82 research outputs found
Understanding the effects of drought upon carbon allocation and cycling in an Amazonian rain forest
Research Grant (NER/A/S/2003/1609)The Amazon rain forest plays an important role in regional and global
biogeochemical cycling, but the region may undergo an increase in the frequency and
severity of drought conditions driven by global climate change, regional deforestation
and fire. The effects of this drought on carbon cycling in the Amazon, particularly
below-ground, are potentially large but remain poorly understood. This thesis
examines the impacts of seasonal and longer-term drought upon ecosystem carbon
allocation and cycling at an Amazon rain forest site with a particular focus upon
below-ground processes. Measurements are made at three one-hectare forest plots
with contrasting soil type and vegetation structure, to observe responses across a
range of Amazon primary forest types. A fourth plot is subjected to partial rainfall
exclusion to permit measurement of forest responses to a wider range of soil moisture
levels than currently exists naturally.
An analysis of the number of samples required to accurately quantify important
ecosystem carbon stocks and fluxes is used to guide the sampling strategy at the field
site. Quantifying root dynamics, in particular, presents methodological challenges.
Thus, I critically review existing methods, and develop techniques to accurately
measure root standing biomass and production. Subsequently, these techniques are
used to record root responses, in terms of standing biomass, production, morphology,
turnover and nutrient content, to variation in soil moisture across the four rain forest
plots. There is substantial environmental variation in root characteristics. However,
several responses remain consistent across plots: root production of biomass, length,
and surface area, is lower where soil is dry, while root length and surface area per unit
mass show the opposite pattern.
The other major component of the below-ground carbon cycle is soil carbon
dioxide efflux. I partition this efflux, on each plot, into contributions from organic
ground surface litter, roots and soil organic matter, and investigate abiotic and biotic causes for observed differences within and between plots. On average, the percentage
contribution of soil organic matter respiration to total soil carbon dioxide efflux
declines during the dry season, while root respiration contribution displays the
opposite trend. However, spatial patterns in soil respiration are not directly
attributable to variation in either soil moisture or temperature. Instead, ground surface
organic litter mass and root mass account for 44 % of observed spatial heterogeneity
in soil carbon dioxide efflux.
Finally, information on below-ground carbon cycling is combined with aboveround
data, of canopy dynamics and stem wood production and mortality, to analyze
the potential effects of drought upon carbon cycling in an Amazon forest ecosystem.
Comparison of the rainfall exclusion plot with a similar, but unmodified, control plot
reveals potentially important differences in tree carbon allocation, mortality,
reproduction, soil respiration and root dynamics. The apparent net consequence of
these changes is that, under drier conditions, the amount of CO2 moving out of the
forest and into the atmosphere is diminished. This synthesis of above-ground and
below-ground data advances understanding of carbon cycling in rain forests, and
provides information which should allow more accurate modelling of the response of
the Amazon region to future drought. Additional measurements at other sites, and of
other ecosystem carbon fluxes, should further refine modelling predictions
Fibre-selective discrimination of physiological ENG using velocity selective recording: Report on pilot rat experiments
An enhancement to velocity selective discrimination of neural recordings:extraction of neuronal firing rates
A Summary of Current and New Methods in Velocity Selective Recording (VSR) of Electroneurogram (ENG)
This paper describes the theory of velocity selective recording (VSR) of neural signals including some new developments. In particular new limits on available selectivity using bandpass filters are introduced and discussed. Existing work has focussed primarily on electrically evoked compound action potentials (CAPs) where only a single evoked response per velocity is recorded. This paper extends the theory of VSR to naturally occurring neural signals recorded from rat and describes a practical method to estimate the level of activity (firing rates) within particular velocity ranges
Supervoid Origin of the Cold Spot in the Cosmic Microwave Background
We use a WISE-2MASS-Pan-STARRS1 galaxy catalog to search for a supervoid in
the direction of the Cosmic Microwave Background Cold Spot. We obtain
photometric redshifts using our multicolor data set to create a tomographic map
of the galaxy distribution. The radial density profile centred on the Cold Spot
shows a large low density region, extending over 10's of degrees. Motivated by
previous Cosmic Microwave Background results, we test for underdensities within
two angular radii, , and . Our data, combined with an
earlier measurement by Granett et al 2010, are consistent with a large supervoid with centered at . Such a supervoid, constituting a
fluctuation in the model, is a plausible cause
for the Cold Spot.Comment: 4 pages, 2 figures, Proceedings of IAU 306 Symposium: Statistical
Challenges in 21st Century Cosmolog
Stellar Spin-Orbit Misalignment in a Multiplanet System
Stars hosting hot Jupiters are often observed to have high obliquities,
whereas stars with multiple co-planar planets have been seen to have low
obliquities. This has been interpreted as evidence that hot-Jupiter formation
is linked to dynamical disruption, as opposed to planet migration through a
protoplanetary disk. We used asteroseismology to measure a large obliquity for
Kepler-56, a red giant star hosting two transiting co-planar planets. These
observations show that spin-orbit misalignments are not confined to hot-Jupiter
systems. Misalignments in a broader class of systems had been predicted as a
consequence of torques from wide-orbiting companions, and indeed
radial-velocity measurements revealed a third companion in a wide orbit in the
Kepler-56 system.Comment: Accepted for publication in Science, published online on October 17
2013; PDF includes main article and supplementary materials (65 pages, 27
figures, 7 tables); v2: small correction to author lis
The Cold Spot in the Cosmic Microwave Background: the Shadow of a Supervoid
Standard inflationary hot big bang cosmology predicts small
fluctuations in the Cosmic Microwave Background (CMB) with
isotropic Gaussian statistics. All measurements support the
standard theory, except for a few anomalies discovered in the
Wilkinson Microwave Anisotropy Probe maps and confirmed recently
by the Planck satellite. The Cold Spot is one of the most
significant of such anomalies, and the leading explanation of it
posits a large void that imprints this extremely cold area via
the linear Integrated Sachs-Wolfe (ISW) effect due to the decay
of gravitational potentials over cosmic time, or via the Rees-
Sciama (RS) effect due to late-time non-linear evolution.
Despite several observational campaigns targeting the Cold Spot
region, to date no suitably large void was found at higher
redshifts z>0.3. Here we report the detection of an R=(192±15)h
−1Mpc size supervoid of depth δ=−0.13±0.03, and centred at
redshift z=0.22. This supervoid, possibly the largest ever
found, is large enough to significantly affect the CMB via the
non-linear RS effect, as shown in our Lemaitre-Tolman-Bondi
framework. This discovery presents the first plausible
explanation for any of the physical CMB anomalies, and raises
the possibility that local large-scale structure could be
responsible for other anomalies as well
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