Aspects of precommercial thinning in heterogeneous forests in southern Sweden

The overall objective of the work underlying this thesis was to suggest and evaluate possible strategies for the tending of young heterogeneous stands of Norway spruce, Scots pine and birch in southern Sweden. Heterogeneity was defined as variation in species composition, height distribution and spatial arrangement of the trees. The influence of stand density after precommercial thinning and timing of thinning on the diameter of the thickest branch was studied for naturally regenerated Scots pine. The branch diameter was found to decrease with increasing number of remaining stems after precommercial thinning. However, leaving very dense stands (> 3000 stems ha-1) resulted only in a minor reduction of the branch diameter. Late precommercial thinning, compared to early, reduced the branch diameter. The influence of the precommercial thinning regime on the crown ratio (living crown length/tree height) was also analysed. To be able to simulate the influence of different management options on the development of the young forest, single-tree growth models was developed for Scots pine, Norway spruce and birch. Height growth and diameter was estimated as a function of tree height, stand and site variables. Growth reduction due to competition was estimated using individual, distance independent indices as well as expressions of the overall stand density. In the third study the influence of stand structure after precommercial thinning on the development of mixtures between Norway spruce and silver birch was simulated. The aim was to identify mixtures that allowed both species to develop well until the first commercial thinning. By leaving birches with an average height slightly greater than spruce at precommercial thinning, a large proportion of competitive birches were available at first commercial thinning, at the same time as the relative diameter distribution of spruce in the mixture was equal to that of a pure spruce stand of the same density. The height difference between the species as well as the species proportion had a decisive impact on volume production. In the fourth study different precommercial thinning strategies were identified and applied to a heterogeneous stand including Scots pine, Norway spruce and birch. Stand development and economical returns over a rotation was estimated using a set of empirical models. The aim of the long-term strategies was: (i) a conifer dominated stand with focus on high production, (ii) a conifer dominated stand with focus on high timber quality, (iii) to preserve the heterogeneous stand structure, (iv) a mosaic pattern by tree species, (v) to reduce the precommercial thinning cost, without jeopardizing the future stand development. The difference in total volume production was found to be relatively small between the strategies. The lowest production was found for the strategies promoting species mixture at tree level (iii) and group level (iv). The net present value was highest for the strategy aiming at high production (ii) and lowest for the strategy aiming at preserved heterogeneity (iii). The minimal precommercial thinning (v) was a less profitable alternative, mainly because of an expensive first commercial thinning. Differences in timber quality were not considered in the simulations. The case study illustrates the possibilities for influencing the structure of a heterogeneous stand through precommercial thinning, as well as the limitations imposed by the initial stand structure

Thermoelectric Phenomena in Quantum Dots

Thermoelectricity is being intensively researched as it is believed to hold great promise for applications in power generation and cooling. One way to quantify the electrical power output of a thermoelectric material is the power factor, a function of electrical conductivity and thermopower. There are relationships between these relevant material properties that make efficient thermoelectric materials challenging to produce. The development of methods for creating nanostructured materials has allowed such trade-offs in material properties to be circumvented. Quantum dots are useful as model systems in this context since they have tunable energy filtering effects that are straightforward to characterize. The work described in this thesis explores thermoelectric phenomena in quantum dots. The aim of this work was to gain a better understanding of the most basic thermoelectric behavior of quantum dots. This knowledge can provide deeper insight into which mechanisms may be of interest in increasing the efficiency of a thermoelectric material. A deeper understanding also allows the measurement method itself to be used as a tool for characterization. A thermoelectric measurement can complement the more commonly used electrical conductance measurements, by both confirming and supplementing data. This could be of great importance for the investigation of physical phenomena in nanostructures. The quantum dots used in this work were defined in semiconductor nanowires. They were formed either by heterostructure growth or afterwards during fabrication of devices. The thermoelectric properties of the quantum dots were thoroughly investigated in the Coulomb blockade regime, and both linear and nonlinear responses as a function of the applied thermal gradient were observed and explained. Thermoelectric measurements were also successfully used to characterize different InAs nanowire devices, either with the nanowire as is or covered by a polymer electrolyte. Closer investigations of these devices revealed physical properties of the nanowires that could be used to improve thermoelectric efficiency. In fact, this thesis presents the first measurements demonstrating an increase in thermoelectric power factor at low temperatures

Nonlinear thermoelectric response due to energy-dependent transport properties of a quantum dot

Quantum dots are useful model systems for studying quantum thermoelectric behavior because of their highly energy-dependent electron transport properties, which are tunable by electrostatic gating. As a result of this strong energy dependence, the thermoelectric response of quantum dots is expected to be nonlinear with respect to an applied thermal bias. However, until now this effect has been challenging to observe because, first, it is experimentally difficult to apply a sufficiently large thermal bias at the nanoscale and, second, it is difficult to distinguish thermal bias effects from purely temperature-dependent effects due to overall heating of a device. Here we take advantage of a novel thermal biasing technique and demonstrate a nonlinear thermoelectric response in a quantum dot which is defined in a heterostructured semiconductor nanowire. We also show that a theoretical model based on the Master equations fully explains the observed nonlinear thermoelectric response given the energy-dependent transport properties of the quantum dot.Comment: Cite as: A. Svilans, et al., Physica E (2015), http://dx.doi.org/10.1016/j.physe.2015.10.00

Select or Not? Comparing the Impact of Selective and Schematic Thinning on Scots Pine Tree Growth and Stand Structure

The first commercial thinning usually entails a high cost in harvest operations and a low resulting income. From a practical forestry perspective, a schematic spatial selection might be more efficient than a selective approach. Therefore, this study aimed to compare basal area, total and standing volumes, and periodic annual increment (PAI), as well as stand structure, between different thinning designs (selective and schematic thinning) and strategies (thinning once or more than once) over a long-term monitoring period of Scots pine (Pinus sylvestris) plantations in Sweden. We also evaluated the relevancy of distance-dependent competition indices (CIs) in individual tree growth models by comparing growth model predictions with the use of distance-dependent and distance-independent CIs. Despite higher heterogeneity in schematically thinned stands, there were no significant differences in standing and total volumes (m(3)& BULL;ha(-1)) among treatments in the short or long term. Although the inclusion of a distance-dependent CI improved the model slightly, distance-independent models predicted diameter growth just as effectively. Schematic thinning could be a viable option for a first commercial thinning or one-time thinning if, at least, one more thinning is included in the management plan, or if the motivating interest is mainly volume

Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics

We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and thermovoltage measurements with modeling. Our results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.Comment: Published in Advanced Functional Materials. Includes colour versions of figures and supplementary informatio

A quantum-dot heat engine operating close to the thermodynamic efficiency limits

Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs. As they do not require moving parts and can be realized in solid-state materials, they are suitable for low-power applications and miniaturization. It was predicted that PE engines could reach the same thermodynamically ideal efficiency limits as those accessible to cyclical engines, but this prediction has not been verified experimentally. Here, we demonstrate a PE heat engine based on a quantum dot (QD) embedded into a semiconductor nanowire. We directly measure the engine's steady-state electric power output and combine it with the calculated electronic heat flow to determine the electronic efficiency $\eta$. We find that at the maximum power conditions, $\eta$ is in agreement with the Curzon-Ahlborn efficiency and that the overall maximum $\eta$ is in excess of 70$\%$ of the Carnot efficiency while maintaining a finite power output. Our results demonstrate that thermoelectric power conversion can, in principle, be achieved close to the thermodynamic limits, with direct relevance for future hot-carrier photovoltaics, on-chip coolers or energy harvesters for quantum technologies

InAs nanowire transistors with multiple, independent wrap-gate segments

We report a method for making horizontal wrap-gate nanowire transistors with up to four independently controllable wrap-gated segments. While the step up to two independent wrap-gates requires a major change in fabrication methodology, a key advantage to this new approach, and the horizontal orientation more generally, is that achieving more than two wrap-gate segments then requires no extra fabrication steps. This is in contrast to the vertical orientation, where a significant subset of the fabrication steps needs to be repeated for each additional gate. We show that cross-talk between adjacent wrap-gate segments is negligible despite separations less than 200 nm. We also demonstrate the ability to make multiple wrap-gate transistors on a single nanowire using the exact same process. The excellent scalability potential of horizontal wrap-gate nanowire transistors makes them highly favourable for the development of advanced nanowire devices and possible integration with vertical wrap-gate nanowire transistors in 3D nanowire network architectures.Comment: 18 pages, 5 figures, In press for Nano Letters (DOI below

Pre-Commercial Thinning Increases the Profitability of Norway Spruce Monoculture and Supports Norway Spruce-Birch Mixture over Full Rotations

Pre-commercial thinning (PCT) is a common measure in Norway spruce (Picea abies L. Karst.) stands but the profitability of doing PCT and timing of PCT has not been fully investigated over a full rotation. Further, limited knowledge is available for mixed forest management compared to monocultures. In this study, different PCT strategies were tested to investigate the effect of PCT and timing of PCT on the production and profitability of Norway spruce monocultures and mixed Norway spruce-birch stands. A forest decision support system was used to simulate stand development during the whole rotation. Our study findings show that there is a positive effect of PCT on Norway spruce plantations' long-term profitability but that the timing of PCT has little effect on profitability. However, site variation might influence the effect of PCT timing on the profitability of Norway spruce stands. Moreover, we also found that retaining 1000 Norway spruce ha(-1) and 1000 birch ha(-1) after PCT supports a mixture of Norway spruce and birch over a full rotation, with little or no economic loss compared to pure Norway spruce stands. Therefore, such a mixture can lead to profitable production while still providing other ecosystem services

Thermoelectric characterization of the Kondo resonance in nanowire quantum dots

We experimentally verify hitherto untested theoretical predictions about the thermoelectric properties of Kondo correlated quantum dots (QDs). The specific conditions required for this study are obtained by using QDs epitaxially grown in nanowires, combined with a recently developed method for controlling and measuring temperature differences at the nanoscale. This makes it possible to obtain data of very high quality both below and above the Kondo temperature, and allows a quantitative comparison with theoretical predictions. Specifically, we verify that Kondo correlations can induce a polarity change of the thermoelectric current, which can be reversed either by increasing the temperature or by applying a magnetic field