Jackson Drew: Aging in the Arctic—Insights from a study on woody shrubs

In this new post, Jackson Drew—a PhD candidate in Alaska—presents his work ‘Age Matters: older Alnus viridis ssp. fruticosa are more sensitive to summer temperatures in the Alaskan Arctic‘. Here he shows the importance of age for plant growth, discusses the importance of global change for vegetation, and tells us how Alaska is not as cold as it used to be.

About the paper

Double rainbow at beautiful Toolik Lake Field Station, taken June 22nd, 2017 (credit: Jackson W. Drew)

To many, the Arctic ecosystems’ transformation due to climate change is no longer a matter of if but when. Large portions of the Arctic have already witnessed a transformation from short graminoid and evergreen shrub tundra to taller deciduous woody shrubs. This change has prompted huge interest as taller deciduous woody shrubs can influence numerous abiotic conditions over time.

These woody plants bear witness to their growth through their annual rings, serving as “historical documents” that reveal the abiotic drivers of their growth, often including air temperature and precipitation. The way in which Arctic shrubs respond to these abiotic drivers can vary depending on age-related trends, primarily through plant size. But, how can plant age influence growth?

To investigate, we donned our bug shirts and dragged the relatively unstudied Siberian alder from the toe slopes of Sagwon Bluffs in Arctic Alaska back to our lab. We selected Siberian alder because it is a unique woody deciduous arctic shrub that can fix nitrogen (N). This N-fixing ability enables these shrubs to change local and landscape-level nutrient conditions as they get older.

Processing Siberian alder samples to a finished product that you can measure is difficult. You often need to dice the samples up (top left) and slice them with a microtome (top right). The finished product (bottom) does look amazing! Taken winter 2019 (credit: Jackson W. Drew)
A closer look at Siberian Alder nodules (Taken August 9th, 2018) (credit: Jackson W. Drew).

Through a series of meticulous slicing and dicing of the wood, we measured Siberian alder’s secondary growth and related growth to air temperature. We found that older Siberian alder is more sensitive to air temperature compared to younger shrubs, meaning they grow more when it’s warmer, but also worse when it’s colder. The greater sensitivity with age is likely driven by increased plant size as plants get older and usually larger. Greater size allows for enhanced photosynthetic capture (i.e., more sugar for growth), but also incurs higher maintenance respiration costs to sustain their larger bodies. In addition, Siberian alder can downregulate N-fixation (a costly endeavour) as N accumulates in soils, resulting in more carbon for growth as it improves the N-limited Arctic soils with time, making older shrubs more efficient than younger ones. These mechanisms allow for Siberian alder to respond differently to the same climatic drivers depending on their age.

About the research

Our findings provide novel evidence that—as the Arctic continues to warm—age will play an important  role in the future growth of Siberian alder where older shrubs will grow better than younger shrubs. In addition, the fundamental principles that govern Siberian alder’s age-related trends may be applicable to other arctic shrubs. This highlights the crucial role that plant age may play in predicting  future shrub responses in the Arctic.

The initial processing of the largest sampled Siberian alder (credit: Jackson Drew)

It is crucial to broaden our perspective beyond the Arctic and recognize that all plants provide feedback to their environment—which can change over time—whether within the lifespan of a single plant or across many generations. Understanding the life history of a given species allows us to anticipate the thresholds that signal changes within a system. In the case of Siberian alder, we know that its expansion can cause sweeping transformations in the landscape. By further investigating age-related trends in Siberian alder and other plants, and pinpointing the thresholds of those trends  at both individual and plot levels, we will be better equipped to predict when we might cross those thresholds that signal the initial signs of ecosystem change.

About The Author

One of Jackson’s favourite ways to recreate out on the tundra, catching a huge lake trout at Toolik Lake, AK. Taken June 18th, 2019 (credit: Trevor Surgener)

If you think of a farm, I bet you wouldn’t imagine my home—Interior Alaska. On that small farm we grew not only flowers to make balms and lotions, but also a child’s wonder for nature. I’d ask, “How do plants grow?”

Farming in Alaska is only getting better with increasingly hot summers and mild winters. Rarely do we see -50 ºC during the winter like I remember growing up. And don’t get me started on how smoky our summers are now due to wildfire. Witnessing these changes firsthand really made me wonder, “Why are things changing?”

These questions constantly rattled in my head. Then as an undergraduate I joined a lab studying terrestrial ecology in the Alaskan Arctic. Just north of the Brooks Range, at Toolik Lake Field Station, I got my first taste of field work (and Toolik’s amazing food) and started to learn how to answer my questions. Now a PhD candidate approaching graduation, I finally have some results! Moving forward I hope to continue doing research in Alaska, excited to answer a question that’s relevant to my work and personal life: “what’s next?”

Dark green Siberian alder mottle that landscape leading to the Sagwon Bluffs, Alaska. Taken June 19th, 2019 (credit: Jackson W. Drew)

Enjoyed the blogpost? Read the research here!

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