The establishment and intensive management of forests for the production of timber can have significant effects on the soil carbon dynamics. The establishment of forest on organic soils under grasslands may lead to substantial losses in soil carbon, due to the site preparation for the planting of trees and other disturbances. This is gradually compensated by carbon accumulation in tree biomass as the forest grows until clearfelling at maturity may initiate another substantial carbon loss. This study had two main aims. The first was to investigate the long-term effects of forest establishment on natural grassland as well as clearfelling and re-growth of the forest during second rotation, by looking at the changes in soil carbon stocks and soil carbon balance in a Sitka spruce (Picea sitchsensis) in Harwood (N. E. England). Secondly, to investigate the short-term effects of forest clearfelling on the fluxes of soil CO2, N2O and CH4 and on the environmental factors (soil temperature, water content and water table depth) affecting them. The fluxes were initially measured in two mature stands (40-years old) during one growth season. One of the two stands was subsequently clearfelled while the other was kept intact and fluxes were measured for a further ten months after clearfelling. The relationships between these fluxes and the environmental factors were also examined. The study also investigated the spatial variability of soil CO2 emissions using geostatistical approaches. The soil CO2 fluxes were measured with two methods, a closed dynamic chamber and a closed static chamber, giving the opportunity to compare their relative performance. A performance further investigation on this discrepancy between the two methods took place in lab experiments and on a soil monolith, excavated from the 40-year old stand and kept under controlled conditions in the greenhouse.

The effect of drainage and restoration on the ecology of different types of peatlands in the Šumava Mountains was investigated. The study was focused primarily on peat properties, vegetation dynamics, carbon gas fluxes and their linkages under the affected hydrological regimes.

Global forests cover approximately 30% of land’s surface storing around 45% of above-ground terrestrial carbon. This carbon storage is constantly endangered by anthropogenic activities. Especially, tropical regions like the Amazon rainforest suffer from deforestation taking a great share in global CO2 emissions. In addition, forest dynamics are affected by climatic change like more frequent drought events. Quantifying the impact and feedback mechanisms of such climatic and anthropogenic changes on the global carbon cycle is still a great challenge. In this thesis, we developed a regionalization scheme to apply a forest gap model on the entire Amazon rainforest. Such a forest model has the advantage that it calculates forest growth at the individual tree level. It considers different successional states, that evolve form natural forest dynamics and disturbances, including information on tree height and species. The regionalized forest model thereby allows for integrating forest structure and species compositions into large-scale carbon analyses. The approach is independent of spatial scale and the simulation results can be linked to measurements from field inventory, eddy covariance, and remote sensing at local to continental scales. In a first study (chapter 2), we tested the capability of the forest model FORMIND to simulate gross primary production (GPP), respiration, and net ecosystem exchange (NEE) at daily and yearly time scales. The forest model was applied to spruce forests in Germany in order to analyze how the variability in environmental factors affects simulated carbon fluxes. Simulation results were compared to 6 years of eddy covariance (EC) data at a daily scale. The analysis shows that the forest model described the seasonal cycle of the carbon fluxes correctly, but estimated GPP differed from the observed data on days with extreme climatic conditions. Based on these findings, we developed two new parameterizations. One resulted from a numerical calibration against EC data. The other parameterization resulted from a method where EC data is filtered to extract the limiting factors for productivity. Thereby, new parameter values and even a new function for the temperature limitation of photosynthesis were found. The adopted forest model was then tested successfully at another spruce forest for cross validation. In general, the forest model reproduced the observed carbon fluxes of a forest ecosystem quite well. Although the overall performance of the calibrated model version was best, the filtering approach showed that calibrated parameter values did not necessarily correctly display the individual functional relations. The study has shown that the concept of simulating forest dynamics at the individual tree level is a valuable approach for simulating the NEE, GPP, and respiration of forest ecosystems. The focus of the second study (chapter 3) lied on the simulation of forest structure and above-ground biomass in the Amazon region with the forest model FORMIND. Estimating the spatial variation of biomass in the Amazon rainforest is challenging and, hence, a source of substantial uncertainty in the assessment of the global carbon cycle. On the one hand, estimates need to consider small-scale variations of forest structures due to natural tree mortality. On the other hand, it requires large-scale information on the state of the forest that can be detected by remote sensing. We, here, introduced a novel method that considered both aspects by linking the forest model and a wall-to-wall canopy height map derived from LIDAR remote sensing. The forest model was applied to estimate above-ground biomass stocks across the Amazon rainforest. This allowed for the direct comparison of simulated and observed canopy heights from remote sensing. The comparison enabled the detection of disturbed forest states from which we derived a biomass map of the Amazon rainforest at 0.16 ha resolution. Simulated biomass varied between 20 and 490 t(dry mass) ha-1 across 7.8 Mio km² of the Amazon rainforest (elevation < 1000 m). That equals a total above-ground biomass stock of 76 GtC with a strong spatial variation (coefficient of variation = 63%). The estimated biomass values fit estimates, that had been observed in 114 field inventories, well (deviation of only 15%). Beside biomass, the forest model allowed for estimating additional forest attributes such as basal area and stem density. The linkage of a forest model with a canopy height map allows for capturing forest structures at the individual to large scale. The approach is flexible and can also be combined with measurements of future satellite missions like ESA Biomass or GEDI. Hence, the study sets a basis for large-scale analyses of the heterogeneous structure of tropical forests and their carbon cycle. In a third study (chapter 4), we analyzed the interactions of productivity, biomass, and forest structure that are essential for understanding ecosystem’s response to climatic and anthropogenic changes. We here applied the forest model on the Amazon rainforest, combined simulation results with remotely-sensed data as in chapter 3, and additionally simulated ecosystem carbon fluxes. We found that the successional state of a forest has a strong influence on mean annual net ecosystem productivity (NEP), woody above-ground net primary production (wANPP), and net ecosystem productivity (NEP). These relations were used to derive maps of carbon fluxes at 0.16 ha resolutions (current state of the Amazon rainforest under spatial heterogenic environmental conditions). The Amazon was estimated to be a sink of atmospheric carbon with a mean NEP of 0.73 tC ha-1 a-1. Mean wANPP equals 4.16 tC ha-1 a-1 and GPP 25.2 tC ha-1 a-1. We found that forests in intermediate successional states are the most productive. Under current conditions, the Amazon rainforest takes up 0.59 PgC per year. This third study shows that forest structure and species compositions substantially influence productivity and biomass, and should not be neglected when estimating current carbon budgets or climate change scenarios for the Amazon rainforest. The findings of this thesis set a fundament for future analyses on carbon storage and fluxes of forests. Simulating at the tree level has the potential to investigate carbon dynamics from individual to continental scales. The regionalized forest model allows for the integration of different types of remotely sensed data in order to improve the spatial accuracy of estimates. The insights, we have gained from the eddy covariance study (chapter 2), help to investigate carbon dynamics of forests at continental scale also under changing climate. In combination with the regionalization approach (chapter 3 and 4), the findings of this thesis may be used to complement studies on drought events in forests and to understand feedback mechanisms caused by anthropogenic disturbances.

A considerable part of the global pool of terrestrial carbon is stored in high latitude soils. In these soils, repeated cycles of freezing and thawing creates soil motion (cryoturbation) that in combination with other cryogenic disturbance processes may play a profound role in controlling the carbon balance of the arctic soil. Conditions for cryogenic soil processes are predicted to dramatically change in response to the ongoing climate warming, but little is known how these changes may affect the ability of arctic soils to accumulate carbon. In this thesis, I utilize a patterned ground system, referred to as non-sorted circles, as experimental units and quantify how cryogenic soil processes affect plant communities and carbon fluxes in arctic soils. I show that the cryoturbation has been an important mechanism for transporting carbon downwards in the studied soil over the last millennia. Interestingly, burial of organic material by cryoturbation appears to have mainly occurred during bioclimatic events occurring around A.D. 900-1250 and A.D. 1650-1950 as indicated by inferred 14C ages. Using a novel photogrammetric approach, I estimate that about 0.2-0.8 % of the carbon pool is annually subjected to a net downward transport induced by the physical motion of soil. Even though this flux seems small, it suggests that cryoturbation is an important transporter of carbon over centennial and millennial timescales and contributes to translocate organic matter to deeper soil layers where respiration proceeds at slow rates. Cryogenic processes not only affect the trajectories of the soil carbon, but also generate plant community changes in both species composition and abundance, as indicated by a conducted plant survey on non-sorted circles subjected to variable differential frost heave during the winter. Here, disturbance-tolerant plant species, such as Carex capillaris and Tofieldia pusilla, seem to be favoured by disturbance generated by the differential heave. Comparison with findings from a previous plant survey on the site conducted in the 1980s suggest that the warmer temperatures during the last decades have resulted in decreased differential heave in the studied non-sorted circles. I argue that this change in cryogenic activity has increased abundance of plants present in the 1980s. The fact that the activity and function of the non-sorted circles in Abisko are undergoing changes is further supported by their contemporary carbon dioxide (CO2) fluxes. Here, my measurements of CO2 fluxes suggest that all studied non-sorted circles act as net CO2 sources and thus that the carbon balance of the soils are in a transition state. My results highlight the complex but important relationship between cryogenic soil processes and the carbon balance of arctic soils.

In order to predict how terrestrial ecosystems will respond to global change, there is growing recognition that we need to better understand linkages between plant and soil processes. Previously the factors and processes with potential to influence the terrestrial carbon (C) cycle have been investigated in isolation from each other. This study investigated the interactions of nutrient availability and warming in controlling the soil carbon dynamics, with regards to the fate of already sequestered carbon in soil, under conditions of increasing atmospheric temperatures. The project objectives were addressed by three independent experiments designed to explain specific components of the carbon-nutrient cycle interactions, and the findings brought together to describe the implications for future soil carbon storage. The main measurements collected throughout this project included soil carbon dioxide (CO2) fluxes, partitioned into autotrophic and heterotrophic components, net ecosystem exchange and respiration fluxes, and background soil moisture and temperature data, backed by gas, soil and biomass analyses. In the two field experiments, these measurements were taken from plots with or without any inorganic nutrient additions or in the presence or absence of legumes providing biological nitrogen addition to the ecosystem. In the laboratory, temperature and nutrient availability were manipulated within the ecosystem. The reduction in decomposition rates, without reduction of productivity as a result of inorganic nutrient additions, indicated the potential for increasing C storage. There was also evidence that nutrient availability controls the strength of the link between plant and soil processes in semi-natural grasslands. The yields, decomposition rates and soil C fluxes recorded in the presence and absence of legumes provided some evidence of N2 fixation, improving ecosystem productivity and soil properties while reducing soil C effluxes, in a managed grassland. In the laboratory, the warming of soils from lysimeters with and without plants, receiving or not receiving fertiliser, supported the findings from field experiments regarding the importance of the soil-plant link in controlling C fluxes. However, C stocks and δ13C analyses showed that over a year’s worth of warming and nutrient manipulations made little difference to the amount of C stored in the soil, indicating that edaphic factors have greater control over the response of C dynamics to increased temperatures.

<p>High-latitude watersheds have been regarded as a carbon sink with soil carbon accumulating at low temperature. This sink is now believed to turn into a source, acting as positive feedback to climate warming. However, thawing permafrost soils would allow more water to percolate down to deeper soil layers where some of the carbon could be “consumed” in weathering and exported as bicarbonate to the sea. Using a hydrological mixing model showed that this could counterbalance the predicted positive feedback resulting from thawing soils.</p><p>Vegetation-covered riparian zones in headwater areas appear to have a significant role for the dissolved constituent fluxes. Higher concentrations of weathering products are found in taiga and tundra rivers with larger areas of forest and peat cover in the watershed. These landscape elements can thus be regarded as “hot spots” of river loading with dissolved constituents.</p><p>Comparing a regulated and an unregulated river tested the hypothesis that damming leads to a depletion of major elements also in oligotrophic river systems as a consequence of changes in landscape elements. A loss of upper soils and vegetation through inundation prevents the contact of surface waters with vegetated soil, and consequently reduces weathering fluxes. The hypothesis that the lower fluxes of dissolved silica (DSi) in the regulated river could also be explained by biological uptake was then tested using a model, and budget calculations indicate a significant reduction as a result of regulation. About 10% of this reduction can be attributed to the flooding of the fluvial corridor and the rest to diatom blooms in the reservoirs. A more detailed study of landscape elements for the headwaters of the river Luleälven showed that only 3% of the surface area has been inundated by reservoirs but ca. 37% of the deciduous forest. Such a significant loss of hot spots may indeed explain the observed lower DSi fluxes in the regulated watersheds of northern Sweden.</p>

High-latitude watersheds have been regarded as a carbon sink with soil carbon accumulating at low temperature. This sink is now believed to turn into a source, acting as positive feedback to climate warming. However, thawing permafrost soils would allow more water to percolate down to deeper soil layers where some of the carbon could be “consumed” in weathering and exported as bicarbonate to the sea. Using a hydrological mixing model showed that this could counterbalance the predicted positive feedback resulting from thawing soils. Vegetation-covered riparian zones in headwater areas appear to have a significant role for the dissolved constituent fluxes. Higher concentrations of weathering products are found in taiga and tundra rivers with larger areas of forest and peat cover in the watershed. These landscape elements can thus be regarded as “hot spots” of river loading with dissolved constituents. Comparing a regulated and an unregulated river tested the hypothesis that damming leads to a depletion of major elements also in oligotrophic river systems as a consequence of changes in landscape elements. A loss of upper soils and vegetation through inundation prevents the contact of surface waters with vegetated soil, and consequently reduces weathering fluxes. The hypothesis that the lower fluxes of dissolved silica (DSi) in the regulated river could also be explained by biological uptake was then tested using a model, and budget calculations indicate a significant reduction as a result of regulation. About 10% of this reduction can be attributed to the flooding of the fluvial corridor and the rest to diatom blooms in the reservoirs. A more detailed study of landscape elements for the headwaters of the river Luleälven showed that only 3% of the surface area has been inundated by reservoirs but ca. 37% of the deciduous forest. Such a significant loss of hot spots may indeed explain the observed lower DSi fluxes in the regulated watersheds of northern Sweden.

Studies of trace gas fluxes have advanced the understanding of bulk interactions between the atmosphere and ecosystems. Micrometeorological instrumentation is currently unable to resolve vertical scalar sources and sinks within plant canopies. Inverted analytical Lagrangian equations provide a non-intrusive method to calculate source distributions. These equations are based on Taylor's (1921) description of scalar dispersion, which requires a measure of the degree of correlation between turbulent motions, defined by the Lagrangian length scale (L). Inverse Lagrangian (IL) analyses can be unstable, and the uncertainty in L leads to uncertainty in source predictions. A review of the literature on studies using IL analysis with various scalars in a multitude of canopy types found that parameterizations where L reduces to zero at the ground produce better results in the IL analysis than those that increase closer to the ground, but no individual L parameterization gives better results than any other does. The review also found that the relationship between L and the measurable Eulerian length scale (Le) may be more complex in plant canopies than the linear scaling investigated in boundary layer flows. The magnitude and profile shape of L was investigated within a corn and a forest canopy using field measurements to constrain an analytical Lagrangian equation. Measurements of net CO2 flux, soil-to-atmosphere CO2 flux, and in-canopy profiles of CO2 concentrations provided the information required to solve for L in a global optimization algorithm for half hour intervals. For dates when the corn was a strong CO2 sink, and for the majority of dates for the forest, the optimization frequently located L profiles that follow a convex shape. A constrained optimization then smoothed the profile shape to a sigmoidal equation. Inputting the optimized L profiles in the forward and inverse Lagrangian equations leads to strong correlations between measured and calculated concentrations (corn canopy: C_{calc} = 1.00C_{meas} +52.41 mumol m^{-3}, r^2 = 0.996; forest canopy: C_{calc} = 0.98C_{meas} +276.5 mumol m^{-3}, r^2 = 0.99) and fluxes (corn canopy: F_{soil} = 0.67F_{calc} - 0.12 mumol m^{-2}s^{-1}, r^2 = 0.71, F_{net} = 1.17F_{calc} + 1.97mumol m^{-2}s^{-1}, r^2 = 0.85; forest canopy: F_{soil} = 0.72F_{calc} - 1.92 mumol m^{-2}s^{-1}, r^2 = 0.18, F_{net} = 1.24F_{calc} + 0.65 mumol m^{-2}s^{-1}, r^2 = 0.88). In the corn canopy, coefficients of the sigmoidal equation were specific to each half hour and did not scale with any measured variable. Coefficients of the optimized L equation in the forest canopy scaled weakly with variables related to the stability above the canopy. Plausible L profiles for both canopies were associated with negative bulk Richardson number values. / Funding from NSERC.

No description available.

<p>Landscape-level understanding of forest carbon (C) dynamics is required to quantify the net contribution of forest biomes to the global C cycle and to help forest managers to understand the impacts of forest management activities to the C sequestration in forests. Landscape-level estimation of C exchanges in various ecosystems is also crucial for the validation of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived Gross Primary Productivity (GPP) which may help in improving MODIS GPP algorithm and the estimation of national-scale C budget to meet Canada’s international greenhouse gas inventory reporting obligations.</p> <p>In this study, Carbon version of Canadian Land Surface Scheme (C-CLASS) was used to simulate historic C dynamics of a 2500 ha temperate Douglas fir forest landscape in Oyster River area of Vancouver Island in British Columbia from 1920 to 2005 and a 6275 ha boreal black spruce forest landscape at Chibougamau, Quebec from 1928 to 2005. The impacts of disturbance history and the climate variability on the landscape-level C stocks and fluxes were also investigated. The disturbance matrix of the Carbon Budget Model of the Canadian Forest Sector v3 (CBM-CFS3) was incorporated into C-CLASS to account for the removal of the C stocks by disturbance events. Study results indicate that GPP and autotrophic respiration (R_a) in the temperate Douglas fir forest landscape are sensitive to the air temperature variability. Stand replacing disturbance events can remove large amounts of C in the disturbed year, however, it takes a long period of time for the recovery of landscape-level total ecosystem carbon (TEC) to the initial state, which depends on forest age and the effects of historic climate variability. Our analysis further showed that in undisturbed forest landscape, simulated annual net ecosystem productivity (NEP) deviations were positively related to daily minimum and maximum temperatures in spring, while they were not sensitive to summer temperatures. Study results also showed that simulated landscape-level NEP is less sensitive to the changes in air temperature compared to other simulated C fluxes such as GPP, R_a and and heterotrophic respiration (R_h). Simulated landscape-level C stocks (aboveground biomass, belowground biomass, dead organic matter and soil organic matter) are sensitive to the changes in air temperature. This work suggests that the C-CLASS model can be used to investigate the impacts of climate variability and disturbance events on the historic C dynamics of forest landscapes. This study has also made it possible to analyze the importance of climate drivers and the development of methods for including climate sensitivity into inventory-based models.<strong></strong></p> <p>In addition, C-CLASS simulated GPP overCanada’s landmass (at 1-km resolution) in 2003 and its comparison with the MODIS GPP product (MOD17) indicated overestimation of MODIS GPP compared to the C-CLASS upscaled GPP overCanada’s landmass. This overestimation was attributed to the limitations in the components of MODIS GPP algorithm. It further suggests that the parameterization of light use efficiency in MODIS GPP algorithm is amenable to improvement based on observations of light use efficiency at eddy covariance flux tower sites or the photochemical reflectance index derived from satellite remote sensing data.</p> <p>This study would be helpful in calculating Canada’s national terrestrial ecosystem C budget which is important for making environmental policies and ecosystem management for enhancing the terrestrial C sink.<strong></strong></p> / Doctor of Philosophy (PhD)