By late fall of 2017, I was suffering from slingshot finger. It was caused by routinely firing off an 8-foot version of the weapon traditionally wielded by fiendish little kids. I’d spent much of the season hauling it up mountains around northern New England in search of fertile red spruce trees. When my field assistant and I found a tree bearing cones, often 50 or 60 feet up, she’d steady the base of the slingshot while I took aim, pulling down on the rubber straps with my full body weight. The goal was to launch a line over the branch, hoist up a saw-toothed chain, and saw down a cluster of cones. We’d bring the cones and the seeds they contained back to Professor Stephen Keller’s University of Vermont (UVM) lab, which studies the genetic adaptation of forest trees to climate.
If everything goes smoothly – and that did happen once or twice – the whole process takes about 20 minutes, with a sore finger the only side effect. Add two to three hours if the chain gets stuck in the tree, requiring the construction of a platform out of backpacks and decaying logs, or if the sawn-off cluster is still lodged high in the branches as dusk falls.
Fortunately, 2017 was a banner mast year for red spruce and many other trees, and cones abounded. Field teams like mine managed to collect seeds and needle-tissue samples from 340 trees in 65 locations throughout the range of Picea rubens, from North Carolina to New Brunswick. We had completed the first step of a three-year study funded by the National Science Foundation and managed by the Keller lab, in partnership with the University of Maryland’s Center for Environmental Science and the U.S. Forest Service’s Southern Research Station. The next steps were to extract DNA from each tissue sample so the mother trees’ genomes could be sequenced and then to germinate the seeds – more than 10,000 of them – and to pot them in UVM’s greenhouse.
By looking at spruce growth in research gardens and spruce genes in the lab, the project is investigating how P. rubens declined in abundance and shifted its range in response to climate change during the last 20,000 years, and how we might expect those shifts to continue in the future. The shifts occur in tandem with local adaptation, the Keller lab’s main area of research: the process by which populations within a species vary genetically and evolve over generations in response to their local conditions, be that on a regional scale or the bottom versus the top of the same mountain.
A Perfect Study Species
Red spruce is an ideal species for studying local adaptation to climate change because of its strong sensitivity to climate and its highly fragmented range. It exemplifies many of the knottiest problems in conservation genetics. Anoob Prakash, one of Keller’s doctoral students, said that when he discovered the lab’s research, he threw out his list of other PhD programs and applied only to UVM. “I was interested in forest fragmentation, climate change, genetic adaptation to climate change, local adaptation – all the issues I wanted to study, red spruce has all of them.”
I once asked Keller if he’d chosen red spruce because it was the perfect study system, or if he just liked the species. His official answer, he told me, would be the former. Scientists pose questions about nature and then develop the experiments to answer them. But scientists are also human beings, and there was more to the story. Growing up in Pennsylvania, Keller spent summer vacations in the north woods and dreamed of living there. When he landed his first tenure-track professorship at the University of Maryland’s Appalachian Laboratory, he found himself moving south instead. Still, western Maryland wasn’t without its appeal. In the central Appalachians and farther south, remnants of spruce forests grow in isolated “sky islands” on mountaintops. Keller decided to pursue a research program that both aligned with his scientific interests and allowed him to visit his beloved boreal forests.
We think of red spruce as a montane tree in the Northeast, too, mingled with balsam fir on mountainsides across the region. But the species wasn’t always so rare in the valleys. Vermont ecologist Charlie Cogbill and his colleagues have studied records of “witness” trees (covered in the Summer 2013 issue of Northern Woodlands), which 18th-century surveyors used to mark parcel boundaries as they divvied up the land. From these records, they assembled a picture of what the northern forest looked like before the European settlers cut down most of it. Spruce, they found, used to be about twice as common in New England as it is now – the second most common tree after beech. Slow to take hold in the forest, it has never returned to its former rank.
Logging may be the least of the tree’s problems now. “I wouldn’t recommend reading the literature on red spruce,” Prakash warned. “It’s depressing.” The species took a hard hit from acid rain, which the Clean Air Act eventually curtailed, and has only just started to recover. (UVM biologist Hub Vogelmann famously took the head of the Reagan Administration’s Environmental Protection Agency, along with several members of Congress, on a hike up Camel’s Hump to see the affected spruce trees dying on the mountain.) Other challenges loom. In warm climates, such as we expect to be more prevalent in the near future, the conifer is outcompeted by broadleaf trees, which are more efficient photosynthesizers. And with a long life span, red spruce is slow to colonize new, suitable habitat as climatic conditions change.
Building Bridges to Sky Islands
The sky-island populations in the central and southern Appalachians present a picture of what the future might hold for red spruce in the Northeast, the core of the species’ range. As the last ice age drew to a close, spruce forests followed the retreating glacier northward during thousands of years. In the south, as temperatures climbed, spruce persisted only in the “frost pockets” of the high mountains. Now there is nowhere left for them to go at these latitudes, and intensive logging has isolated them even further in their sky islands. Genetic diversity in these populations has declined, leaving them vulnerable to the effects of inbreeding and even less capable of adapting to rapid environmental change.
Katy Barlow of The Nature Conservancy (TNC) hopes to give them a leg up. She works with the Central Appalachian Spruce Restoration Initiative (CASRI) on the long-term goal of planting corridors of red spruce to reconnect some of the sky islands and to restore unimpeded gene flow between them. Last year, TNC and CASRI received a grant from the Wildlife Conservation Society’s Climate Adaptation Fund to restore 250 acres of red spruce forest in the central Appalachians. Barlow is spearheading the effort, and the Keller lab is behind her.
Barlow’s experience in graduate school motivated her to find a career in which she could scale up science to apply to far-reaching conservation projects. It frustrated her when research presentations so often ended with the obligatory final slide that merely gestured at the implications for on-the-ground management.
“I don’t think we think through those slides very well,” she said. “Are we really challenging ourselves as scientists to be practical about how that science can be applied to conservation?”
Now in a position to act on her career goal, she reached out to Keller’s team for advice. Could they figure out which spruce populations in her region had the highest genetic diversity, so that TNC could collect those seeds? And could they tell her how to choose seed sources that would enable adaptation to the future climate of the central Appalachians? Spruce trees take three decades just to bear cones. By the time trees planted next year reach maturity, the climate will look quite different.
Forecasting the Future
Thibaut Capblancq, a postdoctoral researcher in the Keller lab who studies spruce genomics, has been hard at work on finding the answers to these questions. Although an avid outdoorsman, Capblancq spends most of his workdays in front of the computer, wrangling genetic code. When he first arrived in Vermont from France and went backcountry skiing with new friends, he would tell them about his research: “Oh, I am studying a tree, red spruce – it’s probably all around us, but I have no idea…maybe this one?” Two years later, he’s become intimately familiar with the tree and its genes. One of the project’s key findings so far is that back when the Northeast lay under a mile-thick ice sheet, red spruce huddled in a single, potato-shaped refugium across much of the American South. This type of statistical modeling, known as a “hindcast,” infers a species’ former range from ancient climate conditions and can inform a comparable forecast of its future.
Capblancq has often asked himself if he made the right decision in pursuing a career in research. “I had a big internal conflict where I almost stopped research to just do something completely different,” he confessed, because he wanted his work to “be helpful right now, very concretely, to prevent or at least to adapt to climate change.” TNC’s restoration project gave him his first opportunity to make a direct impact on climate-conscious conservation.
The responsibility weighed heavily on him. “It was very scary,” he admitted, “just because I wanted to be sure.” After analyzing the genomes of the spruce trees sampled in the central Appalachian region, he determined which populations had the highest genetic diversity, an advantage that will help their offspring adapt to environmental changes more easily and to curb the deleterious influence of inbreeding, and passed on his results to Barlow. Of course, he added, “don’t tell anyone, but a model is just a model. We are not sure of anything. No one is ever sure of anything. But we tried to be as sure as possible.”
Common Gardens
To validate the model – to be even more sure – the Keller lab and collaborators have spent two years observing thousands of real-life red spruce trees, the ones grown from seeds I helped to collect back in 2017. They are the basis of a common-garden experiment, the largest one Keller has ever carried out, in which offspring from many different populations are brought together in a common environment. Once the data have been analyzed – a massive undertaking led by Prakash – the results will illuminate how the twin influences of genes and environment interact with each other to affect the growth and health of red spruce.
Prakash envies his colleagues who work with the lab’s other study species, balsam poplar, “which just grows – you just throw it in the field and it just grows. [Spruce] are like human babies: you have to care for them for a long time before you can actually see a result.” The lab coddled the spruce seedlings in the greenhouse for a full year before shipping them off to be planted in outdoor raised beds near the western edges of Vermont, Maryland, and North Carolina – three common gardens in three distinctly different Appalachian climates. Biologists at all stages of their careers, myself included, recorded the timing and height of each plant’s growth during two summers, ending with their harvest this past fall and a final measurement of biomass.
By looking at how trees from throughout the species’ range responded to the climate at these different latitudes, and by comparing the results to the statistical models developed through genomic analysis, Keller and his team will be able to assess how genetically prepared different populations might be to adapt to climate change. In short, they’ll have a good answer to Barlow’s second question: which trees will be best adapted to the future climate? As Capblancq put it: “If we know what genes are good for what climate, we can predict that in 2080, for example, here [in Vermont], if you want to be a healthy red spruce, you need these genes.”
The lab hopes to have that answer ready in time for the next round of restoration. In the meantime, 57,500 red spruce seedlings with the highest genetic diversity in the central Appalachians are sprouting in a nursery. They’ll go into the ground this spring, joined by about 30,000 hardwood trees that typically grow alongside red spruce. CASRI and TNC seek to restore the entire spruce ecosystem, which encompasses far more than just the one species. “I know there is some kind of squirrel involved – a flying one,” said Capblancq with a laugh. Although not yet an expert on North America’s natural communities, he is no less passionate about their fate. Restoration work such as CASRI’s is essential, he believes; red spruce can’t be conserved without it. But we’ll have to do more to prevent climate change in order to save the species. “That’s my problem, that’s my question almost every morning,” he confided. “Are we doing something fast enough?”
Discussion *