backed chickadee and Pacific-slope flycatcher) were found in reduced numbers at the burned site in the first few years; they will likely return only as the alders and willows of the riparian woodlands regrow.

All the above factors would account for increased numbers of birds, but not necessarily for their increased reproductive success. The latter is a better indicator of habitat quality, and hence important to measure (though difficult to quantify). Geupel’s and Gardali’s nest survival and population growth models for song sparrows suggest that the burned site became a population “source”for the region, while unburned sites were “sinks.” What accounts for the vigor that characterizes these numbers? No matter what we choose to call that special ingredient, fire appears to deliver it.

GrasslandsThe coastal prairie, composed of perennial bunchgrasses and sod, once covered vast stretches of Point Reyes, occupying the lower elevations roughly between the scrub and the beach. As with all of California’s native grasslands, this habitat has succumbed to a number of outside forces that disrupted their natural cycles. First came the Europeans, whose introduced annual grasses overwhelmed the natives with their sheer insurmountable numbers. Next came the beef and dairy cattle, allowed by ranchers to overgraze the bunchgrasses. Finally came the roads and the cars and the houses, the growth of the wildland-urban interface, and the need for fire suppresion, in stark contrast to the practices of the indigenous Miwok, who regularly burned grasslands to keep them fresh and productive.

Today the grasslands of Point Reyes are more rangeland than prairie, a weed patch of European annuals and grazing grounds for seven dairy and 15 beef operations. But native bunchgrasses such as hair grass (Deschampsia), oat grass (Danthonia), and fescue still bear up through the invaders, complemented by wildflowers like goldfields, checker-bloom, buttercups, Indian paintbrush, and Douglas iris. What few grasslands remained in the region touched by the Vision Fire lay blackened and fallow for one season of rain, then erupted that first spring with new life-the first and fastest zone of transition.

A Shifting Mosaic

Nothing in nature is cut-and-dried; things occur by degrees. Such is the case with fire, which yields different effects on the landscape at different intensities of burn. Studies after the Vision Fire found that 70 percent of the vegetation burned at low intensity, 20 percent at moderate intensity, and 10 percent at high intensity. The result was a kaleidoscope turned in different ways—soft and easy in the low-intensity burn zone, where many of the old trees survived, vs. hard and fast in the center of the firestorm, where the old trees were incinerated. The uneven distribution of ash and sediment from erosion is blown by the wind and washed by the rain, settling unevenly upon the landscape and further complicating the patterns of regrowth. This long-term life cycle dances to a tune called by fire, ancient destroyer and creator, artist extraordinaire.

Native plants so invigorated push the boundaries of their own communities, dictating the watershed-wide laws of succession. A grassland untouched by fire turns into scrub as coyote brush and other pioneer shrubs move in; scrub unburned may become evergreen forest with the advance of Douglas fir. Small fires along the edges keep the boundaries in flux—or a high-intensity crown fire can torch the entire forest, setting the stage again for grassland.

We find that all roads lead back to the source, where a spark in the shadow of an old-growth forest carries all the metaphor of myth. Such ancient connections are like time-ripened seeds held in store for the calamity, yielding beauty from ashes.

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