Geoffrey Rogers

Taxonomic and ecological profiles of rarity in the New Zealand vascular flora

Abstract We sought to taxonomically characterise New Zealands plant rarity within the context of the entire flora, and then to examine any geographical and ecological predispositions the rare flora might possess in terms of elevational and latitudinal range and ecosystem representation. We found skewed frequency distributions in the taxonomic properties of our rare flora, and its threatened and uncommon components, compared with the entire flora. There are more rare dicotyledonous trees and shrubs, dicotyledonous herbs, both composite and non‐composite, and orchids, and fewer rare grasses and sedges than expected at random. Whereas several families and genera have unexpectedly high representation of rare taxa, there is no consistent trend of proportional rarity across a gradient of taxon‐rich‐ness. Forty percent of New Zealands 433 genera are monospecific and 30% of those contain rare taxa. New Zealand endemism is not significantly different in the rare, threatened, and uncommon floras compared with the entire flora. Taxonomic richness in the rare flora is greater in the coastal and lowland zones than in montane and alpine zones. The majority of rare taxa are confined to one of three latitudinal zones ‐ northern North Island, northern South Island, southern South Island ‐ and the Three Kings, Chatham, and subantarctic offshore island groups. Three broad classes of non‐forest ecosystems below treeline support 70% of the rare flora. Coastal and inland cliffs below treeline feature prominently and provide an incentive for research into neglected ecosystems. Taxonomic and ecological profiles provide information useful in formulating conservation policy and recovery strategies for rare taxa and their ecosystems.

Ecology and conservation status of three “spring annual” herbs in dryland ecosystems of New Zealand

Abstract The biological and ecological significance of the “spring annual” life‐history strategy in the New Zealand flora is examined for Myosurus minimus subsp. novae‐zelandiae (Ranunculaceae), Ceratocephala pungens (Ranunculaceae), and Myosotis pygmaea var. minutiflora (Boraginaceae) in terms of their biogeographies, habitats, population sizes, seed morphologies, plant communities, and relationships with co‐occurring exotic taxa. All three have predominantly eastern South Island and lower North Island distributions, two have contracted modern ranges, and all extant populations are centred on Central Otago. The ephemeral wetland habitat of Myosurus, the “desert” pavement habitat of Ceratocephala, and the turf and gravel habitat surrounding water bodies of Myosotis all share a strong summer soil‐moisture deficit. These genuine non‐forest habitats are compared with farming‐induced scabweed or mat vegetation on dry hillslopes as additional facultative habitat for all three taxa. Selection for this growth strategy probably accompanied mountain building in the mid to late Cenozoic with the creation of seasonally dry eastern climates. Small numbers of sites, small population sizes, and habitat vulnerability to weeds and land conversion result in a conservation status of “threatened” for all three taxa. The problem of nesting conservation goals and management strategies for spring annual habitat within the restoration goals for their surrounding, degraded dryland ecosystems is highlighted.

Pre‐settlement woody vegetation of Central Otago, New Zealand

Abstract The potential woody vegetation of Central Otago, South Island, New Zealand, immediately prior to human settlement, is described using complementary lines of evidence. Generalised additive models based on a database of present species locations and environmental surfaces are used to predict the potential distribution of woody species in relation to the environment across a study area of 15 500 km2. Twelve biogeographic zones are classified on the basis of the predicted distributions of the 15 most common potential canopy tree species. The likely structure and composition of each zone is assessed using (1) model‐predicted occurrences of more common canopy, subcanopy, and shade‐intolerant species, and present‐day locations of rarer woody species, (2) compiled information from the subfossil wood, charcoal, and pollen records, and (3) the presence of plant species and life forms in zones of environmental stress. We conclude that water deficit and winter temperature minima are not sufficiently severe to have excluded tall woody vegetation from the intermontane basin and valley floors of Central Otago, and that low forest and shrublands would have dominated here prior to human settle‐ment. At low elevations, grassland was probably confined to floodplains and local areas of shallow or permanently moist soils, while saline soils supported distinctive herbaceous communities. Tall conifer‐hardwood and Nothofagus forests are predicted to have once covered the montane and subalpine range slopes. Analysis of environmental tolerances suggests that shrublands may also have dominated above the regional treeline (770–1380 m a.s.l. in Central Otago) although tall tussocks probably increased in dominance towards the range summits. We discuss the strengths and limitations of the different approaches to vegetation reconstruction. Remaining uncertainties may be resolved with a combination of palaeoecological and physiological research and the application of spatial process modelling. Implications of the work for conservation management are discussed.

Regional patterns of plant species richness in southern New Zealand

Abstract We examine the proposition that spatial heterogeneity, as modelled by land area, productivity‐influencing climatic variables, and elevation, is a usefd predictor of regional patterns of species richness in 56 regions of southern New Zealand stretching from mid Canterbury to Campbell Island. We also examine whether regional patterns of taxonomic richness support the concept of a Late Cenozoic radiation of upland floras and that this was sourced from just a few genera in a few widespread to cosmopolitan families. We found several contrasting geographical patterns in taxonomic richness. Several mainly mountain regions just east of the main divide in western Canterbury, Otago, and Southland are unusually species‐rich, but not genus‐ or family‐rich. Conversely, several coastal regions of eastern South Island are unusually genus‐ and family‐rich, but not species‐rich. Four geographically distinct groups of regions share relatively species‐, genus‐, and family‐depauperate floras: maritime‐influenced regions of Fiordland, the Chatham and subantarctic islands, inland and lowland basins of Otago and Southland, and eastern Otagos block mountains. Various combinations of elevation range, climatic variability, and land area explain 54% to 61% of the variability in richness of species, genera, families, and life‐form groups. Mean daily temperature of the coldest month appears the strongest climatic variable at depressing floristic richness, although moderate degrees of continentality of climate increase it. We conclude that both propositions are supported by the present study. Databases of regional‐scale patterns of plant species richness should be useful for selecting mainland islands for ecosystem restoration, estimating ecosystem or guild diversity, and testing the ecological distinctiveness of ecological regions and districts.

Identifying pathways for managing multiple disturbances to limit plant invasions

Summary Plant invasions are predicted to accelerate in a world with increased anthropogenic disturbance. Non-native species pre-adapted to these disturbances may especially be poised to invade novel communities. Conservation managers therefore need predictions of how to alter disturbances to maximize the persistence of native biodiversity. We tested a multivariate hypothesis about the causal mechanisms underlying plant invasions in an ephemeral wetland in South Island, New Zealand, to inform management of this biodiverse but globally imperilled habitat. Our approach details among the first applications in ecology of Bayesian structural equation modelling, demonstrating its potential to inform management by disentangling the relative importance of strongly intercorrelated processes. We found that invasion by non-native plants was lowest in sites where the physical disturbance caused by flooding was both intense and frequent. This effect was stronger than the positive response of non-native species to high soil N supply, which was positively related to flooding. Sites flooded over a 4-year period had greater reductions in invasion than those associated with floods in the year prior to plot measurement because non-native species lacked traits for long-term persistence beneath water. Grazer exclusion had a small positive effect on invasion, as non-native species were preferentially selected by the herbivores at our site. Our results show that only species adapted to the dominant disturbance regimes at a site may become successful invaders. Species native to ephemeral wetlands have specially evolved traits that allow them to persist and dominate in these sites. Synthesis and applications. Predictions of invasions in a world of multiple disturbances clearly need to consider whether the evolutionary history of non-native species predisposes them to invade novel communities. Maintaining hydrological and nutrient regimes of ephemeral wetlands will limit the number of introduced species that are pre-adapted to become invasive.

Environment, composition and conservation of coastal turfs of mainland New Zealand

Abstract Coastal turfs are communities of tightly interlaced, ground-smothering, short-statured herbs, grasses and sedges occupying shoreline rock landforms exposed to persistent salt-laden onshore winds. They are recorded from eight mainland New Zealand regions along the west and south coasts of North and South Islands. They display regional compositional distinctiveness best explained by geographic location, landform shape and substrate type. There is evidence that the predominant direction of onshore winds delivering aerial salt is aligned obliquely and not perpendicular to the coastline, and that the topographic aspect of turfs is correspondingly skewed in the opposing direction. Significant differences in soil salinity and pH were found between turfs of native herbs and those of exotic grasses and herbs on the same uniform landform. Turfs support 9 threatened and 24 uncommon plants, with all but 1 of the 33 being herbs, and 21 being recorded from just one region. Turfs exposed to ungulate (mostly farm stock) disturbance showed greater vegetation cover than those without ungulates. Conservation priorities should mostly be set intra-regionally and cautiously view ungulate disturbance as a beneficial management tool.

Evolution of the New Zealand vascular flora: Regional and provincial patterns of richness, radiation, and endemism

Abstract Analysis of species richness, radiation, and endemism in 171 list‐regions and 19 botanical provinces of the New Zealand botanical region indicates that the Late Cenozoic radiation of the New Zealand vascular flora has produced divergent phylogenetic trends in regional and provincial floras. Floristic richness at all taxonomic levels is strongly determined by land area. After accounting for area, most exceptionally species‐rich list‐regions occur immediately east of the South Island main divide, on calcareous substrates in West Nelson, or on several central North Island ranges. Depauperate floras are predominantly in the south on Auckland and Campbell Islands, and four Central Otago and two Southland ranges or basins. Family and genus richness decrease with latitude, but it appears that there has been compensatory radiation from a few herbaceous families in southern island and upland floras, so that species richness shows no latitudinal trend. We present a measure of aggregate regional endemism that is comparable between list‐regions and provinces of different floristic richness. Offshore island groups are the most insular floras, with exceptionally high endemism across different taxonomic ranks; northern island floras have high degrees of family and genus endemism and high phylogenetic diversity, while the subantarctic island floras have high species endemism and high radiation. In contrast, mainland botanical provinces all show average levels of regional endemism after correcting for species richness. With the exception of gymnosperms, the most New Zealand‐endemic plant groups have the most narrow regional distributions. We discuss latitudinal patterns of endemism among plant groups in terms of dispersal and adaptation. Land protection measures will achieve different phylogenetic outcomes in different landscapes across the New Zealand botanical region.

Land use effects on “spring annual” herbs in rare non‐forest ecosystems of New Zealand

Abstract We examined trends over six consecutive spring seasons in three “spring annual” herbs, Myosurus minimus subsp. novae‐zelandiae (Ranunculaceae), Ceratocephalapungens (Ranunculaceae), and Myosotis pygmaea var. minutiflora (Boraginaceae), in terms of their habitats and site land uses in several rare, non‐forest ecosystems. On dry hillslopes, saline soils, and turfs of ephemeral wetlands (including coastal turfs), Ceratocephala and Myosotis declined where sites were managed for conservation by removal of mammalian herbivores. Some populations of Myosurus declined only marginally or remained stable despite high cover of ruderal and weedy, herbaceous exotic plants. Loss of monitored populations during the study was offset by discoveries of additional populations of all three taxa. Statistical models of population trends suggest that in some instances spring annual populations benefit from farm animal and rabbit (Oryctolagus cuniculus cuniculus) disturbance of their habitats perhaps by 1) suppressing transitions to taller vegetation and retarding competition of invasive exotic plants and 2) maintaining nutrient supplementation from faeces lost with the extinction of ground‐dwelling birds. Frequent seeds of all three taxa preserved in moa coprolites from several sites in Central Otago point to 1) the herbs’ probable non‐rarity in pre‐human times and 2) dispersal mutualisms and mediation of community ground cover by extinct ratites.

The woody vegetation of Central Otago, New Zealand

Abstract The present native and exotic woody vegetation of the rainshadow region of Central Otago, in eastern South Island, New Zealand, is described. Fourteen present‐day associations of woody species are related to a gradient in elevation and climate, and to the degree of disturbance by fire and mammalian grazing since human settlement. A few closed‐canopy forest remnants remain only in the montane zone, while more modified forest relicts are scattered across a wider range of environments. Remaining woody plant associations have originated as secondary successions following fire, and range from exotic‐species‐invaded lowland associations on semi‐arid basin floors to prostrate, native‐species‐dominated associations on mountain tops. Indigenous short‐tussock grasslands and seral shrublands are being invaded by exotic trees and shrubs, especially in lowland environments. Dwarf and prostrate low shrub associations on the range tops may have replaced tussock grasslands since the commencement of pastoralism, where taller tussock‐shrubland communities were destroyed by fires soon after human settlement. The regions vegetation is not at equilibrium, and taller, woody communities tend to develop following removal of fire and grazing. Biodiversity restoration goals for deforested eastern South Island regions such as Central Otago should include long‐term succession to indigenous woody communities in representative areas of all land environments.