Soil compaction can affect seedling root development by decreasing oxygen availability and increasing soil strength. However, little quantitative information is available on the compaction tolerances of non-crop native species. We investigated the effects of soil compaction on establishment and development of two New Zealand native species commonly used in restoration programmes; Cordyline australis (Agavaceae) (cabbage tree) a fleshy rooted species, and Leptospermum scoparium (Myrtaceae) (manuka) a very finely rooted species. Seedlings were grown in a range of soil compaction levels in growth cabinet experiments. Low levels of soil compaction (0.6 MPa) reduced both the number and speed of C. australis seedlings penetrating the soil surface. In contrast, L. scoparium seedlings showed improved establishment at an intermediate compaction level. Root and shoot growth of both species decreased with increasing soil strength, with L. scoparium seedlings tolerating higher soil strengths than did C. australis. Despite these results, soil strength accounted for only a small amount of variation in root length (R-2 < 0.25), due to greater variability in growth at low soil strengths. Soil strengths of 0.6 MPa are likely to pose a barrier to C. australis regeneration. This is consistent with adaptation to organic and/or soft, waterlogged soils. Active intervention may be necessary to establish C. australis from seed on many sites previously in farmland.

A ground-based estimate was made of the number of emergent female strobili carried by 74 open-grown stands (10–12 years old) of Pinus radiata growing across a range of latitudes and altitudes of the South Island and North Island of New Zealand. The intent of this study was to develop and evaluate an empirical model to describe the relationship between environmental variables and emergent female strobilus numbers. The model suggested that variability in female strobilus number was necessarily associated with climate and site factors. Temperature and to a lesser extent rainfall variables were associated with the number of seed cone-buds that achieved anthesis in the spring. The step-wise multiple regression analysis selected mean minimum February temperature, integrated soil water stress (February–March), accumulated growing degree days (weighted against spring rainfall prior to seed cone-bud initiation), in that order. The first variable was found to explain around 54% of the variance in strobilus numbers across sites, while taken together all of the variables explained 64% of among-site variability. Model evaluation was carried out across both similar eastern South Island sites and different North Island sites at which it was developed. It was found to predict strobilus counts accurately across the South Island sites, but less accurately across the warmer North Island sites. Mean minimum February temperature was still the most important predictor of strobilus production across sites, but this relationship was non-linear. Thus, a non-linear modelling approach was adopted to better fit the model across regions New Zealand-wide. Long term nation-wide data for mean minimum February temperature were then used in a climate modelling system to generate a temperature map for New Zealand, to assist with seed orchard site selection. This map illustrated that oceanic (as opposed to semi-continental) environments around the New Zealand coast are likely to be the most productive sites for seed orchard production. Contrary to a priori perceptions, the northern regions of the North Island were depicted to offer a greater number of candidate sites for seed orchard siting than the more south eastern semi-arid regions of 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.