92 research outputs found
Aboveground Deadwood Biomass and Composition Along Elevation and Land-Use Gradients at Mount Kilimanjaro
Deadwood is an important structural and functional component of forest ecosystems and biodiversity. As deadwood can make up large portions of the total aboveground biomass, it plays an important role in the terrestrial carbon (C) cycle. Nevertheless, in tropical ecosystems and especially in Africa, quantitative studies on this topic remain scarce. We conducted an aboveground deadwood inventory along two environmental gradients-elevation and land use- at Mt. Kilimanjaro, Tanzania. We used a huge elevation gradient (3690 m) along the southern slope of the mountain to investigate how deadwood is accumulated across different climate and vegetation zones. We also compared habitats that differed from natural forsts in land-use intensity and disturbance history to assess anthropogenic influence on deadwood accumulation. In our inventory we distinguished coarse woody debris (CWD) from fine woody debris (FWD). Furthermore, we calculated the C and nitrogen (N) content of deadwood and how the C/N ratio varied with decomposition stages and elevation. Total amounts of aboveground deadwood ranged from 0.07 +/- 0.04 to 73.78 +/- 36.26 Mg ha(-1) (Mean +/- 1 SE). Across the elevation gradient, total deadwood accumulation was highest at mid-elevations and reached a near-zero minimum at very low and very high altitudes. This unimodal pattern was mainly driven by the corresponding amount of live aboveground biomass and the combined effects of decomposer communities and climate. Land-use conversion from natural forests into traditional homegardens and commercial plantations, in addition to frequent burning, significantly reduced deadwood biomass, but not past selective logging after 30 years of recovery time. Furthermore, we found that deadwood C content increased with altitude. Our study shows that environmental gradients, especially temperature and precipitation, as well as different anthropogenic disturbances can have considerable effects on both the quantity and composition of deadwood in tropical forests
Are rare plant species less resistant than common ones to herbivores? A multi-plant species study using above- and below-ground generalist herbivores.
Rare plant species are suggested to be less resistant to herbivores than common species. Their lower apparency and the fact that they often live in isolated populations, resulting in fewer herbivore encounters, might have led to the evolution of reduced defences. Moreover, their frequent lower levels of genetic diversity compared with common species could negatively affect their resistance against enemies. However, the hypothesis that plant resistance depends on plant regional and local rarity, independently of habitat and competitive and growth strategy, lacks evidence. To test this hypothesis, we assessed the performance and preference of one belowground and three aboveground generalist invertebrate herbivores from different taxonomic groups as indicators of plant resistance. Herbivores were fed a total of 62 regionally and locally rare and common plant species from Switzerland. We accounted for differences in a plant's growth and competitive strategy and habitat resource availability. We found that regionally and locally rare and common plant species did not generally differ in their resistance to most generalist herbivores. However, one herbivore species even performed better and preferred locally and regionally common plant species over rarer ones, indicating that common species are not more resistant, but tend to be less resistant. We also found that all herbivore species consistently performed better on competitive and large plant species, although different herbivore species generally preferred and performed better on different plant species. The latter indicates that the use of generalist herbivores as indicators of plant-resistance levels can be misleading. Synthesis: Our results show that rare plant species are not inherently less resistant than common ones to herbivores. Instead, our results suggest that the ability of plants to allocate resources away from defence towards enhancing their competitive ability might have allowed plants to tolerate herbivory, and to become locally and regionally common
Single-hole transistor in p-type GaAs/AlGaAs heterostructures
A single-hole transistor is patterned in a p-type, C-doped GaAs/AlGaAs
heterostructure by AFM oxidation lithography. Clear Coulomb blockade resonances
have been observed at T=300 mK. A charging energy of ~ 1.5 meV is extracted
from Coulomb diamond measurements, in agreement with the lithographic
dimensions of the dot. The absence of excited states in Coulomb diamond
measurements, as well as the temperature dependence of Coulomb peak heights
indicate that the dot is in the multi-level transport regime. Fluctuations in
peak spacings larger than the estimated mean single-particle level spacing are
observed.Comment: 4 pages, 5 figure
Aharonov-Bohm oscillations in the presence of strong spin-orbit interactions
We have measured highly visible Aharonov-Bohm (AB) oscillations in a ring
structure defined by local anodic oxidation on a p-type GaAs heterostructure
with strong spin-orbit interactions. Clear beating patterns observed in the raw
data can be interpreted in terms of a spin geometric phase. Besides h/e
oscillations, we resolve the contributions from the second harmonic of AB
oscillations and also find a beating in these h/2e oscillations. A resistance
minimum at B=0T, present in all gate configurations, is the signature of
destructive interference of the spins propagating along time-reversed paths.Comment: 4 pages, 3 figures, published versio
Microwave photon-mediated interactions between semiconductor qubits
The realization of a coherent interface between distant charge or spin qubits
in semiconductor quantum dots is an open challenge for quantum information
processing. Here we demonstrate both resonant and non-resonant photon-mediated
coherent interactions between double quantum dot charge qubits separated by
several tens of micrometers. We present clear spectroscopic evidence of the
collective enhancement of the resonant coupling of two qubits. With both qubits
detuned from the resonator we observe exchange coupling between the qubits
mediated by virtual photons. In both instances pronounced bright and dark
states governed by the symmetry of the qubit-field interaction are found. Our
observations are in excellent quantitative agreement with master-equation
simulations. The extracted two-qubit coupling strengths significantly exceed
the linewidths of the combined resonator-qubit system. This indicates that this
approach is viable for creating photon-mediated two-qubit gates in quantum dot
based systems.Comment: 14 pages, 10 figures and 6 table
Strong Coupling Cavity QED with Gate-Defined Double Quantum Dots Enabled by a High Impedance Resonator
The strong coupling limit of cavity quantum electrodynamics (QED) implies the
capability of a matter-like quantum system to coherently transform an
individual excitation into a single photon within a resonant structure. This
not only enables essential processes required for quantum information
processing but also allows for fundamental studies of matter-light interaction.
In this work we demonstrate strong coupling between the charge degree of
freedom in a gate-detuned GaAs double quantum dot (DQD) and a frequency-tunable
high impedance resonator realized using an array of superconducting quantum
interference devices (SQUIDs). In the resonant regime, we resolve the vacuum
Rabi mode splitting of size MHz at a resonator linewidth
MHz and a DQD charge qubit dephasing rate of MHz extracted independently from microwave spectroscopy in the dispersive
regime. Our measurements indicate a viable path towards using circuit based
cavity QED for quantum information processing in semiconductor nano-structures
Aharonov-Bohm oscillations in p-type GaAs quantum rings
We have explored phase coherent transport of holes in two p-type GaAs quantum
rings with orbital radii 420 nm and 160 nm fabricated with AFM oxidation
lithography. Highly visible Aharonov-Bohm (AB) oscillations are measured in
both rings, with an amplitude of the oscillations exceeding 10% of the total
resistance in the case of the ring with a radius of 160 nm. Beside the h/e
oscillations, we resolve the contributions from higher harmonics of the AB
oscillations. The observation of a local resistance minimum at B=0 T in both
rings is a signature of the destructive interference of the holes' spins. We
show that this minimum is related to the minimum in the h/2e oscillations.Comment: Proceedings of EP2DS-17, Genova 2007; Accepted for Physica E; 3
pages, 3 figure
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