Envisaging the future Of California's landscapes In a time of rapid climate change

According to the Intergovernmental Panel on Climate Change (IPCC), global emissions of greenhouse gases resulting from human activity have grown 70 percent since 1970. During that same time, carbon dioxide (CO2) emissions increased by 80 percent, due primarily to the use of fossil fuels and land-use changes. In 2005, atmospheric concentrations of CO2 reached 379 ppm (parts per million), far outside the natural range scientists have documented for the last 650,000 years. The IPCC Report, Climate Change 2007, goes on to state that this increase in anthropogenic greenhouse gases is very likely the cause of the observed increase in global average temperature since the mid-twentieth century, and that this warming has had "a discernible influence on observed changes in many physical and biological systems."

These changes, already underway in California, are certain to accelerate. A recent paper in the journal Geophysical Research Letters, by Purdue University climate-modeler Noah Diffenbaugh and his colleagues, pinpointed the American Southwest, including major portions of California, as an area that will be particularly hard hit by climate change. Once an ecosystem is lost to climate change, it is impossible to restore it to its original condition. However, we must do what we can, and, though bearing in mind our limitations, a number of UC researchers are working at NRS reserves and in UC research forests to identify climate-change impacts and to help government agencies and other land managers respond to these changes and avoid further losses.

A Series of Cascading Consequences
Forestlands cover 45 percent of California. How these forests respond to the current unprecedented levels (at least, since 650,000 years ago) of atmospheric CO2 will have a major impact on the state's environmental and economic viability. Standing forests take in CO2 and sequester it from the atmosphere. But once those forests are destroyed in a fire, or logged, or have died off, the carbon they store is eventually released back into the atmosphere, adding to CO2 levels from other sources.

UC Berkeley professor John Battles and his colleagues have worked closely with the state government's California Climate Change Research Center, part of the state's Energy Commission () to describe the changes now underway in our forests and the potential future course of these changes. "The climate models project very large increases in temperature and more modest decreases in precipitation, certainly for the Sierra Nevada," says Battles. "The models can't anticipate the climate in the coastal forests because it's so fog-driven, and we don't know how the fog is going to play out. But, in the Sierra, the climate changes will have major impacts, both positive and negative."

As an example of a positive impact, Battles notes that increased temperatures are extending the growing season, which adds to tree growth. On the negative side, he counters, "The changes also extend the dry season by 10 to 12 days. And because most forests in California are water-restricted, they suffer more water stress."

The warming temperatures and extended growing seasons also affect the insects that feed on the trees, extending their life cycles. Insect pests, like bark beetles, are now completing as many as three generations in a single season in areas where, in the past, they might have completed only one or two generations. This increased activity causes higher tree mortality. In many areas, pests, pathogens, insects, and diseases that once died off with the onset of cold weather now survive through warmer winters. A mountain pine beetle outbreak in British Columbia, for example, far exceeds any similar bark beetle epidemic recorded in North American history. If the current rate of spread continues, 80 percent of the mature lodgepole pines in the province will be dead by 2013. The epidemic could transform Canada from a net carbon sink - where forests store CO2 - to a net carbon emitter, as dead trees and fallen logs decay and burn, releasing CO2 to the atmosphere.

It is also becoming increasingly clear that tree mortality has increased in established old-growth forests throughout the western United States, including California. In a paper published in Science (Vol. 323, 23 January 2009), USGS forest ecologist Philip J. van Mantgem and his colleagues reviewed long-term datasets on old-growth forests in the southern Sierra Nevada and found that tree mortality rates had risen from 1 percent in 1983 to 1.7 percent today. The team then expanded their study to include 76 other sites throughout the West - and found similar increases. Though the percentage of this increase in tree mortality may seem small, its long-term impact could be a dramatically altered forest structure, with the loss of larger trees and the dominance of ever-smaller ones. This, in turn, could sharply reduce the land's carbon-storage capacity and its ability to support wildlife. In looking for possible causes for the increased tree mortality, the group found a strong correlation with warmer temperatures and water shortfalls.

Climate change is not only reducing precipitation in the Sierra Nevada. It is also altering how that precipitation falls, significantly reducing the mountain snowpack that provides the water for much of the state. "It's pretty clear, going back to the 1980s as a time frame, that the snow is melting 10 to 12 days sooner," Battles notes. "And the rains aren't coming any earlier, so we're extending the super-dry summer drought in the Mediterranean parts of California."

Much of the mortality in young trees happens at the end of the summer drought. The trees do most of their growing in May and June, when the weather is wet and warm enough. Then, by the middle of July, soil moisture in the forests hits rock bottom and remains there until the rains come in October/November. During this period, the trees "just kind of survive." Over the last several decades, that stressful period has gotten longer and longer.

As well as exerting a direct physiological stress on the trees, the extra weeks of dry conditions also increase the threat of catastrophic fire by extending the fire season. With the forests enduring extended periods of extremely dry conditions and carrying tremendous fuel loads that have built up due to the disruption of natural fire regimes, the intensity of the wildfires that do break out often completely destroys a forest's structure, leaving little hope of regeneration for decades to come.

It's also important to keep in mind that all of these climate change-induced alterations are not being drawn on a blank slate. Forests are facing a host of other challenges as well. "It's not just climate change," Battles notes. "It's global change. Anthropogenic air pollution is a good example. The levels vary depending on location, but every forest in the northern hemisphere is being exposed to elevated levels of atmospheric deposition of nitrate and ammonia, and often to ozone, another stressor. These levels wax and wane, depending on where you are, but those are two general stressors. Another is that humans are constantly moving organisms around, often inadvertently bringing into the forest potentially destructive organisms that can spread unchecked."

Battles refers to the combined effects of all these factors as "cascading consequences." He explains: "You have these direct effects on tree growth, direct effects on pests and pathogens, increases in water stress, and then fire. So there's not just one factor - there's a host of compounding perturbations. The likelihood of any one thing happening might be low, but when you have lots of risks out there, that increases the odds dramatically."

The Unknown Impact of CO2 Fertilization Trees in dry environments must constantly balance their intake of CO2 with their need to retain water. Most tree leaves consist of about 98 percent water, and each time a tree opens the stomata in its leaves to take in CO2, moisture transpires out. From this perspective, increased levels of CO2 facilitate tree growth, because the trees don't have to keep their stomata open for as long and moisture loss is reduced. The net effect is that plants are more efficient with their water use because of so-called CO2 fertilization.

"There's been some evidence that trees grow faster because there's more CO2 around," says Battles. "They become more efficient in their water use, and that offsets some of the negative aspects of direct climate change. If there's water stress associated with a warming climate, there's also an efficiency increase."

Scientists have yet to develop a complete understanding of CO2 fertilization. One of the biggest unknowns about plant growth and the carbon budget is how lasting its effects will be. Will plants acclimatize to higher CO2 levels, causing productivity increases to plateau? Will another nutrient limitation curb the increases? Or will they continue to become more efficient as CO2 levels rise, potentially offsetting some of the negative effects of climate change? Answering this question is critical for modelers trying to predict forest efficiency and carbon sequestration for the next century.

Battles watches with interest as other researchers publish their findings. "A new paper comes out almost every month," he notes. "As a consumer of this information and not a producer, I would say that there's accumulating evidence that CO2 fertilization is relatively short-lived or modest. But even if the effect might be short-lived, it could still be significant. Five years of increased growth, especially for trees, will position them better to capture more resources and resist predators. Trees are long-lived, so there's a fair bit of accumulation."

Understanding the impact of CO2 fertilization is important, because it affects models for carbon sequestration. Professional foresters usually project growth models out 30 years, about the time it takes for many trees to reach maturity. Climate policy experts want the trees to hold carbon, at a minimum, for 100 years to have a climate impact. So how can a government agency or a private landowner manage a forest for that length of time, especially in today's changing environment when they can no longer predict how fast the trees will grow?

Predicting forest growth in response to changing conditions is a major focus of Battles's work. He says: "From my perspective as a forest ecologist, I want to understand how forests will change through time. We already know the present drivers of change, but as we move into this unknown future, how do we deal with all of these different factors? How do we make wise decisions that give us a shot at being proactive? We see changes coming, but by the time we understand what's going on, there's not much we can do. Sudden oak death is a great example of that."

Modeling the Future
In research funded by the California Climate Change Center, Battles and other forestry experts are working to develop models that accurately portray what is happening in the forests today. Battles bases his models on data gathered at UC's research forests and from national forests managed by the USDA Forest Service . "We run empirical models," he explains. "We take as much information as we have from the last 30 years and correlate the relationship between tree growth and climate. Then we project the local climate into the future and see how the trees respond. What are the implications of that if you're managing for timber yield, or habitat conservation, or fire risk? What does it mean if you have these kinds of trajectories?"

Other modeling teams are less focused on specific forests. They take a more top-down approach, building on global and regional models to predict forest growth and productivity for all of California. These models show species shifting around and entire forests moving, based on general principles, and are valuable for statewide planners trying to imagine what the state will look like in the next century. However, these models have relatively little impact on individual land managers. As Battles notes: "Land managers aren't interested in global or even regional models - or about what will happen 100 years from now. They're focused on what they have to do today. They'd like to say, ‘I managed a giant sequoia grove or an oak woodland forest, and during my tenure, I made sure it was still there.'"

The goal now is to unify these models. The empirical models, like those Battles is developing, use growth relationships that have existed in the last 30 years, and if things fundamentally change in the future, they won't capture that. But at the same time, they include a wealth of data on species at a wide range of latitudes and altitudes throughout the Sierra. The process models, on the other hand, begin with a fundamental understanding of important functions (e.g., biogeochemical cycling) and their relationship with climate (e.g., temperature, precipitation) and then use changes in the drivers to project future conditions.

"The key," Battles explains, "is not to be singly focused, because there's not just one threat or one driver. There's this potential for compounding or offsetting interactions, so you have to understand the priority stressors that are changing a system. They might be fire, or drought stress, or pests and pathogens. But there might be positives as well, like increased growth or improved habitat for certain species. We need to look at these situations in a balanced way."

Ideally, Battles would like to see a hybrid model that combines his ecological model as a regulator that keeps things "real," along with the process models' openness to letting things get different as forests adjust to unprecedented environmental conditions. "The problem with process models is that there are no constraints," he notes. "If CO2 fertilization increases tree growth by 5 percent, it just keeps going and going. It never shuts off. Based on our field observations, we think it should shut off at some point."

SPLATS at Sagehen
Over the last two to three years, Battles has focused more and more of his work on the NRS's Sagehen Creek Field Station in Nevada County near Truckee. His team's efforts there illustrate the type of detailed, on-the-ground studies he integrates into his ecological models. In 2003, Battles served on the three-campus review team that recommended the inclusion of this site into the NRS. At the time, he realized that the huge accumulation of fuel in the forest surrounding the station made it an ideal site for testing SPLATS (Strategically Placed Area Treatments), a new fuel-treatment strategy being developed by the Forest Service to reduce the risks of catastrophic fires.

In the following years, his crews conducted in-depth vegetation and fire-fuel surveys throughout the watershed, establishing 525 five-hundred-square-meter plots where they surveyed, measured, identified, and tagged all trees over five inches in diameter; compiled canopy-closure data; and counted and classified ground fuels. The team is currently working with the Forest Service to develop an acceptable program that combines the logging of specific sites with controlled burns to achieve fire-hazard reduction, as well as species-habitat protection, forest regeneration, and other ecological goals.

Though the project's initial goal was to reduce fire danger in the watershed, the survey has served other values as well. As Battles notes: "The data provide a great baseline plot network for understanding any changes in the forest, whether the driver is ten years of fire management or ten years of climate change. For us, these add to the data we've collected at UC's research forests, state demonstration forests, and Forest Service experiment stations located from the southern Sierra all the way up and over the crest near Mt. Lassen."

California's Prospects
Though the forests of the Sierra Nevada are facing increasing challenges, they also constitute a tremendously resilient ecosystem. To this point, they've proven large enough, diverse enough, and isolated enough to limit the damage caused by invasive insects and pests, catastrophic fires, or warming temperatures. The significant acreage controlled by the National Park Service, the Forest Service, and the U.S. Bureau of Land Management provides a large enough area for species, both animal and plant, to move around, hopefully buffering some of the pressures.

This natural resilience gives California time to develop plans for responding to the increasing threats faced by the forests. So far, state leaders seem to have the political will to begin dealing with climate change issues. "In some ways, California is uniquely positioned," Battles reasons. "We've been thinking about climate change impacts on the state much longer than the federal government has. Governor Arnold Schwarzenegger made a strategic investment in climate-change research to stimulate advances in knowledge that have positioned UC well for leading the way nationally."

Thanks largely to its universities, the state also has the intellectual capacity and resources to lead the way in developing proactive solutions for land managers. "UC has these reserves and research forests that have proven their value over and over," Battles notes. "The research record and the productivity of these sites have been remarkable. We need to be able to test new ideas on these locales to provide guidance to land managers. If we're not willing to take risks, we can't expect the Forest Service, the Park Service, or the other land managers to do it. It's really our mission, and we must try to get ahead of the game."

As an example of the kinds of risks he is advocating, Battles cites work recently done in the giant sequoia/mixed-conifer forests at UC Berkeley's Whitaker Forest Research Station* adjacent to Kings Canyon National Park. Anticipating that the giant sequoia will need to move in response to climate change, researchers wanted to learn what type of forest environment would be required to regenerate the ancient trees. They designed a "forest treatment" experiment that opened up the canopy for the young redwoods. The design and approval process took over two years, and involved meetings and field trips with university administrators, conservation groups, National Park staff, and other land managers to get their buy-in before the logging began.

Though the project was controversial and risky, the results have been extremely valuable. "The experiment was carefully controlled and thought out," Battles notes. "The execution was perfect. We're now one of the few entities that have real experimental data on how you can use forest-management practices to get giant sequoia to grow back. They're doing great at Whitaker, and we now have a good understanding of how much light and how much water the young redwoods need. We know exactly how big a canopy opening has to be, both the minimum size and the impact of larger sizes on young tree growth."

Battles admits that not all experiments will turn out so positively, and he argues that the public must give researchers leeway to make mistakes, so long as the experiments are based on good science. "It's much better to test SPLATS in the Sagehen Experimental Forest," he argues, "than to do SPLATS across the entire Tahoe National Forest and screw it up. If we can't risk failure in an experimental forest, we're never going to learn."

Active Management
It can be hard for many conservationists to hear talk of "actively managing forests." Land managers who see the dramatic changes taking place on their lands and who hope to maintain resilient forests for 30, 50, or even 100 years, however, are looking for ways to be proactive and to avoid acting only after it's too late to respond in any meaningful way. One of Battles's key pieces of advice to these managers is to avoid following only one conservation strategy. "We just don't know enough," he argues. "So yes, if you hedge your bets, you're likely to lose some of a forest, but if you encourage heterogeneity in almost everything you do, then there's a chance that some of your units are going to survive. Hedge your bets with diversity."

As an example, he points to the work done at UC Berkeley's Blodgett Research Forest, in El Dorado County, where crews, after logging a section of land, replanted the area with six species of trees in order to maintain the forest's heterogeneity. "From a timber-production point of view, it makes much more sense to plant one or two species," he notes. "Planting the mixes back in there is not nearly as efficient in terms of near-term production, but it gives us much less likelihood that a pest will come through there and kill all of our pines. Mixed species provide separation and diversity, so it's harder for destructive insects to build the synergy for a major infestation. And even if a pest gets the pines at Blodgett, it'll only get one-sixth of our trees."

The immense scope of the bark beetle infestation currently devastating Canada's second-growth forests is due largely to the fact that their replanted forests consist primarily of a single species - lodgepole pine - that is highly valued by lumber companies. Heterogeneity, on the other hand, buffers the forest and reduces the chances of such catastrophic losses. Even though a certain percentage of the trees may die due to an insect infestation, the forest will still provide many of the essential services that humans value in terms of water cycling and providing wildlife habitat, recreational opportunities, and lumber. Those values will be retained because the surviving trees will grow bigger, taking advantage of canopy openings left by other trees that have died off. The same situation is created by wildfires. Each species has a different sensitivity to fire, so a noncatastrophic fire might kill some trees, but it will most likely leave others.

Assisted Migration
The founding legislation for Sequoia National Park specifically states that its purpose is to protect giant sequoia. To achieve this goal in today's rapidly changing climate, Battles argues that park managers concerned about the species' long-term survival must do more than simply protect existing groves within the park. They should also look for potential new giant sequoia habitat, whether inside the national park or in nearby national forest land, where they can plant the groves of the next millennium.

Such "assisted migrations" may be the key to helping a number of species survive, and Battles believes that UC research sites should lead the way in developing techniques for facilitating this process. As he points out, while many species are already moving to higher elevations or more northerly latitudes, some are unable to do so because they're dispersal-limited, because established competitors surround them, or because they are isolated from appropriate locations. He explains his thinking as follows: "If we leave this process alone, the most endangered ecosystems will fall apart, open up opportunities, and something will happen. But the process will be kind of stochastic [random]. It might be a good seed year for one species, or another species might not ever get there. So the idea is that we should start trying to find spots, identify the species that can't move, and then help them get there, whether it's a plant species, an animal species, or even an entire ecosystem."

Noting that the statewide NRS system represents an ideal latitudinal and elevation gradient, Battles suggests that each NRS manager could maintain and monitor a cleared area at his or her site and, every year, record which plants arrive and how long they last. "This would give us an early indicator of what's getting there by itself. So, each year we would check these open plots to see what species are moving up the hill or in from surrounding lands. That's part of trying to use the best information we have to make realistic projections."

Conclusions
The general public largely regards forests as unchanging monoliths. Land managers and field scientists, however, know that forests change rapidly when confronted with completely new conditions, whether heightened levels of CO2, warmer climate, altered precipitation, or exotic pests. They understand that simply leaving forests alone will not make them healthy and resilient. "Forests are dynamic," emphasizes Battles. "They may not change much in our lifetime, but they're dynamic in a tree's lifetime, and we have to take that approach to thinking about them. Leaving them alone is not going to make them healthy, not going to keep them intact. Whatever you like about forests, whether it's the aesthetic pleasure, economic gain, or recreational opportunities they offer, could be lost. And, in fact, the drivers of change are intensifying. Since 1970-80, we have documented key changes in the forest. Mortality rates are increasing, fire risk is increasing, species are moving around… . These changes are already happening, and we need to get away from this idea that the best thing is to put a box around the forests and do nothing. If we do nothing, we're insuring failure." -JB

For more information, contact:
Dr. John J. Battles
Environmental Science, Policy, and Management (ESPM)
328 Hilgard Hall
University of California
Berkeley, CA 94720
Phone: 510-643-0684 (office)
Email: jbattles@berkeley.edu