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

Mycorrhizal roots in a temperate forest take up organic nitrogen from 13C- and 15N-labeled organic matter

Background and Aims The importance of the uptake of nitrogen in organic form by plants and mycorrhizal fungi has been demonstrated in various ecosystems including temperate forests. However, in previous experiments, isotopically labeled amino acids were often added to soils in concentrations that may be higher than those normally available to roots and mycorrhizal hyphae in situ, and these high concentrations could contribute to exaggerated uptake. Methods We used an experimental approach in which we added 13C-labeled and 15N-labeled whole cells to root-ingrowth cores, allowing proteolytic enzymes to release labeled organic nitrogen at a natural rate, as roots and their associated mycorrhizal fungi grew into the cores. We employed this method in four forest types representing a gradient of soil pH, nitrogen mineralization rate, and mycorrhizal type. Results Intact uptake of organic nitrogen was detected in mycorrhizal roots, and accounted for at least of 1-14% of labeled nitrogen uptake. Forest types did not differ significantly in the importance of organic uptake. Conclusions The estimates of organic N uptake here using 13C-labeled and 15N-labeled whole cells are less than those reported in other temperate forest studies using isotopically labelled amino acids, and likely represent a minimum estimate of organic N-use. The two approaches each have different assumptions, and when used in tandem should complement one another and provide upper and lower bounds of organic N use by plants

Correcting tree-ring δ13C time series for tree-size effects in eight temperate tree species

Stable carbon isotope ratios (δ13C) in tree rings have been widely used to study changes in intrinsic water-use efficiency (iWUE), sometimes with limited consideration of how C-isotope discrimination is affected by tree height and canopy position. Our goals were to quantify the relationships between tree size or tree microenvironment and wood δ13C for eight functionally diverse temperate tree species in northern New England, and to better understand the physical and physiological mechanisms underlying these differences. We collected short increment cores in closed-canopy stands and analyzed δ13C in the most recent 5 years of growth. We also sampled saplings in both shaded and sun-exposed environments. In closed-canopy stands, we found strong tree-size effects on δ13C, with 3.7 – 7.2‰ of difference explained by linear regression vs. height (0.11 – 0.28‰ m-1), which in some cases is substantially stronger than the effect reported in previous studies. However, open-grown saplings were often isotopically more similar to large codominant trees than to shade-grown saplings, indicating that light exposure contributes more to the physiological and isotopic differences between small and large trees than does height. We found that in closed-canopy forests, δ13C correlations with DBH were nonlinear but also strong, allowing a straightforward procedure to correct tree- or stand-scale δ13C-based iWUE chronologies for changing tree size. We demonstrate how to use such data to correct and interpret multi-decadal composite isotope chronologies in both shade-regenerated and open-grown tree cohorts, and we highlight the importance of understanding site history when interpreting δ13C time series

The role of surface roughness, albedo, and Bowen ratio on ecosystem energy balance in the Eastern United States

Land cover and land use influence surface climate through differences in biophysical surface properties, including partitioning of sensible and latent heat (e.g., Bowen ratio), surface roughness, and albedo. Clusters of closely spaced eddy covariance towers (e.g., \u3c10 \u3ekm) over a variety of land cover and land use types provide a unique opportunity to study the local effects of land cover and land use on surface temperature. We assess contributions albedo, energy redistribution due to differences in surface roughness and energy redistribution due to differences in the Bowen ratio using two eddy covariance tower clusters and the coupled (land-atmosphere) Variable-Resolution Community Earth System Model. Results suggest that surface roughness is the dominant biophysical factor contributing to differences in surface temperature between forested and deforested lands. Surface temperature of open land is cooler (−4.8 °C to −0.05 °C) than forest at night and warmer (+0.16 °C to +8.2 °C) during the day at northern and southern tower clusters throughout the year, consistent with modeled calculations. At annual timescales, the biophysical contributions of albedo and Bowen ratio have a negligible impact on surface temperature, however the higher albedo of snow-covered open land compared to forest leads to cooler winter surface temperatures over open lands (−0.4 °C to −0.8 °C). In both the models and observation, the difference in mid-day surface temperature calculated from the sum of the individual biophysical factors is greater than the difference in surface temperature calculated from radiative temperature and potential temperature. Differences in measured and modeled air temperature at the blending height, assumptions about independence of biophysical factors, and model biases in surface energy fluxes may contribute to daytime biases

Mycena species can be opportunist-generalist plant root invaders

ACKNOWLEDGEMENTS We thank Karl-Henrik Larsson and Arne Aronsen for provisions of specimens from the Natural History Museum of Oslo and help with the identification of field specimens from Svalbard. We further thank Cecilie Mathiesen and Mikayla Jacobs for technical assistance in the laboratory, Brendan J. Furneaux for valuable input to the R script, and the curators of H, TUR, and OULU. The Mycena ITS sequences originating from the specimens deposited in H, TUR, and OULU were produced as part of the Finnish Barcode of Life Project (FinBOL) funded by the Ministry of Environment, Finland (YM23/5512/2013), Otto A Malm's Donationsfond, and the Kone Foundation. We thank the European Commission (grant no. 658849) and the Carlsberg Foundation (grant no. CF18-0809) for grants to C.B. Harder that made this research possible. C.B. Harder was financed by a grant from the Danish Independent Research Fund DFF/FNU 2032-00064B (SapMyc) at the time of writing. Research Funding Carlsbergfondet. Grant Number: CF18-0809 Danish Independent Research Fund. Grant Number: 2032-00064B European Commission. Grant Number: 658849 Ministry of Environment, Finland. Grant Number: YM23/5512/2013Peer reviewedPublisher PD

Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ15N) data, patterns of 15N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum 15N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The 15N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling 15N in soil profiles, fractionation against 15N during N transfer by mycorrhizal fungi to host plants, leads to 15N-depleted plant litter at the soil surface and 15N-enriched nitrogen of fungal origin at depth. The preferential preservation of 15N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large 15N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ15N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site

Insights into root growth, function, and mycorrhizal abundance from chemical and isotopic data across root orders

Background and aims: Detailed analyses of root chemistry by branching order may provide insights into root function, root lifespan and the abundance of root-associated mycorrhizal fungi in forest ecosystems. Methods: We examined the nitrogen and carbon stable isotopes (δ15N and δ13C) and concentration (%N and %C) in the fine roots of an arbuscular mycorrhizal tree, Fraxinus mandshurica, and an ectomycorrhizal tree, Larix gmelinii, over depth, time, and across five root branching orders. Results and conclusions: Larix δ15N increased by 2.3 ‰ from 4th order to 1st order roots, reflecting the increased presence of 15N-enriched ECM fungi on the lower root orders. In contrast, arbuscular mycorrhizal Fraxinus only increased by 0.7 ‰ from 4th order to 1st order roots, reflecting the smaller 15N enrichment and lower fungal mass on arbuscular mycorrhizal fine roots. Isotopic and anatomical mass balance calculations indicate that first, second, and third order roots in ectomycorrhizal Larix averaged 36 %, 23 %, and 8 % fungal tissue by mass, respectively. Using literature values of root production by root branching order, we estimate that about 25 % of fine root production in ECM species like Larix is actually of fungal sheaths. In contrast to %N, %C, and δ15N, δ13C changed minimally across depth, time, and branching order. The homogeneity of δ13C suggests root tissues are constructed from a large well-mixed reservoir of carbon, although compound specific δ13C data is needed to fully interpret these patterns. The measurements developed here are an important step towards explicitly including mycorrhizal production in forest ecosystem carbon budgets