How to Become a Paleobiologist

by Caitlin MacKenzie

A few weeks ago, in the middle of a busy rush of lab work on a sediment core, I spent a morning curled up in a comfy chair with a giant mug of coffee. While snow fell outside my window, I was reminiscing about spring in the Maine mountains. It wasn’t exactly daydreaming — I was reading the proofs for a paper from one of my dissertation chapters, ‘Trails-as-transects: phenology monitoring across heterogeneous microclimates in Acadia National Park, Maine. ’ The open granite ridges of Acadia were a second home during my PhD. Today, my relationship with Acadia’s plants is a little different: instead of recording their fresh flowers in the field, I’m counting the grains of pollen they left behind thousands of years ago. Reading through the proofs for this paper brought me back to my biology and ecology roots — somewhere along the way toward my chosen career as a conservation biologist, I became an accidental paleoecologist.

Map of high-elevation and alpine areas of northeastern United States, with locations of my master’s research (White Mountains) and PhD research (Acadia National Park). The three major high-elevation paleoecological study sites in the northeast are the Adirondacks, Mt Washington, and Sargent Mountain Pond. Map created by Kevin Berend.

An alpine masters and a ridge top PhD

I’ve worked above treeline in the northeast United States for over a decade. Both my master’s and my PhD research were grounded in plant phenology — I spent many springs compiling detailed records on the timing of leaf out and flowering. My perception of time was short — the y axes of my figures were ‘day of year’. Change over time was measured in years, or in the case of very long records, decades. There was an immediacy and a precision to my fieldwork — I monitored for changes every few days to catch each plant right as it bloomed. The field sites were incredible: hiking my favorite mountains week after week, wearing out the treads on my trail runners on long treks across open granite ridges. There were bad days, to be sure — sudden rain storms, blisters, sunburns, black fly bites, and granola bar fatigue — but I loved being above treeline. My master’s work in New Hampshire’s White Mountains and my PhD research in Acadia allowed me to spend long spring days in these incredible alpine and subalpine habitats each year. Climbing to my field sites and transects meant hiking through forests and watching the maples and beeches give way to birches, firs, and stunted spruces before emerging above the trees into a world of cushion plants and tiny tangled shrubs. Sitting back against a summit cairn and soaking in the sun while clouds cover the valley below in a pillowy undercast is among my favorite experiences, and throughout graduate school I got to do this morning after morning on peaks from Mt Washington to Sargent Mountain.

My last hike for PhD research in Acadia National Park,
with my daughter as my 'research assistant.'
Photo credit: Mike MacKenzie. 
An untested assumption

During my many hikes monitoring phenology, I often marveled at the alpine plants. Survival above treeline is impressive: the growing season is short, the wind and ice are intense, and the soil is shallow and poor. Alpine and subalpine habitats are scattered across the highest peaks in the U.S. northeast, like little islands above the forest. In Acadia National Park, on the coast of Maine, the mountains are quite small — only 466 m on the summit of Cadillac — and subalpine plant communities persist at these low elevations. The northeast’s alpine and subalpine habitats have been interpreted as post-glacial tundra relicts, the first plant communities to colonize this barren landscape as the Laurentide Ice Sheet retreated ~15,000 years ago. As the climate warmed in the Holocene, forests moved in at lower elevations, and only the coldest microclimates continued to support tundra plants. This is the explanation I remember from college, and from the seasons I worked in environmental education in the White Mountains.

During my PhD research in Acadia, I started following Maine scientists on Twitter. I first met Jacquelyn Gill when she was a new faculty member at the University of Maine. Our paths crossed online and then in person, when she visited Acadia to give a talk on her paleoecological work. I remember asking her for some paleo-insight into the low-elevation subalpine communities in Acadia — why was I able to find plants like mountain cranberry and crowberry nearly 900 m below their elevation limit in New Hampshire? She confirmed my memory that these were regarded as tundra relicts but noted that this was an untested assumption. Most paleoecological work in the northeast is based on sediment cores from low elevation ponds. Very little work has been done above treeline.

At low elevations across the northeast, and especially southern New England, decades of research based on pollen and macrofossils from sediment cores reveal a dynamic mosaic of vegetation assemblages responding to postglacial climate change over the past 14,000 years. Pollen and plant macrofossil records from Lakes of the Clouds, perched just above treeline on Mt Washington, suggest that this site — the highest peak in the northeast — maintained alpine vegetation throughout the Holocene. The Lakes of the Clouds research has been cited as evidence for the projected persistence of alpine habitat on Katahdin (Maine’s highest peak), though all state-listed threatened or endangered plant species in Maine associated with alpine habitats had medium or high vulnerability to climate change in the same report. I am aware of only one other high elevation pollen record from the region, a study of vegetation patterns in the Adirondacks of New York. These Lakes of the Clouds and Adirondacks studies are over 25 years old and analyzed at temporal resolutions too coarse to capture responses to rapid warming events since deglaciation. With the available paleoecological data, it is impossible to know if the current distribution of alpine and subalpine habitats in the northeast — which comprise not just the highest elevations but are also scattered across isolated peaks and low elevation coastal ridges — represent persistent refugia for tundra vegetation throughout the Holocene.

Collecting sediments at Sargent Mountain Pond with Jacquelyn Gill in September 2017. 

Same place, different time

After my conversations with Jacquelyn, I became slightly obsessed with the mystery of northeast alpine and subalpine vegetation. During the final field season of my PhD, I was still hiking my phenology monitoring transects, but I began to look at the mountains with a new focus, and my time scale suddenly unspooled from year-to-year shifts in flowering to one hundred- or thousand-year shifts in vegetation assemblages. In between hikes on the ridges, I sat down with Jacquelyn and sketched out a proposal to study these paleoecological dynamics. I began to eye the kettle hole Sargent Mountain Pond that sat just below treeline on my transect hike down the south ridge of Sargent Mountain — I knew that researchers had collected sediment cores there in the past, but the pollen had never been worked up for the full Holocene. Towards the end of a PhD, it’s common to think about the future. Somehow, at the end of my PhD, I became immersed in the deep past. I wrote a postdoc proposal to return to Acadia National Park and collect sediment cores. I would be hiking up Sargent Mountain one more time — now with coring equipment, an inflatable kayak, and a raft — but instead of studying this year’s flowers, I would be coring for evidence of past plant communities. Somehow, this postdoc has rushed by and I’m already approaching the end of my paleoecological fellowship. While I completed coring fieldwork and began pollen and macrofossil analysis, I slowly shepherded my dissertation chapters through peer review. Re-reading the proofs from my dissertation work brings me back to my life before paleoecology and reminds me of my first encounters with the alpine plants I now search for in my sediment cores.

Working on one of my sediment cores
in Jacquelyn Gill's lab. Photo credit: Britte Heijink

Caitlin MacKenzie, Ph.D.
David H. Smith Postdoctoral Research Fellow, Climate Change Institute, University of Maine

If you have questions or comments concerning Caitlin's post, please leave a comment below, or send her an email. You can also connect with her on Twitter.


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