Trees confer a variety of benefits on cities and suburban areas; they reduce local temperatures, provide shade and provide food and shelter for wildlife. Given a few basic caveats, such as selecting the right tree species for the available space and planting the trees properly, most tree professionals heartily encourage the planting of more trees near our homes, schools, playgrounds and shopping malls.
Given the “pro tree” attitude of most industry professionals, it is perhaps surprising that many researchers are suggesting that some trees are making the drought worse for us. In fact, they are directly competing with us for that ever-precious resource – fresh water. Many of these species have been finding water in the western United States for far longer than people have been trying to squeak out a living in this arid corner of the world, so we definitely have our work cut out for us.
“But, wait,” you may say, “humans and trees have been coexisting fairly well (at least, from the human perspective) for several decades, and this hasn’t become a problem until recently. What changed?”
To answer that, we must look at both the path water takes en route to our reservoirs, crops and taps, as well as the effects prior environmental management strategies have had on the Sierra Nevada ecosystem.
Forest to Faucet
Much of California’s water supply comes from the Sierra Nevada Mountains. As explained by University of California Merced hydrologist Roger Bales in an interview with NPR, the snowpack is responsible for about 60 percent of the state’s consumable water. It falls as snow during the winter, and often coats the region in a 10-foot-thick blanket of fluffy powder. As the snow melts in the spring, it trickles through the substrate, drips into streams and rivers, and pours down the watershed, eventually reaching the Pacific Ocean. Along the way, we draw some of this water for our own needs.
Trees of the Sierra Nevadas also draw their share of this melted snow water, by pulling water from the soil via their roots. This process benefits the environment in several ways, such as reducing the amount of runoff water rushing down the mountains, stabilizing the soil, and so on. In fact, we typically embrace the ability of trees to accomplish these tasks and plant them in our cities to do so.
But there is an important distinction: Unlike the trees of the urban forest, who in large part get their water after humans have withdrawn that which is necessary for our own needs, these trees of the Sierra Nevadas get first access to the water. Normally, this is not a big problem, but it looks like we have made a few mistakes, and we are paying the costs now.
Forest Fire Rears Its Head
Fire is a natural component of many Californian ecosystems. It helped shape the individual species native to the land, as well as the ecosystem as a whole. Consider, for example, the acorn-caching habits of scrub jays (Aphelocoma californica), who help to replant the next generation of coast live oaks (Quercus agrifolia) in the ashes of charred landscapes, or the cones of lodgepole pines (Pinus contorta), which only open when fires warm the air enough to melt the wax on their cones, thereby releasing the seeds.
These periodic fires kept the forest understories from becoming overcrowded. However, due to the fire suppressive strategies embraced over the last century or so, the forests are becoming very dense. In some places, the forests have grown twice as dense as historical patterns indicate they should.
With twice as many trees wicking moisture from the ground, less water is able to runoff the mountains and into our supply chain. It is important to note that the trees did not cause the drought, and water would be tighter these days anyway. However, with a properly balanced forest, we would certainly have more water.
While no one yet knows exactly how much more runoff water we would have without these excessively dense forest stands, Bales suggests that the number is between 20 percent and 40 percent. Further research is necessary to nail down these figures better, but it is clear that the quantity in question is clearly significant.
The irony of the situation is that in developed, urban or suburban areas, we need trees because they absorb runoff water. Clearly, it is crucial to find a happy medium between our cities, which would benefit from far more trees and canopy cover, and our current forests, many of which are overcrowded.
Fortunately, fire suppression philosophies are changing, and many scientists are looking at ways to thin the forests, thereby allowing more water to find its way down the watershed. However, there are no quick fixes for this particular problem.