Water is a crucial and scarce resource in the West. It is important to humans for drinking water, irrigated agriculture, industry, energy, recreation, and the natural resources we manage and care about. While most citizens understand the basics of the water cycle, they are unaware of the leading role that forests play in delivering fresh water to streams and cities.
Where do you think water started its journey to your faucet after raining or snowing to the ground? Many people might respond “a local reservoir or stream,” but in fact, almost 80 percent of the nation’s freshwater resources originate from forested areas. In the west, most forests occur in high-elevation areas where there’s enough snow and rainfall to support tree growth. The water that originates on these forests may then feed into the rivers, streams, and reservoirs that supply water to local communities.
Precipitation landing in a forest can experience several fates. Some water is captured by tree branches, where it remains until evaporated by the sun or pulled to the forest floor by gravity. Water making it to forest floor hardly has time to rest. Much is evaporated from the soil surface or sucked up by plant roots. Plants retain some of this water to hydrate their cells, but they let most of it escape through their leaves and back into the atmosphere, a process called transpiration.
Water molecules that are not evaporated or transpired are dragged deeper into the soil by gravity or they run across the soil surface into nearby streams. This “runoff” is then available as habitat for fish, nourishment for vegetation growing along the riverbank, and resources for human use.
The way we manage forests can greatly impact the balance between water entering and leaving a forest. However, the largest regulator of forest-water relationships will always be the climate. Resource managers and the general public need to understand and appreciate these linkages if we are to enjoy forests, streams, and a supply of clean drinking water into the future.
Over a century of data collection and research show that average temperatures are increasing across the globe. Minimum and maximum temperatures are also rising, increasing the amount of moisture sucked out of plants by the atmosphere. These changes are reducing the ability of some plant and animal species to persist in their current environments.
Many researchers and policymakers are focused on the temperature aspect of climate change, but according to Charlie Luce, research hydrologist with the Rocky Mountain Research Station, “The ecological effects of changing precipitation may well outweigh changes related to temperature. Ecosystems and economies are both very sensitive to precipitation, and they will go wherever the water takes them.”
Luce and fellow scientists with the Forest Service and University of Idaho published a comprehensive report in 2012 documenting many of the changes facing forests in the coming decades. Their report shows substantial declines in annual precipitation projected for the southwest United States, with estimates ranging anywhere from a 6 to 20 percent decline. Researchers are less certain about how precipitation will change in the Northern Rockies and Pacific Northwest. Some models predict as low as a 10 percent decline in average precipitation while others project as much as a 21 percent increase.
Overall, climate projections for the West generally agree that precipitation will become more variable in the coming decades. Larger rainstorms could be punctuated by more severe droughts. Record snowfall one year could be followed the next by conditions that disappoint skiers for an entire season. Winter droughts will deepen the summer droughts much of the West is already experiencing due to warmer temperatures and earlier snowmelt.
These changes in precipitation are not just looming in the future, some are already here today. Charlie Luce and Zack Holden, a researcher with the Northern Region of the Forest Service, recently published an analysis of streamflow gages in the Pacific Northwest from 1948 to 2006. They noticed a consistent decrease in annual streamflow over this period, with flows decreasing as much as 20-50% during the driest of years. An increase in extreme conditions, especially the drying of dry years, is of great concern for forest and water managers. “Dry years test the trees—determine if they live, die, or catch on fire. They test the fish. They also test the farmers and the water managers,” says Luce.
When forests receive abundant precipitation, trees grow rapidly and excess water fills nearby streams. Periods of drought, on the other hand, slow tree growth and may result in tree mortality. Many trees have adaptations to cope with drought, such as closing the tiny pores on their leaves to reduce water loss from transpiration, but prolonged drought can cause starvation because trees cannot effectively photosynthesize when pores are closed. Severe drought can also cause hydraulic failure, which is when cells that transport water up tree trunks like a straw fill with air bubbles—generally an irreversible and fatal process.
A recent publication from researchers with the Forest Service, U.S. Geological Survey, and several universities across the West suggests several additional ways that drought can impact forests. For example, thirsty trees are also more vulnerable to insects and disease. The current mountain pine beetle epidemic started during some of the most extreme droughts observed in recent history. Trees could hardly grow given the dry soil conditions, and many were unable to produce the resin that can expel beetles. Droughts during the early 2000s are also implicated in the deaths of thousands of aspen trees across the West.
Prolonged and severe droughts set the stage for wildfires. Years of low annual streamflow in the Pacific Northwest also saw the greatest number of acres burned in wildfires from 1984 to 2005, according to new research by Holden, Luce, and their colleagues with the University of Idaho and The University of Arizona. Less annual precipitation leads to less runoff and it also leads to earlier snowmelt, simply because there is less snow to melt. These conditions dehydrate forests throughout the summer, with desiccated vegetation and dry fuels making perfect tinder for larger wildfires.
While it seems rather obvious that fires are more likely to ignite and grow large when things are dry, much less is known about how fire severity might change under a drying climate. Recent research suggests that fires burning under relatively dry conditions have a higher probability of burning as stand-replacing wildfires. According to Holden, “These findings have important implications for how we manage fires in the West. Currently, we tend to put out fires when we can, causing most forests to burn only during droughts. Continuing this trend may have important consequences for tree mortality and post-fire erosion.”
At the same time, University of Idaho professor Penny Morgan cautions that we don’t want to always view increased acres burned as a bad thing. “Many forests, woodlands, and grasslands in the west are healthier when they have fire,” Penny explains, “fire that’s under controlled conditions and safe.”
Streamflow can benefit from forest die-off. For example, annual water yield increased 15% following a spruce beetle outbreak in the 1940s on the White River Plateau of western Colorado. The beetles caused extensive mortality in Engelmann spruce and lodgepole pine, and it took over 25 years for the vegetation to recover and streamflow to decline back to pre-outbreak levels.
Findings from a research paper co-authored by Luce suggest that streamflow often increases when more than 20% of the trees die in a watershed and/or annual precipitation is greater than 20 inches (500 mm). When a large portion of trees die in a forest, there are fewer roots sucking up water and fewer branches capturing precipitation. This leaves more water in the soil that can eventually end up in streams.
However, if annual precipitation is low, most of the excess water will be taken up by surviving vegetation or evaporated from the soil surface. This means that increased streamflow is less likely to follow extensive mortality in low-elevation forests. In fact, water yield from a piñon–juniper forest in northern New Mexico actually declined following substantial morality of piñon trees due to drought. Researchers attribute the decline in water yield to thirsty grasses and forbs that sucked up the excess water to support rapid growth.
Not only does widespread tree mortality often increase streamflow, it can also increase rates of erosion. Erosion occurs when soils cannot resist the force of water running downhill. The death of trees and other deeply-rooted plants increases an area’s susceptibility to erosion by weakening the net of roots in the soil.
Wildfires can cause massive erosional events by denuding an area of vegetation. A publication by Luce and fellow Forest Service scientists Jaime Goode and John Buffington reports that high rates of erosion can even persist 10 years after severe fires. Heat from wildfires can also create water-repellent layers in soil. These layers temporarily change the water-absorbing capacity of soils from that of a sponges to that of a rain jackets, yielding rapid runoff over the soil surface. The amount of sediment reaching streams during post-fire erosional events can be 100 times larger than long-term average values, and about 1000 times greater than the amount of erosion resulting from road construction.
Yet again, climate change inserts itself into the relationship between forests and water. “We expect climate change to lead to higher streamflows and/or rates of erosion in some areas by inducing forest die-offs through drought stress, reduced resistance to insects and pathogens, or more extensive wildfires,” reports Luce. Increased rates of erosion are even more likely in areas where the future climate includes more intense rainstorms.
A future with higher rates of erosion comes to most people as unwelcome tidings. Muddy rivers are displeasing to look at and cause problems for drinking water treatment. Sediment and debris also clog reservoirs, reducing how much water they can hold.
Negative connotations with erosion muddy the fact that this natural process also has beneficial impacts on fish and aquatic insects. Large woody debris often increases the abundance and diversity of fish habitat. Fine sediment can clog the gills of fish and increase mortality, but it also carries nutrients into streams and increases the productivity of algae and aquatic plants.
Stream ecosystems can handle a certain amount of sediment, but increased rates of erosion in the future could put some native fish species “up the creek without a paddle.” This is especially true as climate change raises stream temperatures and pushes cold-water species to their limits in smaller and smaller habitats.
While completely preventing the flow of sediment into streams is not feasible, there are ways that land managers can intervene to help stabilize the relationship between forested slopes and nearby streams. A great start is restoring riparian ecosystems and connecting aquatic habitats today. That way, stream ecosystems can recover should severe fires and massive erosion occur.
Intentionally putting large logs in streams can create deep pools and provide cover for fish, and planting vegetation along riparian areas can provide shade and decrease stream temperatures. Maintaining and even decommissioning roads can also go a long way towards restoring streams. Erosion rates following fires are magnitudes larger than erosion from roads, but roads deliver a consistent supply of sediment to streams over the long term. Removing these additional sources of sediment can put streams in a better position to handle debris from fires.
Researchers are confident that rainfall and snowfall patterns are changing and will continue to change and that these changes will impact forest and water resources. We cannot predict exactly how much annual precipitation will decrease or increase in specific regions across the West, but the general trend is a drying of the dry years. This means less water to support tree growth. It means drier forests that more easily ignite and carry wildfire. It means less water to fill streams, provide habitat for fish, and fill reservoirs for human use.
Humans have only a modest ability to change the amount and quality of water leaving forested areas. The biggest players are the amount and type of precipitation falling from the sky. This means that a crucial role for managers is monitoring changes, anticipating future conditions, and prioritizing when and where to intervene with actions such as fuel treatments, fire suppression, road reclamation, and riparian restoration.
Resource managers and communities in the West can start talking today about how to respond intelligently to major changes in ecosystems and water supplies in the future. Since there is uncertainty about how much and when changes in precipitation will happen, it helps to plan for “worst case” scenarios as well as more moderate conditions. Communication among diverse resource specialists is also crucial. When foresters, aquatic ecologists, and fuel specialists work together, it encourages holistic planning that keeps in mind the important role that forests play in the water balance.
Adams, H.D.; Luce, C.H.; Breshears, D.D.; Allen, C.D.; Weiler, M.; Hale, V.C.; Smith, A.M.S.; Huxman, T.E. 2012. Ecohydrological consequences of drought- and infestation- triggered tree die-off: Insights and hypotheses. Ecohydrology 5:145-159.
Dillon, G.K.; Holden, Z.A.; Morgan, P.; Crimmins, M.A.; Heyerdahl, E.K.; Luce, C.H. 2011. Both topography and climate affected forest and woodland burn severity in two regions of the western US, 1984 to 2006. Ecosphere 2(12):1-33.
Goode, J.R.; Luce, C.H.; Buffington, J.M. 2012. Enhanced sediment delivery in a change climate in semi-arid mountain basins: Implications for water resource management and aquatic habitat in the northern Rocky Mountains. Geomorphology 139-140:1-15.
Holden, Z.A; Luce, C.H.; Crimmins, M.A.; Morgan, P. 2011. Wildfire extent and severity correlated with annual streamflow distribution and timing in the Pacific Northwest, USA (1984-2005). Ecohydrology 5:677-684.
Luce, C.H.; Holden, Z.A. 2009. Declining annual streamflow distributions in the Pacific Northwest United States, 1948-2006. Geophysical Research Letters 36(16).
Luce, C.H.; Morgan, P.; Dwire, K.; Isaak, D.; Holden, Z.A.; Rieman, B. 2012. Climate change, forests, fire, water, and fish: Building resilient landscapes, streams, and managers. General Technical Report RMRS-GTR-290. USDA Forest Service, Rocky Mountain Research Station, Fort Collins, CO. 207 pp.
Sedell, J.; Sharpe, M.; Apple, D.D.; Copenhage, M.; Furniss, M. 2000. Water & The Forest Service. FS-660. USDA Forest Service, Policy Analysis, Washington, DC. 26 pp.
CHARLES LUCE is a research hydrologist with the Rocky Mountain Research Station, working out of the Boise Aquatic Science Laboratory in Idaho. His current research centers on long-term trends in streamflow, linkages among hydrology, forests, and wildfire, and the impact of forest roads on rates of erosion. Charles holds a MS in forest hydrology from the University of Washington and a PhD in civil engineering from Utah State University.
ZACK HOLDEN is a scientist with the Northern Region of the Forest Service. He is interested in mountain climatology, namely how fine-scale temperature and precipitation patterns influence mountain ecosystems. Zack also has a strong interest in the development of tools to help bridge the gap between research and management. He graduated with his PhD from the University of Idaho.
PENNY MORGAN is a professor of fire sciences at the University of Idaho and director of the University’s Wildland Fire Program. Her current research focuses on some of the broad challenges facing people in the West, such as the potential impacts of climate change on wildfire occurrence and severity. She also investigates the process of vegetation recovery following large fires, including the effects of post-fire management. Penny holds a PhD from the University of Idaho in fire ecology and management.
EMILY HEYERDAHL is a fire ecologist with the Rocky Mountain Research Station based out of the Missoula Fire Sciences Lab. Her research explores how climate, forest type, topography, and land use have affected spatial and temporal variation in fire regimes over the past several centuries. Emily’s PhD is in forest ecology from the University of Washington.