Central South Island Research Articles

ONSET OF GLACIATION DROVE SIMULTANEOUS VICARIANT ISOLATION OF ALPINE INSECTS IN NEW ZEALAND

The origin of the New Zealand "beech gap," a low-diversity zone in the central South Island corresponding with a disjunction in the distribution of many taxa, has been the focus of biogeographical debate for many decades. Here, we use comparative phylogeographic analysis (COI; H3) of six alpine stonefly genera (116 individuals, 102 localities) to test a vicariant evolutionary hypothesis for the origin of this "biotic gap." We find strikingly similar phylogeographic patterns in all six genera, with the deepest genetic divergences always found between samples north and south of the beech gap. The magnitude of north-south genetic differentiation for COI is similar across all six genera (ranging from 0.074 to 0.091), with a test for simultaneous vicariance confirming that divergence is consistent with a single evolutionary event. The concordant cladogenesis detected across multiple taxa is consistent with vicariant isolation caused by the onset of glaciation in the late Pliocene. This study thus indicates an important cladogenetic role for glaciation, an abiotic evolutionary process that is more typically associated with loss of biodiversity.

Stratigraphy, optical dating chronology (IRSL) and depositional model of pre-LGM glacial deposits in the Hope Valley, New Zealand

A 110 m thick succession of glacial valley fill is described from Poplars Gully, central South Island, New Zealand. The section consists of eight lithofacies assemblages that represent different stages of ice occupation in the valley. Basal sediments record an ice retreat phase followed by a glacial re-advance which deposited mass flow diamictons and till. A subsequent ice retreat from the site is indicated by the stratigraphic transition from till to thick glacio-fluvial gravels. This is followed by a probably short-lived glacier re-advance that caused folding and thrusting of proglacial sediments. Final glacial retreat from the valley led to the formation of a large proglacial lake. In total, Poplars Gully holds evidence for two major ice advances, separated by a glacial retreat that resulted in complete ice evacuation from the lower Hope Valley. Infrared stimulated luminescence (IRSL) dating on ice-proximal sediments from Poplars Gully yielded six ages between 181 and 115 ka BP. Our stratigraphic logging and dating results show that the fill sequence was not, as previously thought, deposited in association with ice advances during the Last Glacial Maximum (LGM) nor indeed during the last glacial cycle. LGM glaciers later overran the fill but we find that the older glacial sequences are considerably more voluminous than those deposited during the last glacial cycle. We also show that the mid-Pleistocene glaciers carved a much deeper valley trough than did glaciers during the LGM. Taken together these features are likely to reflect a significant difference in the magnitude of successive Pleistocene glaciations in the valley, with the mid-Pleistocene ice advances having been considerably larger than those of the last glacial cycle. The recognition of the in-situ survival of extensive pre-MIS 5 (Marine Isotope Stage) deposits in valley troughs that were later occupied by LGM glaciers represents a new feature in the Quaternary stratigraphy of the Southern Alps. The results demonstrate that New Zealand's commonly very large soft-sedimentary valley fills provide a valuable, yet largely unexploited, terrestrial sedimentary archive of successive glaciations in the region.

Diversification ofChionochloa(Poaceae) and biogeography of the New Zealand Southern Alps

Aim We test hypotheses regarding the origin of diversity and patterns of species richness in and around the New Zealand Southern Alps with 25 species of Chionochloa (Poaceae, Danthonioideae). Location New Zealand. Methods We inferred a well-resolved and mostly robustly supported chloroplast phylogeny based on multiple DNA sequence markers (trnT–L–F, rpl16, trnD–psbM, atpB–rbcL, matK and ndhF), sampling 92% of the recognized species and 82% of the subspecific taxa. Nuclear ribosomal internal transcribed spacer sequences were also sampled, but proved uninformative. Biogeographic reconstruction and character optimization were done using both parsimony and likelihood approaches, and molecular dating used relaxed clock approaches. Results Most of the species diversity in Chionochloa stemmed from a common ancestor in the southern South Island with subsequent dispersal between areas. One clade of apparently cryptic taxa diversified within the central South Island ‘endemism gap’, persisting there throughout at least the latter half of the Pleistocene. Exclusively alpine and other habitat specialist species originated independently, the former relatively recently (between 7.6 Ma and the present). Main conclusions The phylogeny of Chionochloa and other published phylogenies of New Zealand plant groups demonstrate that the higher degree of endemism in the north and south of the New Zealand South Island relative to a central endemism gap cannot be explained by Alpine Fault displacement. Furthermore, our results suggest that if extinctions resulting from glaciations played a role in the origin of the central endemism gap, their impact was less than might be presumed on the basis of the distribution of taxa as they are currently defined. The diversification of Chionochloa and a number of New Zealand plant groups, such as Ranunculus, was contemporaneous with the initiation of the uplift of the Southern Alps. In contrast to patterns of diversifications within the alpine regions typical of the hyperdiverse Andes, exclusively alpine species in New Zealand arose independently from ancestors distributed in more lowland areas. Similarly, habitat specialists in Chionochloa arose independently from more generalist ancestors. Thus, although diversification in these groups may have been stimulated by mountain building and Pleistocene climatic oscillations, cladogenesis did not occur within the high alpine habitat itself.

Processing and preliminary interpretation of noisy high-resolution seismic reflection/refraction data across the active Ostler Fault zone, South Island, New Zealand

Abstract In an attempt to understand the structure of active faults as they emerge from bedrock into shallow semi-consolidated and unconsolidated sediments, we have recorded a comprehensive high-resolution seismic reflection/refraction data set across the Ostler Fault zone on the central South Island of New Zealand. This fault zone, which absorbs 1–2 mm/yr of compression associated with oblique convergence of the Pacific and Australian tectonic plates, consists of a series of surface-rupturing N–S trending, west-dipping reverse faults that offset a thick sequence of Quaternary glacial outwash and late Neogene fluvio-lacustrine sediments of the Mackenzie Basin. Our study focuses on a region of the basin where two non-overlapping fault segments are separated by a transfer zone. Deformation in this area is accommodated by offsets on multiple small faults and by folding in their hanging walls. The seismic data with source and receiver spacing of 6 and 3 m and nominal CMP fold of 60 was acquired along twelve 1.2 km long lines orthogonal to fault strike and an additional 1.6 km long tie-line parallel to fault strike. The combination of active deformation and shallow glacial outwash sediments results in particularly complicated seismic data, such that application of relatively standard processing schemes yields only poor quality images. We have designed a pre- and post-stack reflection/refraction processing scheme that focuses on minimising random and source-generated noise, determining appropriate static corrections and resolving contrasting reflection dips. Application of this processing scheme to the Ostler Fault data provides critical information on fault geometry and offset and on sedimentary structures from the surface to ∼ 800 m depth. Our preliminary interpretation of one of the lines includes complex deformation structures with folding and multiple subsidiary fault splays on either side of a ∼ 50° west-dipping primary fault plane.

Glacial refugia in a maritime temperate climate: Cicada (Kikihia subalpina) mtDNA phylogeography in New Zealand

Understanding the biological significance of Pleistocene glaciations requires knowledge of the nature and extent of habitat refugia during glacial maxima. An opportunity to examine evidence of glacial forest refugia in a maritime, Southern Hemisphere setting is found in New Zealand, where the extent of Pleistocene forests remains controversial. We used the mitochondrial phylogeography of a forest-edge cicada (Kikihia subalpina) to test the hypothesis that populations of this species survived throughout South Island during the Last Glacial Maximum. We also compared mitochondrial DNA phylogeographic patterns with male song patterns that suggest allopatric divergence across Cook Strait. Cytochrome oxidase I and II sequences were analyzed using network analysis, maximum-likelihood phylogenetic estimation, Bayesian dating and Bayesian skyline plots. K. subalpina haplotypes from North Island and South Island form monophyletic clades that are concordant with song patterns. Song divergence corresponds to approximately 2% genetic divergence, and Bayesian dating suggests that the North Island and South Island population-lineages became isolated around 761,000 years BP. Almost all South Island genetic variation is found in the north of the island, consistent with refugia in Marlborough Sounds, central Nelson and northwest Nelson. All central and southern South Island and Stewart Island haplotypes are extremely similar to northern South Island haplotypes, a 'northern richness/southern purity' pattern that mirrors genetic patterns observed in many Northern Hemisphere taxa. Proposed southern South Island forest habitat fragments may have been too small to sustain populations of K. subalpina, and/or they may have harboured ecological communities with no modern-day analogues.

Three‐dimensional attenuation structure of central and southern South Island, New Zealand, from local earthquakes

The three‐dimensional attenuation properties of the South Island, New Zealand, are obtained from local earthquake t* (spectral decay) data. We extend the spectral fitting method used to obtain t* by enabling limited frequency dependence for Q. Observed Qp heterogeneity relates to both active geological processes and to constituent tectonic blocks, with high Qp for the Haast schist. The most pronounced low Qp features in the upper crust correspond to zones of recent seismicity, rather than the most significant faults. The central Alpine fault, a dipping, oblique‐slip fault, forms the western boundary to a low Qp volume in the ductile crust. Particularly low Qp is imaged at the base of the Alpine fault at 30‐km depth and may be related to metamorphic fluid release in the crustal root, a region of predicted high strain. Conversely, the southern Alpine fault, which is a vertical strike‐slip fault, forms the western boundary to the high Qp schist region. In the southern South Island, the plate boundary becomes a subduction zone, exhibiting unusual Qp character. Moderately high Qp is associated with the subducted Australian plate. There is also a high Qp, high Vp feature in the Pacific mantle to 160‐km depth that appears to cause the subducting slab to bend to near vertical. This feature may represent mantle shortening since the Miocene. Regional attenuation rates for Fiordland earthquakes, calculated from the 3‐D Qp model, show large variations over the South Island; the low Qp in the crustal root has a significant effect on passing raypaths.

Evolution and biogeography of New Zealand's longjaw galaxiids (Osmeriformes: Galaxiidae): the genetic effects of glaciation and mountain building

Summary1. The biological impact of glaciation in Southern Hemisphere freshwaters is poorly understood. Several large rivers of eastern South Island, New Zealand, represent a mosaic of glaciated and non‐glaciated regions, and are thus well‐suited for studies of post‐glacial recolonization.2. We conducted mtDNA analyses of South Island's endemic non‐migratory longjaw galaxiids Galaxias prognathus and G. cobitinis (Osmeriformes: Galaxiidae) to test hypotheses of post‐glacial recolonization, and to assess the vicariant effects of Pleistocene mountain building.3. We analysed the phylogeography of longjaw cytochrome b sequences from 38 sites in central South Island (n = 83). On the basis of our sampling it seems that G. prognathus and G. cobitinis have a parapatric distribution in the Waitaki River system, their disjunction broadly coinciding with three large post‐glacial lakes. Waitaki clades of both species are deeply divergent relative to conspecific taxa in drainages to the north and south.4. Tests for recent population growth – predicted under post‐glacial expansion of G. prognathus– do not refute recent recolonization of streams above glaciated lakes in the Waitaki River drainage. The apparent absence of potential ‘source’ populations from non‐glaciated regions suggests a post‐glacial population decline for G. prognathus below the Waitaki lakes.5. Molecular clock calibrations based on several freshwater vicariant events elsewhere in New Zealand supported the geologically‐derived hypothesis of Waitaki–Canterbury drainage isolation approximately 300 ka.

The linkage between riparian predators and aquatic insects across a stream‐resource spectrum

Summary1. Spatial subsidies, defined as the flow of energy, nutrients, organisms or pollutants from one habitat to another, have been shown to affect the food–web dynamics in a wide range of ecosystems. An important subsidy to riparian communities is the contribution of adult stream insects to terrestrial predators such as birds, bats and lizards, but also invertebrates including ground and web‐building spiders.2. We surveyed 37 first‐ and second‐order forest streams across differing environmental gradients in the Central South Island, New Zealand, to investigate the relationship between potential aquatic prey subsidies and predatory riparian arachnids. We anticipated that stream‐insect biomass would be positively associated with riparian arachnids, as a result of emergent adult aquatic insect subsidies to the adjacent habitat.3. We confirmed positive associations between stream‐insect biomass as a predictor variable and riparian arachnid biomass (R2 = 0.42, F1,34 = 25.2, P < 0.001) and web densities (R2 = 0.45, F1,14 = 11.5, P < 0.01) respectively as dependent variables after adjusting for the confounding effects of environmental variables. Hierarchical partitioning confirmed the importance of stream insect biomass as a statistically significant contributor to the total explained variance in analyses calculated for arachnid biomass, abundance and web density.4. A concurrent survey of spider‐web density along 20‐m transects from the stream edge into the forest indicated a strong decline in web‐building spider density moving away from the stream (R2 = 0.41, F1,158 = 109, P < 0.001), with stream‐insect biomass as a significant covariate (F1,149 = 17.7, P < 0.001).5. Our results suggest that productivity gradients present in the donor system affect the magnitude of the interaction between adjacent habitats. Productivity gradients may lead to increased reciprocal subsidies through a positive feedback loop involving the predation of spiders and other predatory terrestrial invertebrates by aquatic predators. However, terrestrial insectivores such as birds, bats and lizards that are not readily used as prey by aquatic predators may circumvent the feedback cycle by consuming a large proportion of emergent aquatic‐insect biomass. This may lead to asymmetry in the strength of food–web linkages between aquatic and terrestrial habitats.

Crust and mantle thickening beneath the southern portion of the Southern Alps, New Zealand

An 8.54 ± 0.20 km s−1Pn speed is estimated on a line oriented ca. N5°E from the Alpine Fault beneath the Southern Alps of South Island based on a refraction experiment that uses the Fiordland earthquake of August 2003 as a source. This high Pn speed results from both strong anisotropy in the mantle lid of 7–13 per cent and a high Pn speed average of 8.3 ± 0.3 km s−1. A maximum crustal thickness of 48 ± 4 km is calculated for the southern South Island near the town of Wanaka. This represents a crustal root of about 18 km, compared to measured crustal thicknesses at the east and west coasts of the South Island. The average topography in the southern Southern Alps is of the order of ∼1000 m, which is less than half that predicted by Airy isostasy for an 18 km crustal root. As recently proposed for the central South Island ∼120 km to the north, we propose that thickened cold, and therefore more dense, mantle lithosphere exists beneath southern South Island, and that this excess of mass is an effective load that pulls down the overlying crust. The load is similar to that beneath the central Southern Alps, despite the predicted convergence across the Alpine Fault there being nearly twice that at Wanaka. Gravity modelling of crustal structure along a profile through Wanaka suggests that this mass excess has a minimum density contrast of 35 ± 5 kg m−3 between thickened mantle and asthenosphere, assuming an across-Moho density contrast of −300 kg m−3. We speculate that the reason for the enhanced thickening beneath Wanaka is that the subducted Australian Plate at the southwestern corner of the South Island acts like a backstop onto which Pacific mantle collides at ∼26 mm yr−1, ca. 3/4 the full plate speed.

Impacts of introduced brown and rainbow trout on benthic invertebrate communities in shallow New Zealand lakes

Summary1. Brown and rainbow trout have been introduced to many inland waters in New Zealand, but research on the impacts on native communities has focused mainly on streams. The purpose of this study was to compare the benthic communities of trout and troutless lakes. Based on previous studies in North America and Europe, we predicted that the benthic biomass, and especially the abundance of large invertebrates, would be lower in lakes with trout as compared to those without. We surveyed the invertebrate fauna of 43 shallow, high‐elevation lakes (26 with and 17 without trout) in four geographic clusters on the central South Island and then conducted a detailed quantitative study of invertebrate biomass and community structure in 12 of these lakes.2. Benthic community composition and diversity of lakes with and without trout were nearly identical and biomass was as high or higher in the lakes with as without trout. There was no evidence that trout have caused local extinctions of benthic invertebrates. Although the proportional abundance of large‐bodied aquatic was slightly lower in lakes with than without trout, the abundance of several groups of large‐bodied benthic taxa (dragonflies, caddisflies and water bugs) did not differ.3. Our findings are in contrast to those in North American and Europe where trout introductions into previously troutless lakes have led to declines in the abundance of benthic invertebrates, especially large‐bodied taxa. We propose that the modest effects of trout in New Zealand could be explained by (i) the high areal extent of submergent vegetation that acts as a benthic refuge, (ii) low intensity of trout predation on benthic communities and/or (iii) characteristics of the benthic invertebrates that make them relatively invulnerable to fish predation.4. Regardless of the relative importance of these hypotheses, our results emphasise that the same invertebrates occurred in all of the lakes, regardless of size, elevation and presence of trout, suggesting habitat generalists dominate the benthic fauna in shallow New Zealand lakes.

Two-dimensional and three-dimensional finite element modelling of mantle processes beneath central South Island, New Zealand

SUMMARY Numerical models that include pre-existing faults, elasto‐visco‐plastic rheology, thermal structure, realistic plate boundary conditions, and surface erosion are built to investigate alternative hypotheses of mantle flow geometry beneath central South Island, New Zealand. Bouguer gravity anomalies, teleseismic traveltime delays and seismic velocity structures are used to constrain models. Model predictions are compared with topography, uplift, magnetotelluric resistivity structure, isostatic gravity anomalies (IGAs), and seismicity. We find that the wedge model, in which the mantle lithosphere of the Australia plate is wedged between crust and mantle lithosphere of the Pacific plate, gives reasonable predictions of topography, uplift rate, IGAs, and stress and strain state in the lithosphere. Coupled erosional-tectonic models suggest that tectonic compression primarily determines the asymmetric surface relief, with erosion influencing its detail and having small effects on shallow stresses. Both 2-D and 3-D models suggest that the neotectonics of central South Island is dominated by weak faults. 3-D model results show that the Alpine fault and mantle shear zone accommodate most convergent and strike-slip plate motion, confirming non-partitioned plate motion in this region. Adding the regional strike-slip component has little effect on the convergent velocity field, and only changes the stress field by rotating the horizontal principal stresses out of the plane of the cross-section.

Habitat use as a predictor of nest raiding by individual hedgehogs Erinaceus europaeus in New Zealand

Predation by introduced hedgehogs Erinaceus europaeus is a significant cause of nest failure in threatened endemic wading birds nesting on the dry gravel beds of braided rivers in New Zealand's central South Island. Night-time movements of 10 hedgehogs (four male; six female) were investigated during the 2002 bird breeding season using spool-and-line tracking and GPS data recording. Nine of the 10 hedgehogs studied used river braids rarely and concentrated their activity in scrub and woodland habitat. One female foraged almost exclusively on the dry river braids. The ranked order of habitat preference (scrub>woodland>river braid) matched the ranked availability of hedgehogs' most common invertebrate prey, estimated by pitfall trapping, in the three habitat types. Male foraging ranges were generally larger than those of females, except for the female that foraged on the relatively resource-poor river braids, which had the second largest range overall. This animal had a much higher probability of encountering, and opportunistically raiding, nests than the others. Trapping programmes targetted at nuisance predators commonly aim to reduce overall predator abundances within a designated area using traps set along logistically convenient lines. This design reflects the implicit assumption that all target animals within the area represent an equal risk to threatened prey. Our results suggest that this assumption may not always be valid. To improve nest protection during the birds' breeding season, we suggest that traps should be focussed in and around the nesting habitat in order to target those predators that habitually forage in the area.

Mantle tectonics beneath New Zealand inferred fromSKSsplitting and petrophysics

SUMMARY We constrain the extent of lithospheric and asthenospheric deformation beneath New Zealand by coupling measurements of shear wave splitting of teleseismic waves and petrophysical analysis of mantle xenoliths. SKS splitting for the central South Island and the eastern part of the North Island confirms earlier observations. The fast S-wave polarization directions are roughly NE/SW, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. This suggests that flow parallel to the plate boundary extends up to 160 km away from the boundary. Departures from this pattern are restricted to the southernmost station and the two northernmost stations and indicate changes in the mantle flow or a more complex anisotropic pattern, such as a dipping axis of symmetry or heterogeneous anisotropy. Analysis of mantle xenoliths from the Raglan (North Island), Dunedin (South Island) and Chatham Island regions allows us to constrain the lithospheric contribution to the observed seismic anisotropy. The intrinsic S-wave anisotropy is 1.5 per cent higher for the South Island samples (∼5.0 per cent versus ∼3.5 per cent for the North Island), indicating a strong lithospheric deformation that could explain the major part of the observed splitting. In the North Island, the intrinsic lithospheric anisotropy is too low to explain the SKS splitting times. This indicates that asthenospheric deformation probably plays a major role there. For the Chatham Island, the anisotropy pattern remains unresolved as the xenolith suggests strong lithospheric anisotropy but no present splitting is measured.

Top‐down and bottom‐up influences on populations of a stream detritivore

Summary1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly,Zelandopsyche ingens(family Oeconesidae), was affected by the presence of predatory brown trout,Salmo truttaand the abundance of their primary detrital resource (Nothofagusleaves).2. The density ofZ. ingensand the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years.3. Physicochemical conditions were similar in trout and fishless streams, butancovaindicated thatZ. ingensnumbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams.4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.

Radar ‘lensing’ by a small river: Can a layer of surface water improve the signal?

ABSTRACTA radar survey was carried out on a gravel bar between the Hope and Boyle Rivers near the Hanmer Basin in the central South Island of New Zealand. One of the profiles crossed a shallow stream, where the depth of penetration and resolution both appeared to improve. The water's high dielectric permittivity, and consequent slower velocity, causes the signal to be directed more vertically into the subsurface. The superior resolution is also due in part to the slower average velocity; the water table is at or above the ground surface in the stream channel. Spectral analysis shows that there is a small increase in the amplitude of the higher frequencies in the stream portion of the profile, so that the improved resolution appears to be real. Multiple reflections from the stream may also contribute to the spectral broadening. The apparent increased depth of penetration is partly due to the increased time required for echoes to return at the slower velocity, but the larger two‐way traveltime does yield a marginally greater depth of penetration.

Farmers as Learners: Evolving Identity, Disposition and Mastery through Diverse Social Practices

This paper reports a study (Allan 2002) that explores the questions of how farmers learn, how they construct knowledge, and how learning informs their practice. It seeks to identify how they gain new ideas, make changes, and develop to new levels of expertise. Six central South Island (N.Z.) farmers were selected purposively as case studies. The range of case profiles provides for comparison and contrast of the relative importance of formal qualifications, differences between sheep/beef farmers and dairy farmers, levels of expertise, age and experiences. The self-rating of the farmers enables a comparison of lower and higher performers, identifying characteristics which enable insight into why some farmers consistently lead new practice and why some are reluctant followers. The farmers are identified as social learners, although working independently, in relative geographical and social isolation and learning through participation in the practice of farming. This practice includes a constellation of communities of practice, which may be resource-rich or resource-poor with mastery developed through evolving identity, disposition and knowledge.

Species novae graminum Novae‐Zelandiae III*. Two diploid species of Rytidosperma (Danthonieae: Danthonioideae)

Two diploid species of Rytidosperma, R. horrens and R. telmaticum, are described from central South Island, the latter as a segregate from R. pumilum. The preferred habitat of R. telmaticum is the margins of kettle‐holes, drainage ponds, and other depressions in glacial deposits as one of the dense turf species there; it occurs more sparingly in seral communities on some montane riverbeds, and in one case on sparsely vegetated schist terrain. Rytidosperma horrens is known from only one locality where it grows on the margins of sloping wet flushes. Both species, and their close associates, occupy relatively fertile sites.

Fault zone fluids and seismicity in compressional and extensional environmen inferred from electrical conductivity: the New Zealand Southern Alps and U. S. Great Basin

Seismicity in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and the distribution of fluids. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward (“U”-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island due to fluids generated from prograde metamorphism within a thickening crust. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. The central South Island is relatively weak in seismicity compared to its more subduction-related northern and southern ends, and the production of deep crustal fluids through metamorphism may promote slip before high stresses are built up. The deep crustal conductivity is highly anisotropic, with the greater conductivity along strike, consistent with fault zone models of long-range interconnection versus degree of deformation. The central Great Basin province of the western U.S. by contrast is extensional at present although it has experienced diverse tectonic events throughout the Paleozoic. MT profiling throughout the province reveals a quasi one-dimensional conductor spanning the lower half of the crust which is interpreted to reflect high temperature fluids and perhaps melting caused ultimately by exsolution from crystallizing underplated basalts. The brittle, upper half of the crust is generally resistive, but also characterized by numerous steep, narrow conductors extending from near-surface to the middle crust where they contact the deep crustal conductive layer. These are suggested to represent fluidised/altered fault zones, with at least some fluids contributed from the deeper magmatic exsolution. The best-known faults imaged geophysically before this have been the listric normal faults bounding graben sediments as imaged by reflection seismology. However, the major damaging earthquakes of the Great Basin appear to nucleate near mid-crustal depths on near-vertical fault planes, which we suggest are being imaged with the MT transect data, and where triggering fluids from the ductile lower crust are available. In both compressional and extensional examples, the fluidised fault zones are hypothesized to act to concentrate slip, with major earthquakes resulting in asperities along the fault surface.

Late-glacial and Holocene vegetation and climatic history of the Cass Basin, central South Island, New Zealand

Lithology, pollen, macrofossils, and stable carbon isotopes from an intermontane basin bog site in southern New Zealand provide a detailed late-glacial and early Holocene vegetation and climate record. Glacial retreat occurred before 17,000 cal yr b.p., and tundra-like grassland–shrubland occupied the basin shortly after. Between 16,500 and 14,600 cal yr b.p., a minor regional expansion of forest patches occurred in response to warming, but the basin remained in shrubland. Forest retreated between 14,600 and 13,600 cal yr b.p., at about the time of the Antarctic Cold Reversal. At 13,600 cal yr b.p., a steady progression from shrubland to tall podocarp forest began as the climate ameliorated. Tall, temperate podocarp trees replaced stress-tolerant shrubs and trees between 12,800 and 11,300 cal yr b.p., indicating sustained warming during the Younger Dryas Chronozone (YDC). Stable isotopes suggest increasing atmospheric humidity from 11,800 to 9300 cal yr b.p. Mild (annual temperatures at least 1°C higher than present), and moist conditions prevailed from 11,000 to 10,350 cal yr b.p. Cooler, more variable conditions followed, and podocarp forest was completely replaced by montane Nothofagus forest at around 7500 cal yr b.p. with the onset of the modern climate regime. The Cass Basin late-glacial climate record closely matches the Antarctic ice core records and is in approximate antiphase with the North Atlantic.

The influence of N addition on nutrient content, leaf carbon isotope ratio, and productivity in a Nothofagus forest during stand development

To test whether increased nitrogen (N) availability might increase productivity in maturing mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in central South Island, New Zealand, we applied N to 25-year-old sapling and 125-year-old pole stands. Nitrogen fertilizer increased foliar and fine-root N concentrations, fine-root growth, and leaf litter production in both sapling and pole stands but had no effect on stem basal area increment or individual leaf area, and it decreased individual leaf mass marginally. Heavy flowering and seeding occurred in the second year after fertilizer was applied, and N increased production of both. Leaf litter production and flowering responded similarly to N in sapling and pole stands, but N increased fine-root and seed productivity more in pole stands than in sapling stands, confirming our hypothesis that productivity of pole stands was more limited by low N availability. Resource allocation to fine roots and seed production may have restricted stem basal area increment response to N in the short term. Pole stands had higher leaf δ13C values than sapling stands. It is concluded that both low N availability and moisture stress may contribute to the decline in productivity and wood biomass previously found in mature mountain beech stands.