Editor’s Note: This is part two of a series about the indigenous forests that blanket Sonoma County — their past, present and threats to their future.
Our forests “are undergoing a sea change,” observes Mark Tukman, founder of Tukman Geospatial, who is spearheading the Sonoma County Agricultural Preservation and Open Space District’s development of a fine-scale vegetation map.
“I’ve spent over a year looking at aerial photos and coordinating field teams. Many of our oak woodlands are disappearing rapidly, transitioning to Douglas fir and California bay,” he said.
In many places this is visible at ground level — dead manzanitas scattered beneath oaks dying in the shade of Douglas firs — a century of change visible in a glance. Of course, oaks and firs represent just a few of our native trees. Sonoma County’s wide range of geology, soils, landforms and climate has been described as “where Alaska meets Mexico.” With 10 species of oaks and 19 conifers, our forests reflect this diversity.
Close up, they can seem infinitely complex. But if you pull back, larger patterns emerge. Moving west to east, conifers grow in parallel bands — Bishop pine along the cool coast, then redwoods, and finally Douglas fir reaching warmer areas far inland. Interspersed are woodlands of oak, bay, madrone and other hardwoods. There are no hard boundaries between any of these types — in fact mixed conifer-hardwood forests are more common than either alone.
By the early 20th century, forested lands had seen severe impacts. Logged-off tracts of redwood and Douglas fir were now brushy and crowded with young trees. Oak and madrone woodlands, leveled for firewood, had become grassland. Settlements replaced oak savannahs. By all indications, there were far less trees 100 years ago than today.
Even so, in some areas, a mix of surviving forest, brushy areas recovering from timber harvest, and chaparral stands created conditions ripe for catastrophe. One of the county’s first large-scale wildfires occurred in September 1923. Pushed by strong winds, it burned 10,000 acres, destroying many homes and structures in Sonoma Valley. Afterward, citizens formed several rural fire departments. Rather than treating it as a tool, as Native Americans had with intentional, frequent burning, fire became the enemy. Suppression was the goal.
In the short run this was fairly successful, but there were long-term consequences. Without regular fire, more seedlings became full-size trees. Meanwhile, increasing fossil fuel use led to cutting fewer trees for firewood and charcoal. Even where woodlands had been cleared, trees often re-sprouted from their stumps. Over decades, forests returned and the land healed.
But today’s regenerated forests are different from the ones that were cut. Without harvesting and burning, trees grew more densely. Douglas fir and bay became more common and widespread, expanding into areas that had been oak woodland, grassland and chaparral for hundreds or thousands of years.
Crowded forests are less healthy than more open ones, as trees compete for limited sunlight, water and nutrients. Just as human sicknesses spread quickly under crowded conditions, so do plant diseases. Bay trees carry the sudden oak death pathogen but rarely succumb to it. In dense conditions, the disease spreads easily to coast live and tan oaks, which are now dying off in large numbers.