QnAs with James S. Clark

All of nature is connected, and every entity has its purpose. So ecologist James S. Clark tends to look at the big picture rather than focus on a single aspect. A professor of environmental and statistical science at Duke University, with a visiting professorship at France’s National Research Institute for Agriculture, Food, and Environment in Grenoble, Clark aims to understand how communities of species, namely trees, respond to environmental change. Often, he searches for global patterns using data collected over the course of decades. Clark’s endeavors include leading a US Forest Service report on drought (1) and testifying before the US Congress on behalf of the Ecological Society of America and the National Science Foundation. Elected to the National Academy of Sciences in 2020, Clark spoke with PNAS about his recent work on tree fecundity—the potential to reproduce—and the race against climate change.

James S. Clark collecting seed-crop data on Mont Blanc, France. Image credit: Chantal Reid (Duke University, Durham, NC).

PNAS: When did you decide to pursue ecological research as a career?

Clark: When I was a child, I was always out in the field collecting, learning, and reading. I knew that I wanted to be a scientist, but my family background provided no guidance on academic fields. I was very interested in insects, so I got an undergraduate degree in entomology. It’s a great field, and I still use the information, but the emphasis on pest control didn’t seem quite right for me. Then, I thought, I’ll get a Master’s in forestry because I’m really interested in forests. And that’s where I learned that forestry has a lot to do with harvesting trees, albeit to provide important forest products. I’m glad I have a degree in forestry, which grounds much of the way I think in the real world, but it took until my PhD to realize that my interest was ecology. So, I finally got a PhD in ecology, and that was the right thing to do.

PNAS: Many researchers study trees. What makes your work unique?

Clark: There’s a quote that I like from climatologist Steve Schneider. When he was getting cancer treatment, he said, “I trust my oncologist to know the probabilities. I don’t trust him to put them together” (2). And ecologists have the same issue: If we want a model of an ecosystem responding to climate change, how do we estimate the process in a unified way? If we estimate how A affects B, and then we estimate how B affects C, we never learn about the indirect connections between A and C. So, we’re involved with syntheses that treat processes as a system (3). A meta-analysis of several independent studies would be a common approach, but we did something quite different, for instance, in the Inaugural Article (4). To understand the controls on seed production, we took raw data from across the globe and modeled it as just one system.

PNAS: In your Inaugural Article (4), why did you focus on the ability of trees to reproduce?

Clark: As one reviewer of our paper said, senescence is a fact of all multicellular life—and that’s absolutely true. But there’s still this widespread belief embedded in models of climate change effects on forests, which is that reproductive output increases as trees age. I’ve always been skeptical of this, but what brought my skepticism to a head was a study of ours on how changes in climate translate into changes in tree fecundity across the US continent (5). The reason why fecundity wasn’t accelerating in the West in the ways you would expect from climate change is because the trees, on average, were bigger and older than trees in the East. That was an overwhelming pattern in the data. And because tree fecundity slows down with size and age, it shows this geographical difference in how forests respond to climate change. Yet, everywhere, we’re still seeing this assumption about tree fecundity increasing with size and age. So, we decided to write a paper about it.

PNAS: How did you create such expansive masting, or synchronized seed production, datasets for your Inaugural Article (4), and what was the most unexpected result of your analyses?

Clark: Seed production data look like noise. The notion of masting is that you can have a tree producing thousands of seeds one year and zero the next, so it’s extremely difficult to extract signals from masting data that tell us about the role of climate or the condition of individual trees. You need very large datasets, which we had, but the fact that we had this global synthesis of masting data and could detect signals at the geographic scale was very interesting. We got so many incredible datasets from collaborators; everyone on that paper contributed data. I didn’t realize we’d have such an overwhelming set of data to work with, so being able to do this analysis on hundreds of species and show a pervasive pattern with broad geographic coherence was a surprise.

PNAS: Large, old trees are vital to ecosystems, even though tree fecundity declines with size. How should conservationists balance the need for such trees with the need for arboreal reproduction?

Clark: The important thing is not to equate more seeds with good. In terms of reproductive potential, of course, more seeds can be better. But looking across the globe at places with the highest fecundity rates, you wouldn’t necessarily say high fecundity is always a great thing. High seed production can support your natural enemies and produce a range of negative feedbacks. If you clear-cut a forest and let it grow back, it’s initially very boring; all the trees are the same age and there aren’t many species. But with time, forests develop structure, which is critical for all kinds of ecosystem functions. For example, if you walk in a pine plantation, which is basically a monoculture of pine after a clear-cut, there’s not much habitat or food for wildlife. Many microhabitats for wildlife come from larger, older trees. The fact that they don’t produce as many seeds as younger trees is balanced against other ecosystem services they’re providing. Actually, one of the main challenges of forest management is simultaneously maintaining forest productivity and structure.

PNAS: In your opinion, what is ecology’s most pressing concern in current times?

Clark: If we can’t fix the climate crisis, then nothing else we do will matter. The destabilization of ecosystem function goes hand in hand with destabilizing cultures and socioeconomic systems. We can’t fully imagine the impacts we’re headed for in terms of agriculture and water supply. We’ve tended to think about what climate was like in the past, how that allowed ecosystems to function, and what it will be like in 2050. But we’re not going to have a stable climate in the future that we can compare to the past. Species living where they are because of past climates are now continually out of sync. We really need to think about how ecosystems respond to this trajectory with no identifiable endpoint. Hopefully, we’ll change that. But for the moment, we don’t have a future that we can anticipate.