ALBERT

Description: Aims Understanding the relationships among disturbance, invasion and species change is essential for effective management of many systems. We investigated relationships among fire history, invasion by a native tree species, Allocasuarina huegeliana , and diversity change to understand the potential drivers of plant community alteration in a complex and biodiverse system. Methods We used plant species surveys from 1983 and 2011 to quantify species loss/gain and thence compositional changes. Additionally, we surveyed population densities of the invasive species and collated long-term fire history data for each site. General linear models and non-parametric models were used to assess the strength of relationships between the three variables of interest. Important Findings Within the last 30 years, ~11% of the plant species richness was lost from the reserve. At an individual site level, we found only a 4% average decrease in overall plant species richness, but large species losses and gains that imply considerable compositional shifts. Though such shifts might be expected over 30 years, many of the gained species were common, potentially opportunistic species, while those lost were often locally rare woody perennials. In addition, gained species tended to be expanding their recorded range westward suggesting that they may be responding to the regional drying climate. The relationship between invasion density and species loss was strong over all spatial scales. We identified a potential state change to dominance by the native invasive particularly as high densities prevented species gain at the site scale. In these extreme cases of high invasive density and high biodiversity loss, we argue that there may be a need to directly address the expanding native population.

Description: 〈span〉〈div〉SUMMARY〈/div〉Ocean-bottom seismograph and multichannel streamer wide-angle seismic data are jointly analysed and compared with reflection images, bathymetric maps and potential field data, to reveal the detailed structure of layer 2 of the oceanic crust formed at the intermediate spreading Costa Rica Rift (CRR). Separate modelling of each wide-angle data set independently reveals a gradual increase in 〈span〉P〈/span〉-wave velocity with distance (hence crustal age) from the ridge axis, with a model derived from their joint inversion, in turn, displaying a pattern of shorter-wavelength structural complexity in addition to a background flow-line trend. Normalizing against a ridge-located reference velocity–depth model reveals that, off-axis, velocity perturbations are correlated with trends in basement roughness and uplift; regions of rougher and uplifted basement correlate with slower layer 2 velocity, 〈0.5 km s〈sup〉−1〈/sup〉 faster than at the ridge axis, and thinner sediment cover, while smoother basement and locations where sediment cover forms a continuous seal over the oceanic basement, are mirrored by regions of relatively higher velocity, 1.0–1.4 km s〈sup〉−1〈/sup〉 faster than at the CRR. These velocity variations are interpreted to reflect periodic changes in the degree of magma supply to the ridge axis.Using a combination of global and shipboard magnetic data, we derive a spreading history model for the CRR which shows that, for the past 5 Ma, spreading has been asymmetric. Comparing the seismic model structure with variations in full spreading rate over this period, reveals a correlation between periods of slower spreading and slower layer 2 velocity, basement roughness and uplift, and faster spreading, higher velocity and smoother basement structure. Zones of slower velocity also correlate with lows in the residual mantle Bouguer anomaly, interpreted as most likely reflecting corresponding regions of lower density in the lower crust or upper lithospheric mantle. Using ODP borehole 504B as ground-truth, we show that periods of faster spreading are associated with phases of magmatic accretion, interspersed by phases of increased asymmetric tectonic extension that likely facilitates fluid flow to the deeper crust and results in metamorphic alteration, manifest as the modelled density anomalies.Overall, our study shows that the mode of CRR crustal formation is sensitive to relatively small changes in full spreading rate within the range of 50–72 mm yr〈sup〉−1〈/sup〉, that tips the balance between magmatic and magma-dominated crustal formation and/or tectonic stretching, as characterized by significant variation in the fabric and physical properties of layer 2. We further hypothesize that this inherited structure has a direct influence on the subsequent evolution of the crust through secondary alteration. We conclude that descriptive phrases like ‘ocean crust formed at an intermediate-spreading rate’ should no longer be used to describe an actual crustal formation process or resulting crustal structure as, over the full range of intermediate spreading rates, a fine tipping-point dictates an episodic transition between primarily magmatic accretion and magma-dominated crustal formation coupled with enhanced faulting, with their asymmetry recorded in either ridge flank.〈/span〉

Description: This paper reports an approach for estimating thinning-induced changes in N and P budgets in jarrah ( Eucalyptus marginata ) forest in the Wungong catchment of Western Australia. Two thinning strategies, herbicide injection and selective removal, were tested and nutrient budgets were constructed for soil, litter and tree biomass. The effects of thinning were evaluated based on pre-thinning biomass allocation and on reductions in biomass after thinning. Tree above ground biomass was 399 ton ha -1 , from which the selective logging removed 18.7 ton ha -1 or 5 per cent of the N and 4 per cent of the P. Thinning residues from stem injection of herbicide contained fivefold more nutrients than the ground litter. Top soil was the primary nutrient store but only 1–2 per cent of total N and P were in available forms. In contrast, fine litter materials in thinned sites may release 4.8–5.7 kg P ha –1 via leaching over the rainy months. Cut branches and dead stems stored 176 kg N ha –1 and 7.0 kg P ha –1 but would decompose over many decades. Our results indicate that both thinning strategies would increase nutrient cycling in the forest, while the implications of thinning-induced nutrient supply for the growth of remaining vegetation, understorey competition and ecosystem health need further examination.

Description: 〈span〉〈div〉Summary〈/div〉Ocean-bottom seismograph and multichannel streamer wide-angle seismic data are jointly analysed and compared with reflection images, bathymetric maps and potential field data, to reveal the detailed structure of layer 2 of the oceanic crust formed at the intermediate spreading Costa Rica Rift (CRR). Separate modelling of each wide-angle dataset independently reveals a gradual increase in P-wave velocity with distance (hence crustal age) from the ridge axis, with a model derived from their joint inversion, in turn, displaying a pattern of shorter-wavelength structural complexity in addition to a background flow-line trend. Normalising against a ridge-located reference velocity-depth model reveals that, off-axis, velocity perturbations are correlated with trends in basement roughness and uplift; regions of rougher and uplifted basement correlate with slower layer 2 velocity, 〈 0.5 km s〈sup〉−1〈/sup〉 faster than at the ridge axis, and thinner sediment cover, while smoother basement and locations where sediment cover forms a continuous seal over the oceanic basement, are mirrored by regions of relatively higher velocity, 1.0–1.4 km s〈sup〉−1〈/sup〉 faster than at the CRR. These velocity variations are interpreted to reflect periodic changes in the degree of magma supply to the ridge axis.Using a combination of global and shipboard magnetic data, we derive a spreading history model for the CRR which shows that, for the past 5 Ma, spreading has been asymmetric. Comparing the seismic model structure with variations in full spreading rate over this period, reveals a correlation between periods of slower spreading and slower layer 2 velocity, basement roughness and uplift, and faster spreading, higher velocity and smoother basement structure. Zones of slower velocity also correlate with lows in the residual mantle Bouguer anomaly, interpreted as most likely reflecting corresponding regions of lower density in the lower crust or upper lithospheric mantle. Using ODP borehole 504B as ground-truth, we show that periods of faster spreading are associated with phases of magmatic accretion, interspersed by phases of increased asymmetric tectonic extension that likely facilitates fluid flow to the deeper crust and results in metamorphic alteration, manifest as the modelled density anomalies.Overall, our study shows that the mode of CRR crustal formation is sensitive to relatively small changes in full spreading rate within the range of 50–72 mm yr〈sup〉−1〈/sup〉, that tips the balance between magmatic and magma-dominated crustal formation and/or tectonic stretching, as characterised by significant variation in the fabric and physical properties of layer 2. We further hypothesise that this inherited structure has a direct influence on the subsequent evolution of the crust through secondary alteration. We conclude that descriptive phrases like ‘ocean crust formed at an intermediate-spreading rate’ should no longer be used to describe an actual crustal formation process or resulting crustal structure as, over the full range of intermediate spreading rates, a fine tipping-point dictates an episodic transition between primarily magmatic accretion and magma-dominated crustal formation coupled with enhanced faulting, with their asymmetry recorded in either ridge flank.〈/span〉