Helping forests migrate: Planners race to plant trees adapted to the future climate

Researchers from UC Davis collect acorns in arid west Texas to plant on their campus in northern California. They estimate their climate in 2100 will be similar to that of Barstow or even Phoenix today. City staff from a town near Portland, Oregon travel to California and Arizona for seedlings they can take home and plant along their city streets. They are preparing for Portland’s weather to become like Sacramento today.

The range of Arizona oak. For one town near Portland, Oregon, the list of potential future street trees includes this species, as well as California buckeye, California laurel, and silverleaf oak.

With these regions breaking new heat records annually – Sacramento just topped 90 degrees for the 110th day (and counting) in 2020—and given that trees take decades to mature, the race is on. Birds can fly, mammals can walk, but trees expand their ranges very slowly. Most acorns from an oak end up within a few hundred yards from their home tree.

Climate velocity, the speed at which ecotones are shifting north, is much faster than that. Our climate is changing ten to one hundred times faster than during a global warming event 55 million years ago known as the Paleocene-Eocene Thermal Maximum (PETM). During that “rapid” spike, palm trees successfully migrated to the Arctic circle, but they had thousands of years to make it there.

Dead blue oaks in Fresno County, California. They experienced excessive mortality during the 2012-16 drought. These hills may revert to grassland. Researchers want to use the genes of the survivors as stock for the future in the north. For a full presentation of blue oak gene-assisted migration see this presentation by the California Department of Fish and Wildlife.

While trees can’t walk, they can die. Range contraction of trees along their southern xeric (dry) edge is happening in the American West right with the speed of climate change. Blue oak die-offs are widespread in the southern third of their range. From California to Colorado, conifers such as Ponderosa pine and Douglas-fir are disappearing from lower elevations. To quote Davis et al (2019), “In areas that have crossed climatic thresholds for regeneration, stand-replacing fires may result in abrupt ecosystem transitions to nonforest states.” When people talk about California becoming Arizona, the cleanup hitter in that process may be fire, but the first batters are heat, drought stress, and bark beetles. After fires, decreased soil moisture and increased vapor pressure deficit (VPD) associated with climate change are leading to reduced probability of regeneration (Davis et al 2019). In short, many forests are not coming back.

Ponderosa pines are disappearing from lower elevations of the Sierra in California. This has been documented in Colorado as well.

Range expansion of trees northward has been documented, but the pace is anemic, insufficient to keep up with the changing climate. One study in the east found that ranges in adult trees expanded north less than 150 yards per year (Sittaro et al 2017). They concluded, “our results add to the body of evidence suggesting tree species are mostly limited in their capacity to track climate warming…”

Recent mega fires include many of the drought-killed conifers in the southern Sierra. Research suggests regeneration may be imperiled due to a warming climate.

Researchers have discussed facilitating tree migration due to climate change for over a decade (Aitken et al 2008). For over a hundred years, botanists have recognized regional differences within the same plant species, and simple garden experiments have shown that local varieties do better. The standard rule of thumb has always been that local varieties are best; they are adapted to the local ecological niche. Now that is changing.

Recent research is showing that trees are now in the wrong places; the climate has shifted past them. Valley oaks, white fir, Douglas fir, ponderosa pine, Western hemlock, and lodgepole pine seedlings all do better when removed from their original home and moved north (Aitken and Bemmels 2015).

The local trees are becoming misfits in a world that is changing around them. Many researchers are hesitant to fully embrace assisted migration; introducing non-native species has a horrid track record. But they are beginning to study “assisted gene flow”, moving hardy trees from the southern end of a species’ range to the north end. Cities, on the other hand, are beginning to see trees as more than just aesthetically pleasing; they are critical infrastructure, providing shade and reducing urban temperatures. So the cities and towns are moving faster, boldly cultivating trees from the dry Southwest into the Pacific Northwest.

This photo from Aitken and Bemmels (2015) shows a series of Sitka spruce, all eight years old, planted together in British Columbia. The trees from the south, adapted for a warmer and drier environment, are out-competing the locals.

Tree migration is also critical for the range expansion of animals. Without the trees and other vegetation, many birds, mammals, and other forms of life have no habitat rungs on the ladder to enable them to move north as well. Anna’s Hummingbirds now winter in Canada and even Alaska, largely due to ornamental plantings. The Oak Titmouse, on the other hand, is dependent on oaks, tightly constraining its ability to expand north. It may be that, in the coming decades, oaks and other tree species planted in cities and towns will provide critical refugia for a wide variety of birds and insects seeking cooler climes.

Hell in Paradise: Why the Camp Fire was the largest climate-induced mass mortality event in modern history

fires Paradise

The Camp Fire was started by downed power lines, spread from a forest, and then became a structure-to-structure urban fire in which houses burned but many trees did not.

I grew up in southern California on the edge of the San Fernando Valley. Fires, usually fueled by Santa Ana winds on dry grasses in the hills, were a fact of life. I evacuated. I ran the pump to squirt water from the pool onto the roof. I helped neighbors on the edge of the hills water their roofs. Back in those days, in the 1970s, fires over 100,000 acres (~150 square miles) were rare. The worst fires destroyed 200 to 300 homes. Rarely, someone died. That was then.

fires infographic

Infographic prepared in 2013, before most of the recent mega-fires.

Twelve years ago, four researchers examined a comprehensive database of all large wildfires in western United States forests since 1970. They detected a signal and concluded that “large wildlife activity increased suddenly and markedly in the mid-1980s.” Looking at weather data and land-use history, they concluded the driving factor was “increased spring and summer temperatures and an earlier spring snowmelt”. Forest management, on the other hand, had “relatively little effect”. Published in the journal Science in 2006, they had found the “force multiplier” that climate change brings to the West. We all knew that forests had been mismanaged for nearly a century, and that too many homes and towns had been built up against wildlands.  But now there was a new factor driving fires—longer hotter drier summers. If you’re looking for numbers, the data show that fires really ramped up in 1987.  The so-called “force multiplier” of climate change was big—about 3 or 4. That is, wildfire frequency was triple in dry years when compared to moderate years, and quadruple that in wet years. Ominously, they noted the effect was non-linear, meaning that, in warmer years, fires really increased. Thus, the multiplier was not just a constant number—it increased with temperature and lack of rain. The Berkeley Tunnel fire, which was exceptional because it killed 24 people trying to evacuate and was the first fire in California history to burn over a thousand homes (it actually burned over 2,000 homes), occurred in 1991.

In the twelve years since the Science article, mega-fires, in terms of acres burned, structures destroyed, and people killed, have gone off the charts—literally. When it comes to fires that burn more than 200,000 acres (~300 square miles), destroy more than a thousand homes, and kill dozens, if not hundreds of people, climate change is not really a “force multiplier”, it’s an on-off switch. Such fires were very rare or non-existent before the year 2000. Now, they appear to be annual.

CA fires 2b

We’ve crossed a threshold, tripped a wire.  In 2017, after the astounding loss of 6,000 homes in Santa Rosa, we hoped that was an outlier, a blip in the data. A few months later, in December, when fires were previously unheard of, the Thomas Fire became the largest in state history. Barely six months later, the Carr Fire made Redding “the new Santa Rosa”. After that fire I posted a chart showing that 16 of the state’s largest 20 fires had all occurred in the past 20 years. We had a reached a “new abnormal”. Then, a few months after that, in November, when by all historical standards the fire season should be over, the Camp Fire literally wiped out the entire town of Paradise, population 26,000. Hospitals, high schools, stores, and houses, all gone. The death toll is without historical precedent. While that was burning, the Woolsey Fire became the largest and most destructive fire in the history of the Los Angeles area. At the present rate, next year the Paradise inferno will be surpassed by some hell unimaginable.

CA fires 2a

When I say the Camp Fire is the largest climate-induced mass mortality event in modern history, I’m not counting hurricanes. Hurricanes, even large hurricanes, have always occurred and always will. A Category 5 hurricane striking a major city is an inevitability. Yes, climate change has made hurricanes larger and more numerous, thus increasing the risk, but nothing like the change we’ve seen with fires in the West. Thus, attributing any one hurricane to climate change is like attributing a single specific cancer case to an environmental contaminant causing a cluster of cases.

CA fires 2c

When I say the Camp Fire was caused by climate change, I’m not saying that future destructive fires are inevitable. The fires will come, but we can do things to mitigate the


Graphic from

destructive aspects. A full post-mortem on the Camp Fire and other recent mega-fires is of course required. Such analysis should look beyond the political rhetoric of Trump, exclusively blaming management practices in national forests. Most of these fires in these graphs, spreading across dry grass and oak woodland, had nothing to do with forests. Furthermore, in the face of massive tree death from drought, simply removing dead trees from forests has serious feasibility limitations. To quote a forest fire expert colleague, “Yes, fuels reduction is needed same as 30 years ago, but the mills are all full from the tree mortality supply, much of the fuels are not merchantable, and we are not going to cut our way out of longer fire seasons and deadly MegaFires.”

In the short run, we can’t stop the changing climate, the record low humidity and record high dry vegetation, or the longer summers. We can’t shorten the fire season, now 80 days longer than in 1970s. But we can modify power lines, conduct preventative burns, revise urban fire perimeter requirements, and re-evaluate evacuation routes. Those things won’t turn off the switch we’ve triggered, but they might at least save some homes and lives in the coming years.


The CalFire damage inspection map of Paradise. The latest interactive map can be found here