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In a recently published paper, PhD student Ellen Waddle and her coauthors provide some clarity on a decades-old problem

When researching what drives the growth of small populations, ecologists consider several factors, says��Ellen Waddle, a PhD student in the University of Colorado Boulder’s Department of Ecology and Evolutionary Biology.

“There’s climate. There’s density, which can be thought of as both the total number of individuals in a population or how crowded or spread out individuals are. And then there’s stochasticity, which is this big word that just means variance” or random chance.��

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CU Boulder scientists Ellen Waddle (left), a PhD student in ecology and evolutionary biology, and Dan Doak (right), a professor of environmental studies, and their research colleagues found "that climate data alone did a pretty poor job of predicting population growth (in small tree populations)."��

But whether any of these drivers matters more than the others is a question that has challenged researchers since at least the 1950s, and one that Waddle and her coauthors��, Christopher Steenbock and��Dan Doak take up in a�� recently published in Ecology and Evolution.

Time and perspective

Researchers have tended to fall into opposing camps with this question, Waddle explains.

“There’s a lot of people that think if we can perfectly predict what the climate’s going to be in an area, we’re going to be able to perfectly predict how that population is going to grow through time. And then you have another set of ecologists that argue, well, it also really matters how many individuals you have in the population.”

Yet in their paper, Waddle and her coauthors come to a less divisive conclusion. By analyzing the rings of two long-lived tree species, Ponderosa pine and limber pine, “we found that climate data alone did a pretty poor job of predicting population growth. We needed to include other drivers (in our predictive models), like competitive density effects and stochasticity, to accurately reconstruct population dynamics over time.”

This means that no individual driver proved more influential than the others. They all mattered.

Which was somewhat surprising, Waddle says, considering the long timescale she and her colleagues were dealing with—many hundreds of years. (The oldest tree they sampled dates back to 1470, half a century before Queen Elizabeth I was born.)

“We're averaging over such a long timeframe that you might be tempted to think that random fluctuations and stochasticity are less important, but this sort of study highlights that that's not always true. There's a lot of uncertainty in how long it's going to take small populations to grow.”

“The most important aspect of our work, to my mind,” adds Doak, professor of environmental studies at CU Boulder and head of the��Doak Lab, “is showing that simplifying assumptions we often make about population growth don’t seem to hold up.”

‘The entire history of a tree’s life’

Tree rings, says Waddle, are a gold standard for measuring a tree’s history, one with which most people are familiar. The center, or pith, signifies when the tree established, or secured its roots and became capable of growing on its own, and each concentric ring around it represents a year of growth.

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CU Boulder researchers studied small populations of Ponderosa pine (seen here) and limber pine to better understand how drivers such as climate data and competitive density affect growth. (Photo: Wikimedia Commons)

But for their study, Waddle and her coauthors used tree rings—in the form of tree cores, or centimeter-wide rods extracted from living tree trunks—a little differently.

“What we did, which has not been done often, was to core every single tree in the population,” says Waddle, which enabled her and her coauthors to get a clearer picture of how tree populations changed over time than they would have gotten coring only a handful of trees.

“Another way to put it: The tree core data basically allows us to reconstruct annual censuses of population from start (1400s-1500s) through present day because we can know exactly how many individuals were alive in each year and when each individual first established.”

The tree-core samples themselves came from Bighorn Basin, a mountain-encircled plateau region in north-central Wyoming about 500 miles from Boulder. Waddle collected some of the tree cores herself in 2017, while an undergrad at CU, for what turned out to be her first camping experience.

Yet the bulk of the core samples owe their existence to Lesser and Steenbock. Lesser alone cored around 1,100 Ponderosa pines between 2007 and 2008, in hot, sometimes tense conditions.

“We (Lesser and an undergraduate field technician) would start hiking to the first trees of the day typically around 5 a.m. to avoid the worst of the heat,” Lesser recalls. “Trekking��up dry streambeds to reach the trees we would encounter multiple rattlesnakes each morning and on one occasion a mountain lion that set us on edge for the rest of the day! Many days we would core fewer than 20 trees due to the low density of the population��and the ruggedness of the terrain—getting from one tree to the next often took an hour or more negotiating��cliff faces, ravines and steep slopes.”

But the effort, he says, was worth it.

“Coring the trees itself was an incredibly rewarding experience—sizing up the tree to get a sense of its shape and where the pith was and then extracting the entire history of its life!”

Pick a species, any species

This research on small-population growth is no small matter, says Doak, “because all populations start small,” and “understanding what controls the growth of new populations has a new urgency as we try to predict whether wild species can shift their ranges to keep up with climate change.”

“Pick some species you care about,” says Waddle, who is currently writing her dissertation on how mountain terrain affects plant species’ ability to follow their preferred climate. “What I care about might be different than what someone else cares about, but there’s probably a species that matters to you, whether it’s a food species or your favorite animal.

“If we want to help keep those populations on the landscape, we need to know how small populations grow and how they persist.”

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