To look into the future of forests in Sonoma County requires finding a similar time in the past — or at least one as close as possible. And for that, scientists have turned to studying minute specks of pollen at the bottom of Clear Lake.
“When considering the future of our forests,” says Professor David Ackerly of UC Berkeley, “the closest analogy for the present moment was the warming at the end of the last ice age, 14,000 years ago.” Pollen preserved in sediment at the bottom of Clear Lake recorded a dramatic change at that time from conifer-dominated forests to oak woodland. That transition took place over 4000 years, as temperatures rose about 10 degrees Fahrenheit.
Today the Earth is heating up many times faster and, depending on who you talk to, warming over the coming century could match the rise after the ice age. Changes in rainfall are also expected for Sonoma County, although it’s uncertain whether we’ll see more precipitation or less. But even with more rain, warmer overall temperatures may increase evaporation to the point where less water will be available to plants. The timing of rainfall is important, too — a few torrential storms bank less soil moisture than the same amount falling slowly and steadily over the winter months. Soil moisture is an important factor controlling where trees can grow.
In this scenario, the survival of living things depends upon their ability to adjust to the changes more quickly than in the past. Animals are lucky — they can move to other locations; individual trees and plants are stuck to one spot. In the short term, many are already shifting their yearly cycles — the beginning of spring, as marked by budding and flowering, is two weeks earlier than it was 50 years ago. Long-term, trees move between generations, their seeds spread by wind, water, gravity and animals to more suitable places.
“Seeing the future may be as simple as paying attention to what’s already growing on the forest floor,” says Lisa Micheli, President and CEO of Pepperwood, which operates a 3200-acre reserve located northeast of Santa Rosa. A study there by Ackerly’s UC Berkeley lab is monitoring all the plants in more than fifty scattered plots. “It does seem,” Micheli observes, “that today’s baby trees tend to be the more drought tolerant species in some of those plots.” Whether those saplings will reach maturity is unknown. In some cases, the young trees may perish while older trees hang on.
“Trees have strategies we don’t even know of yet,” says Micheli. “We recently discovered that the roots of some oaks go all the way down into fractured bedrock, where a lot of water is stored. Our tree species have survived droughts beyond anything we’ve ever seen. Yet we also saw many large, older trees, weakened by recent drought, come down during storms this winter, victims of both too little and too much water.”
No one knows how long it will take our forests to shift and adjust to the changing climate. The forest always lags behind the present; today’s trees sprouted and matured successfully under past conditions. The shifts may be fairly subtle, such as Oregon oak being replaced by more drought-tolerant coast live oak. On the other hand, catastrophic events like wildfire or disease may cause sudden, unpredictable shifts or “type conversions.” A forest unable to recover from wildfire can become shrub land in just a few years.