Date of Award
Doctor of Philosophy
Craig E. Tweedie
Almost half the global soil organic carbon pool is found in northern high latitudes. The majority of this soil carbon is stored in a greenhouse inert state within permafrost. As pronounced warming of high northern latitudes ensues, there is a strong likelihood that this substantial soil organic carbon pool will be mobilized to the atmosphere where it is likely to positively enhance greenhouse warming. Modeling studies suggest this positive feedback could alter future climate states of the Arctic and the globe, and affect how humans may need to adapt to climate change. Accordingly, understanding the patterns and controls and the fate and transport of carbon in arctic terrestrial ecosystems has become well recognized as a research priority.
This dissertation focuses on improving knowledge of decade-time scale plant community change and its impact on ecosystem function (i.e. biogeochemical cycling, and energy balance) in select sites throughout the North American Arctic. The studies presented largely take advantage of historic research activities conducted between the late 1950's and mid-1970s that have been resampled to assess change in ecosystem structure (i.e. makeup of plant species and abundance) and ecosystem function. The overarching goal of this dissertation is to determine how ecosystem function has changed in central Baffin Island, Canada, and in northern Alaska near Barrow in response to changes in climate, herbivory, nutrient availability, and plant community change over the past 30-60 years.
In collaboration with Dr. Patrick J. Webber, this International Polar Year Back to the Future (BTF) project's primary objective was to determine how structure and function of high latitude arctic terrestrial ecosystems have changed over the past 25 or more years. At two sites, Baffin Island Canada, and Barrow Alaska, we used newly developed multivariate-geostatistical analytical procedure to asses change in ecosystem function over time. At our site in central Baffin Island, the greatest change in ecosystem function was noted for plant communities with high soil moisture. For example, two pond margin communities: Campylium-Aulacomnium-moss meadows and Eriophorum-Pleuropogon wetlands increased in biomass by 178% and 46%, while NDVI increased 35% and 16%, respectively. Soil moisture was found to decrease in Carex stans wet meadows and Campylium-Aulacomnium-moss meadows by 30% and 24%, respectively. Other changes were heterogeneous and largely plant community specific. Similarly, in Barrow Alaska, the greatest change in ecosystem function was found for aquatic and wet plant communities, where productivity varied and soil moisture increased along with methane flux over time. Interestingly, our functional models appeared to be responsive to years with high lemming populations, which were found to decrease albedo and NDVI, while increasing methane fluxes for both aquatic Carex graminoid and wet graminoid tundra. Further, moist and dry communities which decreased in soil moisture between 1972-2010 appeared more stable through time, with little change found in structure and function.
During the summer of 2010, a range of ecosystem functional properties were measured in replicate 2 x 2 m herbivore exclosure plots, and adjacent control plots established in wet, moist, and dry land cover types (LCTs) near Barrow, Alaska in 1959. We sought to determine the functional implications of long-term lemming exclusion in wet, moist and dry LCTs, and extrapolated results to the landscape scale. We found herbivore effects to vary between LCTs and the strongest differences between exclosures and control plots were found in wet graminoid tundra. Herbivore exclusion in wet tundra increased albedo (+10%), but decreased thaw depth (-37%), saturated soil layer (-28%), normalized difference vegetation index (-20%), methane flux (-23mgC m-2 day-1), and increased loss of CO2 to the atmosphere through increased net ecosystem exchange (+1.75gC m-2 day-1). Further, lemming exclusion decreased saturated soil layer (-47%) for moist, and decreased thaw depth (-20%) for dry LCTs, while no other significant effects were detected in these LCTs. Measurements of ecosystem function for grazed (control) and ungrazed (exclosure) states were extrapolated over the Barrow Peninsula (421 km2) using a high spatial resolution land cover map. In the presence of lemmings, tundra maintained a historically accurate peak growing season carbon sink at -219 tonnesCeq day-1, respectively, while the modeled ungrazed tundra state was estimated to function as a small source +8 tonnesCeq day-1. Thus, a shift in lemming herbivory has the capacity to dramatically alter ecosystem function at plot to landscape scales and should be regarded as a more important component of the changing Arctic System than what has been the case in the past.
The concluding study of this dissertation was motivated by recent evidence suggesting the extent of aquatic tundra near Barrow, Alaska dominated by Arctophila fulva, has increased over the past half-century, which also appears to be concurrent with increases in nitrogen (N) and phosphorus (P) in aquatic ecosystems in the same area. This study examined the response of ecosystem carbon dioxide (CO2) and methane (CH4) flux from A. fulva dominated tundra under elevated N and P levels. We extracted monoliths from pond margin aquatic tundra near Barrow, dominated by A. fulva and placed these in a continuous flux monitoring system, that controlled environmental conditions (light, air temperature, water table height) at 3 nutrient levels (control: 0.0 mgN L-1, 0.0 mgP L-1, low: 1.5 mgN L-1, 0.6 mgP L-1, and high: 7.5 mgN L-1, 3.0 mgP L-1). In response to the high nutrient treatment, we found A. fulva biomass and steady state CH4 emission (SE) to increase with increased A. fulva tiller biomass which acts as gas conduits from the soil to the air. Contrary to expectations, GEE decreased as vascular plant biomass increased. The increase in canopy thickness via biomass growth and a reduction in light use efficiency is likely to be related to this reduction in photosynthetic capacity and carbon sink strength. There were no differences in CO2 and CH4 flux between control and low nutrient treatments. No differences in gas ebullition (GE) among nutrient treatments were found, however, a negative relationship between GE and biomass was documented (R2= 0.34, p< 0.001), which to our knowledge is the first time a correlation between vegetation leaf biomass and GE has been recorded. Collectively, short-term experimental results suggest A. fulva biomass, CO2 and CH4 fluxes in aquatic habitats have likely been altered by high levels of fertilization, which supports long-term field based observations and provides substantial functional implications for future aquatic vegetation change in the Barrow area, and potentially tundra elsewhere in the Arctic.
Received from ProQuest
Mark Jason Lara
Lara, Mark Jason, "Implications Of Decade Time Scale Arctic Plant Community Change On Ecosystem Function" (2012). Open Access Theses & Dissertations. 1861.