Master's Thesis on Forest Fire and Climate Dynamics (2022-2024)

"Sedimentary archives in the Western United States document a significant relationship between fire and climate. However, post-glacial geochemical records that capture fire dynamics are infrequent outside of the Western U.S. Echo Lake, CT and Lost Pond, NH are kettle ponds that received continuous sediment input since both sites experienced deglaciation, capturing the evolution of the climate and the ecosystem of New England in lacustrine sediments. I collected two cores from Echo Lake and Lost Pond to recover geochemical biomarkers that sequestered over >15,000 yrs and >12,000 yrs of changes in both Northern and Southern New England. Both cores have experienced consistent sequestration of organic material for organic biomarker analysis, providing unique archives of coupled climate and fire evolution.

Following the Last Glacial Maximum (LGM), glacial retreat began around 21 ka in Southern New England and slowly progressed northward through the Late Pleistocene. Estimates of the timing of deglaciation indicate Connecticut deglaciated around 18.3 ka while New Hampshire experienced significant glacial retreat around 14.5 ka. After the LGM, the climate generally warmed towards present-day with a brief return to glacial conditions during the Younger Dryas (12.8–11.6 ka). Pollen records throughout New England show a dramatic shift from tundra to a pine/spruce boreal forest in the late Pleistocene and early Holocene. Associated with increasing summer insolation, drier and warmer summers drove vegetation change from 12 to 8 ka, transitioning to the northern hardwood forest adept for both dry and wet climate. This transition in climate and ecosystem imparted significant changes to the drought and fire resistance of the New England landscape and may have exerted large impacts on fire recurrence through time.

For this project, I sampled lake sediment cores from kettle lakes in Northern and Southern New England that span the deglaciation to examine the organic molecular record of environmental change and fire history. Specifically, I examined molecular distributions of terrestrial leaf waxes (n-alkanes) to evaluate long-term changes to vegetation, climate, and hydrology. I quantified polycyclic aromatic hydrocarbons (PAHs), benzene ring structures generated during the combustion of organic material, to constrain long-term estimates of fire activity. Amongst generated PAHs, low molecular weight PAHs (LMW PAHs) represent both local and regional fire activity that are transported as aerosols. High molecular weight PAHs (HMW PAHs) form in hotter, local forest fires and erosion transports them in sediments within watershed basins. Organic molecular data show high ratios of HMW PAHs relative to LMW PAHs, demonstrating that both Northern and Southern New England were characterized by frequent and hot fires in the early Holocene from 11.6–9.5 ka. Sediments from Lost Pond and Echo Lake also record a shift in the average chain length (ACL) of plant-derived n-alkanes, reflecting an increase in temperature in the early Holocene followed by late-Holocene climate cooling until modern day.

PAH concentrations in Northern and Southern New England kettle lake sediments indicate a prominent increase in fire events in the period after the Younger Dryas between 11.6–9.5 ka. This was caused by a warm, dry climate and the presence of fire-intolerant species such as white pine. White pine forests peaked in abundance during a dry, warm period in New England as a result of increasing summer insolation which led to an increase in production and influx of PAHs from 11.6–9.5 ka, reflecting an interval with high fire frequency. White pine population decreased after 9.5 ka, where they were outcompeted by drought-resistant species such as oak. LMW PAH abundances indicate that after 9.5 ka, smaller and/or less intense proximal forest fires characterized the regions around Lost Pond and Echo Lake. After the mid-Holocene warm period and the peak of the effects of summer insolation, the environment surrounding Echo Lake had more regional fire activity coinciding with the expansion of pitch pine and Native American populations in New England. PAH concentrations dramatically increased in sediments that date to the period of European colonization and expansion of agriculture during the 18th and 19th centuries in Southern New England, nearly tripling observed PAH concentrations, while Lost Pond, which was comparatively unaffected by land clearing, logging, or population growth, showed little change in sedimentary PAHs. This study reflects a combination of climate and vegetation influencing fire dynamics in New England and could be applied to how climate change and anthropogenic vegetation change effects fire dynamics in the future."

-Lapham N., Laura. (2024). Characterizing Late Pleistocene and Holocene Fire Dynamics and Climate in New England [Master's Thesis, University of Connecticut].