Traveling the western U.S. state of Nevada in the 1860s, a young American writer named Mark Twain heard a “world of talk” about the beauty of Lake Tahoe and so set out one August day to see the lake perched high in the Sierra Nevada Mountains. Determined to make the 11-mile journey on foot, Twain and his companions became briefly discouraged after toiling up one mountain and then another to no avail. But they trudged on until “at last the Lake burst upon us,” Twain wrote in his 1872 book, Roughing It.
“As it lay there with the shadows of the mountains brilliantly photographed upon its still surface,” he famously continued, “I thought it must surely be the fairest picture the whole earth affords.”
One hundred and fifty years later, visitors to this border region between California and Nevada still marvel at Lake Tahoe, but sadly the “jewel of the Sierra” is not the same gem it was in Twain’s time. Boating on the lake during his trip, Twain said he could see rocks at least 80 feet below the surface—a degree of clarity that scientists attribute to “ultra-oligotrophic” conditions, meaning exceptionally nutrient-poor and pure. Since the 1960s, however, Lake Tahoe’s renowned water quality has been in slow decline, and exactly why “is the $64,000 question,” says soil scientist Dale Johnson of the University of Nevada in Reno.
Construction of hotels, resorts, and casinos to accommodate the region’s millions of contemporary tourists has undoubtedly contributed nitrogen and phosphorus to the lake, which cloud the water by encouraging the growth of algae. Ammonia and other airborne sources of nutrients likely feed the microscopic plants, as well.
But when Johnson arrived in the area nearly 25 years ago, a third possibility caught his eye as he sat in his campus office one day wondering what new research challenge to tackle. “I looked out the window, and there were billows of smoke from a wildfire,” he recalls. “And I thought, ‘Oh! I’ll work on fire.’”
Damaging blazes sweep the Tahoe region regularly, and by the time Johnson teamed up with his University of Nevada colleague, Wally Miller—an expert on Tahoe’s water quality—scientists already knew how catastrophic fires could affect downstream streams and lakes. Wildfire can leave slopes bare of vegetation and unprotected from erosion. Burning also releases nitrogen, phosphorus, sulfate, and other biochemicals locked in plants, wood, and soils, allowing the nutrients to run off downslope or seep into groundwater.
But when Miller and Johnson began their studies, they discovered something that few, if anyone, had suspected: Not only could a major disturbance like fire affect Lake Tahoe, but so too could a lack of disturbance—the absence of fire. “The results were very surprising and therefore very hard to publish,” Johnson says. “No one believed it.”
Natural Part of the Ecosystem
When people think of wildfire today, their minds usually go to catastrophic events sparked by people, such as the 2002 Gondola Fire, which charred 675 acres of a slope facing Lake Tahoe after someone riding in a gondola flicked a smoldering cigarette into the forest below. But for centuries before white settlement, wildfire was a natural part of Sierran ecosystems, with minor fires sweeping through at intervals of 10 to 25 years.
Thanks to these frequent blazes, the forests surrounding Lake Tahoe were historically open and park-like, with large, scattered trees, such as Jeffrey pine and white fir, and little to no understory vegetation or woody debris. When white settlers arrived in the region, however, they began fighting and suppressing wildfires wherever they sprang up, both to safeguard their homes and lives and the region’s burgeoning timber industry.
There has in fact been a policy of near-total fire suppression in the Lake Tahoe Basin and across the American West for more than 100 years now, and naturally forests have changed as a result. Trees crowd together, shrubs and saplings are profuse, and dead branches, logs, and other “fuels” exceed 600,000 pounds per acre in some locations, Miller and Johnson say, now that periodic fires aren’t occurring to clear them away.
Moreover, when fire is suppressed for long periods, the layer of organic matter on the forest floor—what soil scientists term the “O” horizon of soil—thickens substantially; in some places in the Tahoe region, it’s now 6 to 8 inches deep or more, Miller says.
At the time he and Johnson began their work, scientists and forest managers assumed that this thick bed of accumulated material, or litter, would intercept and trap any available nitrogen and phosphorus and that trees and understory plants would then quickly take the nutrients up. Most available nutrients would stay cycling within the forest, in other words, leaving little extra to run off into Lake Tahoe. Or so the thinking went. Miller and Johnson soon learned otherwise.
Most studies of nutrient runoff focus on water that streams overland during rainstorms or snowmelt, but what Miller and Johnson focused on instead was something called “litter interflow.” As its name implies, litter interflow of water doesn’t occur visibly on the soil surface, but subtly within the organic, ground litter—specifically at the zone where the O horizon meets the often water-repellent mineral soil underneath.
Litter interflow moves so differently from surface runoff that the researchers’ first challenge was figuring out how to capture it (they eventually did so with a special collector). Then came the surprise: When they measured nitrogen and phosphorus in the interflow water, they found concentrations of ammonium-nitrogen and phosphate at least 100 times greater than the average levels in nearby streams.
Their findings suggested that if this water was reaching Lake Tahoe, it could be a substantial source of nutrients to the lake—and thus another source of the lake’s water quality problems. But convincing others has taken some doing. Litter inflow is slow-moving and invisible, for one. More importantly, the idea that large amounts of nutrients could leak from an undisturbed forest ran counter to common belief.
But the forests around Lake Tahoe are in fact disturbed, Miller and Johnson argue: They’ve been taken from their natural, fire-prone state to one in which fire is uncommon, and this change could explain why interflow water is so replete with nitrogen and phosphorus. Back when wildfires swept through the area every decade or so, the organic, O horizon layer was much thinner, Miller explains: maybe just an inch thick or so. Today, in contrast, “we have up to a foot of O horizon material that has a lot of nutrients in it,” he says. “And,” he adds, “no roots.”
The exceptional dryness of the O horizon in Sierran forests is what keeps trees from rooting in it, adds Johnson, and to him this is the key. “In other places where I’ve worked, like eastern Tennessee and western Washington where it’s wet, the O horizon is full of roots because that’s where the nutrients are being released [by microorganisms],” he says. In the Tahoe region, on the other hand, bacteria and fungi still decompose organic material and make nutrients available. The difference is that no roots are present to take those nutrients up.
“So, there the nutrients sit,” Johnson says, “waiting to get washed away.”
A Careful Balance
Or burned away. It’s well known that forests crowded with trees, downed branches, and other combustible materials are extremely vulnerable to severe wildfires. And these fires not only torch trees, but consume nitrogen and other nutrients needed by trees, as well.
Johnson, Miller, and others have found, for example, that intense burning of the O horizon can volatilize large amounts of nitrogen: The nutrient literally goes up in smoke. If enough nitrogen is lost over time, it can become a concern for forest managers, who want to maintain the productivity of new trees after each round of timber harvesting.
Meanwhile, the flames break down proteins and other organic constituents in living and dead plant material to release the nitrogen compound, ammonium. Later on, microbes known as nitrifying bacteria convert this new supply of ammonium to nitrate, a form of inorganic nitrogen that readily runs off.
Consequently, fire not only burns up nutrients, but also transforms relatively stable, long-term stores of nitrogen and phosphorus into “mobile” forms that can be carried away into tributary streams feeding Lake Tahoe—where the nutrients feed algae instead of trees. Such runoff is minimal in a dry year following fire. But when heavy rains occur, “you can have a very significant transport into the lake,” Miller says, “and a big impact on water quality.”
The findings lead to a dilemma. On the one hand, wildfire is beneficial in that it burns away decades of accumulated organic material, which Johnson and Miller have shown can potentially be a slow but significant source of nutrients to Lake Tahoe. On the other hand, severe wildfire can also deplete forests of nitrogen and send pulses of nutrients downstream—not to mention the damage it does to timber and private property.
“So what do you do?” Miller asks. The answer lately embraced by forest managers is to mimic the natural, Sierran fire regime that existed before aggressive fire suppression began. Managers will purposely light small, controlled fires to burn away accumulated woody debris. Or they’ll remove wood with harvesting equipment or chip it on site.
But managers don’t want to remove too many nutrients from the system, since they’re needed to support future generations of trees. So Miller and Johnson have been studying the practices’ impacts on nutrients and on water quality, just as they earlier investigated catastrophic wildfire and fire suppression.
As both scientists move into retirement this summer, however, one important task remains undone: drawing a direct connection between what’s occurring in the forests surrounding Lake Tahoe and the conditions in the lake itself. “We have what’s going on in the watersheds, and we have what’s going on in the tributaries and the lake,” Miller says. “But we haven’t been able to make the link.”
They may not have established that link, but their work does reinforce another connection: the indelible tie between people and water bodies like Lake Tahoe. For years, people have recognized urban development, fire, agriculture, and other obvious disturbances as threats to water quality; now Johnson and Miller have shown that even actions performed in the name of protecting ecosystems—such as suppressing fire—can take their toll, as well.
That’s the irony of Lake Tahoe, Johnson adds. While decades of controlling wild fire has preserved timber resources and the nitrogen that sustains the region’s forests, the practice has also produced a nutrient-laden system that is likely helping pollute Tahoe’s once-pristine waters.
“We like our water to be oligotrophic, and we like our vegetation to be ‘eutrophic’,” Johnson says, or nutrient-rich and productive. “And that just doesn’t work out.”