Sonoma County’s forests face uncertain future

Editor’s Note: This is the final?installment in a series about the ?indigenous forests that blanket Sonoma County - their past, present and threats to their future.

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 4,000 years, as temperatures rose about 10 degrees.

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.

It was these and other unknowns that led Ackerly and Micheli to found the Terrestrial Biodiversity Climate Change Collaborative (TBC3) to build a bridge between climate science and natural resource managers, who are making on-the-ground decisions. One aspect of this is Ackerly’s research, which combines climate and vegetation modeling. By scaling climate predictions to a grid fine enough to capture our diverse local topography and plugging in limiting factors for vegetation, his models predict the most suitable vegetation at any particular spot over a range of possible future conditions. Taken together, the various scenarios sketch out the story of where our forests are likely going.

Ackerly’s work suggests that if the magnitude of change is less than 45 degrees above a 1950 - 1980 baseline, Sonoma’s forest lands will expand a little. If it gets warmer than that, many forested areas will begin to shrink. Among individual species there also will be winners and losers. Redwoods are expected to remain stable in the western half of the county, but decline in places more distant from the coast. Douglas fir shows a similar pattern. The biggest loser may be Oregon or Garry oak. The real winner may not be a tree at all, but chamise, a shrubby chaparral species. Some areas will be more sensitive to climate change than others.

Along with changes in the climate, future forests will also affected by the choices humans make and by the ability of science to support those values.

“The cultural forces around fire are particularly complex,” observes Micheli, who has reintroduced the often-controversial topic of prescribed burning at Pepperwood, in part thanks to input from her Native Advisory Council. “We need to become better friends with fire.”

The benefits of regular burning - a technique used by indigenous people here for millennia - are increasingly recognized as a means to both promote ecological health and to reduce fuel loads, which helps protect homes from much more catastrophic wildfires.

Questions no one would have thought of 20 years ago are now being asked, such as: If thinned forests can better withstand the predicted extreme droughts, should we be removing healthy trees? If it appears that climate change will outpace the transition of the forest, should we begin planting a different kind of forest, one adapted to the new conditions? Does the risk of making a mistake outweigh the potential benefits of introducing plants tolerant of the coming climate? If your land is moving toward a future of shrubs instead of trees, should you plant toward that future?

It may well turn out that Sonoma County’s saving graces are the very things which brought many of us here in the first place: the landscape’s varied topography and biodiversity provide a depth of resilience. Many Sonoma County native trees can take the place of ones that can’t survive warmer temperatures and less available water.

As Lisa Micheli says, “Nature will respond by choosing trees tolerant of the new conditions. The diversity of our mixed forests makes me cautiously optimistic about the future.”

Arthur Dawson is a Glen Ellen-based historical ecologist. Contact him at baseline@vom.com.