Neil Mitchell

Generalized additive models in plant ecology

Generalized additive models (GAMs) are a non- parametric extension of generalized linear models (GLMs). They are introduced here as an exploratory tool in the analysis of species distributions with respect to climate. An important result is that the long-debated question of whether a response curve, in one dimension, is actually symmetric and bell- shaped or not, can be tested using GAMs. GAMs and GLMs are discussed and are illustrated by three examples using binary data. A grey-scale plot of one of the fits is constructed to indicate which areas on a map seem climatically suitable for that species. This is useful for species introductions. Further applications are mentioned.

Microclimate and vegetation edge effects in a fragmented podocarp-broadleaf forest in New Zealand

Abstract Microclimate (photosynthetically active radiation; air temperature; vapour pressure deficit) and vegetation edge effects were investigated in five podocarp-broadleaf forest fragments in the North Island of New Zealand. Distinct forest edge microclimate regimes were found to exist for part of the year and these were associated with differences in vegetation composition and structure between the forest edge and interior. Some species-specific edge/interior differences could be related to timing of critical life history stages (e.g. germination and early establishment) relative to the temporal stability of edge microclimate regimes. Penetration of gross microclimatic edge effects was approximately 50 m regardless of fragment size. These results suggest that regularly shaped forest fragments

The derivation of climate surfaces for New Zealand, and their application to the bioclimatic analysis of the distribution of kauri (Agathis australis)

Abstract Modelling techniques based on Laplacian smoothing splines were used to derive climate trend surfaces for each of four parameters: monthly mean rainfall, minimum temperature, maximum temperature and solar radiation. The use of analytical procedures, based on the program BIOCLIM, are described; these allow the estimation of climate parameters at individual locations from the trend surfaces. These techniques were used to analyse the geographic distribution of kauri with respect to contemporary climate. For each location where kauri has been recorded, an estimate was made of various climate parameters, which by summation allowed the derivation of a ‘climate profile’. This profile may be considered a multivariate definition of the climatic regime within which the species now grows. To analyse the role of climate in determining the geographic distribution of kauri, each location in a North Island database was tested against the profile, to assess whether it could support kauri growth. There was a close...

Forest pattern, climate and vulcanism in central North Island, New Zealand

. A quantitative study of relationships between forest pattern and environment in the central North Island, New Zealand, is based on forest composition data from ca. 2000 existing plots distributed throughout the forests of the region. Estimates of mean annual temperature, rainfall, and solar radiation are derived for each plot from mathematical surfaces fitted to climate station data. Estimates of the depth of the last major rhyolitic eruption, (Taupo Pumice, ca. 130 AD) are derived from isopach maps. A classification procedure is used to identify broad compositional groups. Generalised linear models are used to examine relationships between major species and climatic and other physical factors. Significant relationships are identified between the distributions of both plot groups and species, and climate, vulcanism, topography and drainage. Among these factors, temperature and/or solar radiation are indicated as major determinants of the regional forest pattern, with rainfall, topography, and drainage acting at a secondary level. The role of the Taupo Pumice eruption is more difficult to interpret, and its effects seem to have been greatly influenced by topography. Deep extensive deposits of tephra on flat-to-rolling sites close to the eruption centre have probably favoured the current dominance of these sites by more rapidly dispersing conifers. In contrast, on adjacent steep sites where forest destruction was likely to be less severe, slow-dispersing Nothofagus species are largely dominant. Further work is needed to understand the factors favouring conifer dominance of the central basins and the degree to which Nothofagus species might expand their range in the future.

Environmental factors controlling germination of Leptospermum scoparium (manuka)

Abstract Laboratory studies using seed of Leptospermum seoparium J. R. et G. Forst. collected from tagged trees in the Waitakere Ranges near Auckland showed that there was a low overall germination percentage but substantial variation in germination from tree to tree. Germination rate but not final percentage was affected by temperature and by alternating periods of dehydration WIth periods of adequate availability of water. Only a very low total incident radiation was necessary for germination but germination was reduced 10 the dark. It is probable that the low germination percentage is a result of a high proportion of non-VIable seed. There was no evidence of dormancy and, in the Auckland region, it is likely that the major ‘seed bank’is present in the unshed capsules rather than on the ground surface.

The germination ecology of Dysoxylum spectabile (Meliaceae)

Abstract Environmental conditions promoting the germination and establishment of kohekohe (Dysoxylum spectabile) are described. It was found that in the population examined, there was a very high level of seed viability and there was no evidence of dormancy. Seeds would germinate successfully in either the light or dark and seedlings could continue to grow for at least 18 weeks in the dark. The effect of different temperature regimes was examined and it was found that cooler temperatures slowed the rate of germination and seedling development. The role of leaf litter in seedling germination and establishment was examined. A covering of litter was found to be essential for successful establishment. Seeds that germinated on a substrate surface had a much reduced chance of survival.

Rational landscape decision making: the use of meso-scale climatic analysis to promote sustainable land management

Planning for sustainable land management and the development of land classification systems has become a very complex process with the increasing sophistication of GIS techniques. This has resulted in the emphasis moving to detailed micro-scale description and planning, with a loss of perception of the physical processes and hierarchy of constraints that control local activities. We have developed a meso-scale climate-landscape classification technique which would enable policy makers to develop simplified guidelines for rural activities and to help landholders identify appropriate sustainable land uses. It would also provide a simplified tool for assessment of the effectiveness of a protected areas network. We propose that unless the basic physical possibilities and limitations of the environment are properly understood it is not possible to have sustainable land management, nor stay within the carrying capacity of the local environment.

Seasonal variation in seedfall and germination of Leptospermum scoparium (manuka)

Abstract Field studies on the seedfall and germination of Leptospermum scoparium J. R. et G. Forst showed that viable seed is shed throughout the year. It was found that there is a significant seasonal pattern to both seedfall and germination. Peak seed fall occurred in October and January, although capsule-borne seeds made an important contribution to the October peak. Germination was found to be highest in mid-winter and early spring. Analysis is described of the possible role of climatic factors in controlling seed fall and germination. No simple model could be found to account for the observed pattern of seed fall. However, a highly significantmodel (involving temperature and moisture regime) could account for the observed pattern of germination. The significance of these relationships is discussed in the context of the ecology of the species.

Ark 2020: A conservation vision for Rangitoto and Motutapu Islands

Our vision for the development of Rangitoto and Motutapu Islands is that they be restored to the state of functioning native ecosystems. In order to achieve this, all mammals and invasive weeds must first be eradicated. Following this, past ecological and culturally‐influenced communities can be restored by the revegetation of large areas of Motutapu and the reintroduction, or translocation, of native fauna. There is a need for open public access and provision for full interpretation of the Maori and Pakeha culture and history, as well as of the ecological values associated with these two islands. This document outlines a strategy whereby such a project could be achieved. While this document is not intended to be a definitive working plan for the restoration of these islands, we believe that a strategy along these lines is one that would best meet the needs of both native flora and fauna and also of the New Zealand public.

The growth responses of Dysoxylum spectabile (Meliaceae) to a shaded environment

Abstract The effect of shaded conditions on the growth of kohekohe (Dysoxylum spectabile (Forst, f.) Hook, f.) seedlings is described. Kohekohe regenerates under shaded conditions and shows many of the features, described by other authors, which are characteristic of shade-tolerant species. Shade tolerance of the seedlings helps explain some of the known ecology of the species.