In March of 2023, PhD candidate Baxter Worthing shared his research on poplar trees and what it is revealing about how they may respond to our changing climate. It was particularly special to host Baxter because he is an alumni of several CREA programs and served on CREA’s Board of Directors, including filling the role of Board Secretary in his senior year of high school.
Since then, Baxter has received a BS and MS in Biology from Clark University, and is currently pursuing a PhD in the Plant Biology Dept at UVM. Baxter spoke at CREA’s 2012 Annual Meeting, as he prepared to head off to college. His comments reveal how perceptive he was at the tender age of 18, and speak to the role that organizations like CREA can play in the lives of young people.
…it is refreshing to be involved with an organization like CREA: to see kids outside learning about the world they have been separated from; to watch an endangered toad hop across a trail, and to know that this toad and countless other forms of life will forever have a safe haven right here in Topsham; to sit at a board meeting and watch adults, kind enough to include me in their fold, make the world a better place through simple discussion and cooperation; to be able to escape the world of desks and pop quizzes and learn about something real, tangible, and special.
CREA has affected me in countless ways, defined the educational path I took through high school and will take in college, but perhaps the biggest thing I have gained is the understanding that all has not gone wrong. That there are, in fact, people and organizations like CREA (and its faithful members) that are working to make both the present and future better for me and for all of us.
– Baxter Worthing
As you will see from Baxter’s talk, he is doing his part to make the present and future better for all of us.
For context, Baxter described a 1948 experiment conducted by three botanists (Clausen, Keck, and Hiesey) at Stanford University. They hiked the Sierra Nevada range collecting samples and seeds of the Mountain Yarrow, then planted the seeds in a ‘common garden’. Despite being the same species, the seeds grew into plants that had significant variations in size. The researchers concluded these variations reflected differences in each seed’s original environment and referred to these differences as ‘ecotypes’ or local adaptations.
Similar research has been carried out with Sitka spruce growing in very different climates. In common gardens in Vancouver, spruce grown from spruce seed collected in warmer California were much taller than those from seed collected in Alaska. There are several possible explanations (e.g. trees adapted to cold may be stressed by a warmer climate and they may enter dormancy earlier). Either way, long-lived organisms like trees may experience drastic changes within their lifetime — up to 3 degrees of warming. There is concern that the rapid pace of climate change may disrupt these local adaptations.
Baxter’s interest is in understanding how populations of trees adapt to their local environments, particularly as the climate is changing fairly quickly. His focus is on the Populus genus, specifically cottonwoods and aspen (some of which – cottonwoods – are referred to by some as ‘tacamahacs’). He studies mostly cottonwoods, of which there are two species in Maine: Balsam poplar (P. balsamifera) and Eastern cottonwood (P. deltoides), both of which have fluffy cotton-like seeds.
These two species are very shade intolerant, fast growing, and are common along rivers and in mountain valleys. They are a ‘pioneer species’, meaning they are often the first trees to populate areas that have suffered a disturbance (fire, logging, storm damage). Poplar is useful to study because they are widespread across the U.S., with different species occupying different areas. Some hybridization occurs across species.
They are valuable ecologically, supporting over 360 species of moths and butterflies. The balsam resin is valued by indigenous people for medicinal and other uses. Bees use poplar sap as an anti-fungal agent in their hives. And humans utilize poplar trees for salicylic acid (used in aspirin), their fast-growing wood, and many other uses.
Baxter described his research which involves collecting samples from poplars growing wild in diverse environments, getting them established as seedlings, then planting them in common gardens and monitoring their growth. They are excellent for research because they grow fast — a two-year tree can be five meters tall (~ 16 ft)!
Common gardens of varying sizes have been established in multiple states and Canada, including some in warmer locations that are not traditional poplar habitat. Baxter is focusing on phenological changes — changes in trees’ life cycle (such as when they leaf out). Getting the timing of ‘bud flush’ (when the leaf bud starts to open) right is critical. The tree wants to leaf out early enough to take advantage of favorable spring conditions, but late enough to reduce risk of tissue damage from late frost. This is one area of concern and study for Baxter — investigating whether bud flush can be adversely affected by changes in the climate.
He cited one recent examples of adverse impact. There was a warm week in April of 2021, followed by snow and freezing temperatures. Some poplars had flushed and some of their newly formed leaves died.
Poplars protect against by having two stages of winter dormancy in which they ‘count’ cold and warm days. In the fall, they enter endodormancy. After they have encountered a certain amount of cold temperatures, they enter ecodormancy. Ecodormancy ends after a period of warming temps, resulting in bud flush.
Baxter measures the timing of bud flush in all the trees in the common gardens. Some of what he’s learned:
- In Virginia, trees from colder climes don’t need much warming and tend to flush earlier.
- Poplars from colder climates flush earlier, perhaps to maximize growth in the short growing season (i.e. worth the risk).
- Trees from intermediate environments tend to delay their bud flush.
- Their hypothesis is that poplars from intermediate climates remain dormant longer to avoid premature bud flush because they typically experience great variability in weather.
- Endodormancy period
- Warm winters delay bud flush as chilling requirements are not satisfied.
- Bud flush in southern mini gardens occurs earlier, because it’s warmer than typical for poplar.
- In the warmest common gardens (Texas), all the trees delay bud flush because the trees are confused by the lack of cold days.
The results to date have raised two concerns. One is that the cold-adapted poplars can be tricked into flushing prematurely by winter warm temps, risking injury if freezing temperatures return. They are now testing the trees’ tolerance to freezing events. The warmest-adapted poplars may delay bud flush too long, flushing after spring rains have dried up.
They are beginning to test tolerance to early summer drought and are seeing good resilience to premature freezing.
There was great interest in how trees ‘count’ cold and warm days. Baxter gave an excellent explanation of how they do this and it’s complicated. The gist of it is as follows. At the cellular level, information is transmitted through pores in the cell wall. During dormancy, which begins in the fall, these pores are filled with a substance (referred to as a callous) that blocks information from being transmitted. This callous, comprised of fatty acids, starts to melt during warm days. When the callous is gone, information is transmitted initiating bud flush. So it’s not counting in the way that we practice, but it has the same effect.