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.

Why are so many Eurasian Collared-Doves leucistic?

0V2A6850In 2006, I wrote a paper about the spread of the non-native Eurasian Collared-Dove into the Central Valley of California. At that time, there were about 43 records. Now, of course, the species is widespread and common. Quoting from that paper, here’s the backstory of their spread throughout North America:

The Eurasian Collared-Dove was first observed in Florida in the late 1970s. These birds likely originated from an accidental release in the Bahamas in 1974. Since then, their spread has been well documented by Christmas Bird Count and by state bird record committees. By the mid 1990s, the species had been recorded throughout the southeast United States. By 2000, Arizona, Idaho, Oregon, Utah, and Washington had documented records. On the 106th CBC (December 2005 – January 2006), over 30,000 individual birds were reported nationwide, compared to just 560 fifteen years earlier. Their rate of increase has averaged 34% per year.

Today, it seems that in any large aggregation of Eurasian Collared-Doves, there are one or two that are unusually pale, blotched with white and cream. They seem to be about 1% of the population or more, though it’s yet to be studied. These birds bare a strong resemblance to African Collared-Doves, which is generally this pale.  However, based on the dark outer web of the outermost tail feather (see below), as well as size and vocalizations, these birds are clearly leucistic Eurasian Collared-Doves.

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EUCD diagram

All of the photos on this page involve the same two darker doves and one pale dove. Above, the tail patterns of the dark dove (left) and pale dove (right) both suggest Eurasian Collared-Dove.  Woodland, California, October 2018.

0V2A6835Many birders suspect this is due to the Founder Effect, a phenomenon that occurs when a small population colonizes a large area. Eventually, all of the birds (or other animal species) are descended from few individuals. In this context, certain recessive traits that were once rare may become more common.

See the Wikipedia account of the Founder Effect for examples of this in human populations.

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Here, one of the darker doves is mating with the pale dove.