PATTERNS AND DRIVERS OF CARBON FLUXES IN TEMPERATE FORESTS

Despite decades of carbon cycling research in terrestrial ecosystems, a complex suite of biotic and abiotic interactions make a complete understanding of the natural carbon cycle elusive. This thesis aims to advance our understanding of the carbon cycle, and stems from several ongoing projects aimed at quantifying carbon dynamics in forest ecosystems across a range of scales, with a specific effort to include both above and belowground components of forest ecosystems. I begin with a project using detailed chemical measurements on specific segments of root systems from two different tree species, in order to help refine methods that quantify the production of symbiotic root-associated mycorrhizal fungi. Next, I use top-down and bottom-up approaches to determine a comprehensive carbon budget (including the production of mycorrhizal fungi), as well as interannual drivers of carbon fluxes in a northern temperate forest stand. Lastly, I compare patterns of carbon allocation to plant and fungal components in temperate forest stands spanning a range of species composition. Chapter 1 presents results from a project done in collaboration with Dr. Dali Gou and researchers at the Maoershan research station in China, focusing on fine scale patterns of root anatomy, chemistry, and function. I used patterns in fine root chemistry to assess the importance of symbiotic root-colonizing (mycorrhizal) fungi to two important tree species in China that differ in their mycorrhizal associate type — arbuscular mycorrhizal versus ectomycorrhizal fungi. Results indicated a strong fungal association in ectomycorrhizal Larix gmelinii, with fungal material comprising over 50 % of nitrogen and 36 % of the biomass of root tips in Larix. Data from this work helped refine an approach to quantify the production of mycorrhizal fungi in forest ecosystems using stable isotopes. Chapter 2 is the result of a long term effort to quantify carbon fluxes within northern hardwood temperate forest stands at the Bartlett Experimental Forest, New Hampshire. The stands used in this study are centered on an eddy covariance flux tower (part of the Ameriflux network), and are also part of NASA’s North American Carbon Program (NACP) Tier-2 field research sites. I present a detailed carbon budget of net and gross ecosystem fluxes using measurements collected from 2004-2016. Comparison of interannual fluxes suggested the presence of direct climate controls on wood growth (growing season temperature and moisture), and indirect controls on gross carbon uptake related to conditions in the winter and spring preceding the growing season. The data resulting from this work provide an ideal data set for assessing the capability of ecosystem models to simulate a number of aspects of forest ecosystem carbon dynamics. Chapter 3 is an extension of the carbon measurements around the flux tower at Bartlett, and spans a range of forest stands with varying species composition. This work was unique in its attempt to quantify the production of both plant components and mycorrhizal fungi. Results indicate that as biomass of conifer tree species increased relative to deciduous species, the production of foliage, wood, and fine roots significantly decreased. In contrast, the production of mycorrhizal fungi was more than twice as high in nearly pure conifer stands than in pure deciduous broadleaf stands, at times equaling or exceeding rates of wood production. Stable isotope data indicated that both the tree species present (e.g. conifers), as well as soil nutrient availability were important in influencing rates of fungal production. This dissertation is available at University of New Hampshire Scholars' Repository: https://scholars.unh. edu/dissertation/2301