The effect of fire regime on tropical savannas of north-eastern Australia: interpreting floristic patterns through critical life events

The degree to which fire influences plant species composition and abundance (referred to in this thesis as floristic patterns) in Australian tropical savannas has been the subject of debate. Available information emphasises floristic patterns rather than the mechanisms that determine patterns. The assessment of critical events in a plant’s life cycle (referred to in this thesis as critical life events) is necessary to understand population dynamics and therefore the processes that determine floristic patterns. Fire may affect population size through its influence on one or more critical life events including seed dormancy release, seed bank dynamics, seedling emergence, seedling survival and the survival and growth of juvenile and mature plants. This thesis presents data on floristic patterns in response to fire regimes in savannas of north-eastern Australia, a region with limited quantitative information on fire response at a community level; and contributes to the broader understanding of savanna function, through an examination of life cycle processes. The aims of the research were to examine the effect of fire regime on: a) floristic patterns in eucalypt savannas in the vicinity of Townsville, north-eastern Australia; and b) critical life events, to investigate mechanisms determining floristic patterns. To achieve these objectives, laboratory and shadehouse experiments were carried out and surveys of seedling emergence and vegetative post-fire regeneration were undertaken at four sites in the Townsville region. At one of these sites, Cape Cleveland, floristic patterns and critical life events were studied in detail using a manipulative fire experiment which examined three fire regimes between 1997 and 2001: 1. Control: savanna burnt in the mid dry season (July) 1997 only. 2. Early burnt: savanna burnt in the mid dry season 1997 and early dry season (May) 1999. 3. Late burnt: savanna burnt in the mid dry season 1997 and late dry season (Oct) 1999. Five annual surveys of permanent plots at Cape Cleveland demonstrated a high degree of stability in species composition, however the abundance of several species was affected by fire regime. A significant increase in the abundance of various subshrubs, ephemeral and twining perennial forbs, and grasses was detected in the first year after fire, particularly following late dry season burning. The abundance of these species declined towards pre-fire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance, and tree density increased, with fire intervals of four years. Investigations of critical life events detected mechanisms that determine floristic patterns. A pulse of seedling emergence of subshrubs and herbaceous species occurred in the post-fire wet season, with greatest seedling density following late dry season fires. This pulse of seedling emergence explained the increase in plant abundance documented in the year following fire. It resulted from seed dormancy release, triggered by multiple fire-related agents, including the removal of herbaceous cover, which increased germination of a range of species; and the exposure to: heat-shock, which increased the germination of a range of forbs; smoke, which promoted the germination of upright perennial forbs and grasses; and nitrate, which interacted with heat shock to increase germination of at least two forbs. The germinable soil seed bank was dominated by grasses and forbs, and seed banks of trees and shrubs were scarce and short-lived. Seasonal fluctuations were evident in the seed bank, which increased through the dry season to a maximum density in the late dry season, and declined to a minimum by the mid wet season. The high intensity late dry season fires reduced seed production by the dominant tree Corymbia clarksoniana in the year following burning, however the effect of fire on soil seed bank dynamics was limited due to the annual wet season decline in seed reserves that occurred irrespective of fire. These data indicate that late dry season fires promoted the emergence of a greater density of herb seedlings, compared with early dry season fires, by stimulating seed dormancy release in a higher proportion of seeds, and because more soil seed reserves were available in the late dry season to be affected by fire. Seedling survival of herbaceous species was higher for those that emerged from recently burnt rather than unburnt savanna. High rates of seedling mortality of perennial species, and the short lifespan of ephemeral forbs, explained the decline in plant abundance documented in the second year following fire. A moderate proportion of seedlings of common sprouter species developed the capacity to survive fire within one year of germination. Only five percent of C. clarksoniana seedlings survived in unburnt, and fewer in burnt savanna, three years after germination. In savanna that remained unburnt for four years, growth of juvenile trees above 2 m in height accounted for the observed increase in tree density. The capacity of the majority of species to sprout after fire, and rapid seed production of non-sprouter species, explained the stability in species composition documented in the annual floristic surveys at Cape Cleveland. Fire-promoted stem reproduction via root-suckering explained the increase in abundance detected in the subshrub Breynia oblongifolia following burning. This thesis provides important information on the processes that drive tropical savanna function. It indicates that seedling recruitment is a process that limits population growth. Seedling recruitment of trees and shrubs is scarce and episodic, apparently linked to high rainfall years. The seedling recruitment of subshrubs and herbaceous species is driven by opportunities for seed dormancy release, which are primarily restricted to fire events, and limited by the low to moderate soil seed bank density and poor levels of seedling survival. Growth of juvenile trees beyond 2 m in height, and hence to reproductive maturity, requires fire-free intervals of at least four years and is therefore an event that is restricted by fire and limits population growth. The high proportion of sprouting species enables plant persistence, so that juvenile and mature plant survival are life events that provide population resilience. For ephemeral species, the soil seed bank provides a storage mechanism that provides population resilience. This thesis provides practical information to improve the management of this widespread and important ecosystem. Conservation management of these savannas will need to balance the role of regular fires in maintaining the diversity of herbaceous species with the requirement of fire intervals of at least four years for allowing recruitment, and therefore population maintenance, of trees. While late dry season fires may cause some tree mortality, the use of occasional late fires will promote dense seedling recruitment of herbaceous species, which may be necessary to maintain sustainable populations of many grasses and forbs

The response to high magnetic fields of the vacuum phototriodes for the compact muon solenoid endcap electromagnetic calorimeter

The endcap electromagnetic calorimeter of the Compact Muon Solenoid (CMS) detects particles with the dense fast scintillator lead tungstate (PbWO4). Due to the low light yield of this scintillator photodetectors with internal gain are required. Silicon avalanche photodiodes cannot be used in the endcap region due to the intense neutron flux. Following an extensive R&D programme 26 mm diameter single-stage photomultipliers (vacuum phototriodes) have been chosen as the photodetector in the endcap region. The first 1400 production devices are currently being evaluated following recent tests of a pre-production batch of 500 tubes. Tubes passing our acceptance tests have responses, averaged over the angular acceptance of the endcap calorimeter, corresponding to the range 20 to 55 electrons per MeV deposited in PbWO4. These phototriodes operate, with a typical gain of 10, in magnetic fields up to 4T.PPARC, EC(INTAS-CERN scheme 99-424

Water surface height determination with a GPS wave glider: a demonstration in Loch Ness, Scotland

A geodetic GPS receiver has been installed on a Wave Glider, an unmanned water surface vehicle. Using kinematic precise point positioning (PPP) GPS, which operates globally without directly requiring reference stations, surface heights are measured with ~0.05-m precision. The GPS Wave Glider was tested in Loch Ness, Scotland, by measuring the gradient of the loch’s surface height. The experiment took place under mild weather, with virtually no wind setup along the loch and a wave field made mostly of ripples and wavelets. Under these conditions, the loch’s surface height gradient should be approximately equal to the geoid slope. The PPP surface height gradient and that of the Earth Gravitational Model 2008 geoid heights do indeed agree on average along the loch (0.03 m km−1). Also detected are 1) ~0.05-m-sized height changes due to daily water pumping for hydroelectricity generation and 2) high-frequency (0.25–0.5 Hz) oscillations caused by surface waves. The PPP heights compare favorably (~0.02-m standard deviation) with relative carrier phase–based GPS processing. This suggests that GPS Wave Gliders have the potential to autonomously determine centimeter-precise water surface heights globally for lake modeling, and also for applications such as ocean modeling and geoid/mean dynamic topography determination, at least for benign surface states such as those encountered during the reported experiment

On the nature of the z=0 X-ray absorbers: I. Clues from an external group

Absorption lines of OVII at redshift zero are observed in high quality Chandra spectra of extragalactic sightlines. The location of the absorber producing these lines, whether from the corona of the Galaxy or from the Local Group or even larger scale structure, has been a matter of debate. Here we study another poor group like our Local Group to understand the distribution of column density from galaxy to group scales. We show that we cannot yet rule out the group origin of z=0 systems. We further argue that the debate over Galactic vs. extragalactic origin of z=0 systems is premature as they likely contain both components and predict that future higher resolution observations will resolve the z=0 systems into multiple components.Comment: Submitted to ApJ

Identifying the mechanisms underpinning recognition of structured sequences of action

© 2012 The Experimental Psychology SocietyWe present three experiments to identify the specific information sources that skilled participants use to make recognition judgements when presented with dynamic, structured stimuli. A group of less skilled participants acted as controls. In all experiments, participants were presented with filmed stimuli containing structured action sequences. In a subsequent recognition phase, participants were presented with new and previously seen stimuli and were required to make judgements as to whether or not each sequence had been presented earlier (or were edited versions of earlier sequences). In Experiment 1, skilled participants demonstrated superior sensitivity in recognition when viewing dynamic clips compared with static images and clips where the frames were presented in a nonsequential, randomized manner, implicating the importance of motion information when identifying familiar or unfamiliar sequences. In Experiment 2, we presented normal and mirror-reversed sequences in order to distort access to absolute motion information. Skilled participants demonstrated superior recognition sensitivity, but no significant differences were observed across viewing conditions, leading to the suggestion that skilled participants are more likely to extract relative rather than absolute motion when making such judgements. In Experiment 3, we manipulated relative motion information by occluding several display features for the duration of each film sequence. A significant decrement in performance was reported when centrally located features were occluded compared to those located in more peripheral positions. Findings indicate that skilled participants are particularly sensitive to relative motion information when attempting to identify familiarity in dynamic, visual displays involving interaction between numerous features

Atom gratings produced by large angle atom beam splitters

An asymptotic theory of atom scattering by large amplitude periodic potentials is developed in the Raman-Nath approximation. The atom grating profile arising after scattering is evaluated in the Fresnel zone for triangular, sinusoidal, magneto-optical, and bichromatic field potentials. It is shown that, owing to the scattering in these potentials, two \QTR{em}{groups} of momentum states are produced rather than two distinct momentum components. The corresponding spatial density profile is calculated and found to differ significantly from a pure sinusoid.Comment: 16 pages, 7 figure