Investigating Climate Effect on Vegetation and Carbon in Coastal Alaska
Peatlands store up to one-third of Earth's soil carbon (C) pool and are particularly sensitive to changes in climate. They are abundant at high latitudes where climate change is more strongly felt than at lower latitudes. Future warming will shift the C balance in peatlands through changes in C sequestration rates as well as in carbon dioxide (CO2) and methane (CH4) release. One useful way to predict future carbon storage is to investigate past climate shifts and determine how carbon was stored as both temperature and moisture changed throughout previous centuries and millennia. Alaska provides a sensitive region with abundant peatlands where we can extract sediment cores and analyze the peat for past shifts in both climate and carbon accumulation rates.
In order to investigate past climates, we use fossil pollen, plant macrofossils and leaf waxes preserved in peatland archives below ground. These plant remains allow us to reconstruct vegetational change, from which we can infer past climate. Previous pollen stratigraphy from sediment cores along the Alaskan coastline provides a regional context for the changes that we see, and macrofossils provide detailed, site-specific records of in situ response to changes in moisture which can be reliably dated and then compared with similar records from nearby lakes and bogs.
We extracted a 3.72-meter sediment core from Corser Bog (Fig. 1) in 2010 using a hand-operated, 10-cm diameter, modified Livingstone piston corer. The core was transported to Columbia University's Lamont Doherty Earth Observatory where it was refrigerated at 4°C until analysis.
Plant macrofossil analysis of the sediment reveals the sequence of vegetational history, which began as a shallow pond. Using these macrofossils, we build a chronological history of vegetation change from 11,500 years ago to the present.
Analysis of pollen and plant macrofossils (Fig. 2) indicate that initially a shallow lake formed as glaciers melted. Aquatics plants such as water lilies and pondweed are found along with pioneering alder (Alnus) shrubs that colonized the mineral landscape, fixing nitrogen which made the soil richer for other plants to grow.
The dominant vegetation in the early Holocene (the epoch of Earth's history that began about 11,700 years ago) throughout south-central and southeastern Alaska is Alnus viridis subspecies sinuata (formerly A. crispa subsp. sinuata), which is a foundation species in this environment — defined as an abundant species that dominates community structure and moderates or provides stability for fundamental ecosystem processes.
Alder colonizes deglaciated soils rapidly, fixing nitrogen and providing leaf litter from which a thick layer of organic matter is derived. Alder also has a major influence in initiating the development of microbial communities in soil by promoting microbial growth and facilitating the addition of fungal communities to the soil. Its prolific seed production would have ensured that early Holocene environments from Yakutat northward to Prince William Sound were blanketed with alder thickets with an understory of pioneer ferns as glaciers retreated.
Ultimately Sphagnum moss helped to create the bog peat that was then colonized by Andromeda polifolia, a plant of nutrient-poor, ombrotrophic conditions. Alder continued to thrive on the landscape as a foundational species throughout the early Holocene warming, and as Sphagnum arrived, peak carbon accumulated (27-50 g/m2/yr), a pattern found throughout Alaska and the northern hemisphere (Fig. 3)
About 7500 years ago, a shift from moss to sedge peat in the sediment core is paralleled by an increase in sedge pollen, very few macrofossils preserved, and a generally more evaporative climate with a lower water table. Nearby lake sites have a hiatus due to drier conditions during this interval, and peatland records both to the north and south along the coast indicate similar climatic conditions.
However, carbon accumulation rates slow down considerably during this mid-Holocene sedge interval, with rates as low as 13 g/m2/yr (Fig. 3). About 3700 years ago, a regional shift in climate is indicated at this site and elsewhere, through the change from sedge peat to Sphagnum peat and the arrival of first Sitka spruce (Picea sitchensis) and then western (Tsuga canadensis) and mountain hemlock (Tsuga mertensiana) in the region. Sitka spruce expansion requires abundant moisture and lack of a pronounced summer drought, and its arrival may indicate that conditions prior to 4,000 years ago were not favorable for its development along the coastline, particularly if summers were dry. Alternatively, a migration lag across the Bering Glacier area (probably an embayment) may have been present, as spruce was present at Munday Creek about a millennium earlier, and even further to the southeast trees were already present early in the Holocene.
The presence of mountain hemlock attests to the cool conditions that must have been present. This conifer favors colder environments with deep winter snows, and as western hemlock requires favors mature soils, more advanced soils developed through time. A cooler, wetter climate has been inferred regionally from other studies, extending southward to British Columbia, and is consistent with the more depleted deuterium data and low alder values (Fig. 4). The carbon accumulation in the Neoglacial upper portion of the core is about 20 g/m2/yr, which is substantially higher than the mean on the Kenai peninsula and is similar to the increase in C storage for circumboreal peatlands at this time.
Surprisingly, most recently, in the top 10 cm of the core, Alnus pollen provides an early warning signal for warming and ice melt as it markedly increases due to colonization of bare ground after glaciers recede, just as it did in the early Holocene.
The upper 10 cm of pollen samples (at 1-cm resolution) spans 164 years (1846-2010) and shows a 20% expansion of Alnus pollen in the last 60 years. This expansion correlates with the 1-2°C temperature increase recorded at Cordova airport. We suggest that this alder increase is indicative of recent glacial recession providing new mineral soils available for pioneer colonization, effectively a warning signal of glacial melt.
Peteet, D., J. Nichols, C. Moy, A. McGeachy, and M. Perez, 2016: Recent and Holocene climate change controls on vegetation and carbon accumulation in Alaskan coastal muskegs. Quat. Sci. Rev., 131A, 168-178, doi:10.1016/j.quascirev.2015.10.032.
Please address all inquiries about this research to Dr. Dorothy Peteet.