Seeds of change

As foresters, ecologists and plant geneticists refine which seeds to plant to build climate-resilient forests, some scientists urge caution about importing seedlings from elsewhere. Baldwin, the evolutionary biologist, acknowledges that replanters have improved their methods in recent years by more carefully considering their seed sources, but he’s skeptical of assisting trees in migrating to cooler elevations. “There’s been a lack of appreciation for the potential downsides of some of that activity,” he says. He thinks that foresters should import seed only in extreme situations in which the local flora truly can’t recover on their own. Since California’s diverse flora have had such a long time to evolve, there’s a lot of “cryptic diversity” — including potentially unrecognized species — lurking across the landscape, he says.

Failing to recognize this diversity could lead to what’s known as outbreeding depression, in which finely tuned genetic adaptations get disrupted when a local plant breeds with one adapted to conditions elsewhere. But though this is a theoretical risk, research on the topic has found it’s not a major concern, Moran says. In most cases, foresters are only moving trees up to the next elevation band, which is often within the range of tree-breeding for wind-pollinated species anyway — so the trees just uphill are not genetically distinct.

Plus, as long as a reseeding effort uses both local seeds and those chosen for climate adaptability, young trees that aren’t the right fit will be outcompeted while those better suited for survival will reach maturity. Intentionally burning these reforested lands about 15 to 20 years after replanting can also help select for the toughest trees, North notes.

Introducing new trees from populations that aren’t next door on the seed map is still taboo among US agencies that oversee public lands. In Moran’s experiments in the Sierra and Tahoe national forests and Sequoia National Park — where the planted trees came from all over the state, not just right downhill — officials have required that she return and remove the trees before they fully mature and shed any seeds.

In Canada, however, forest officials have already changed their guidelines such that silviculturists are instructed to pick seeds suitable for a future warmer climate, rather than replanting with local seeds. That includes even shifting the species mix toward tree types better suited to a changing climate.

There may never be a clear answer on the extent to which humans should meddle in the post-fire footprints of forests. Those decisions, after all, involve value judgements about what’s most important to managers. Still, though foresters may disagree on the level of meddling that’s permissible, they are increasingly being convinced that trees might indeed need human help in order to preserve the benefits that forested ecosystems provide. It may be time to radically rethink reforestation and rebuild forests into ones more resilient to the threats of climate change.

“Trees don’t walk, and we are shifting the climate faster than it appears to have shifted in a long time,” says North. “It’s incumbent on us to start researching and experimenting.”