Fiona E. Carswell

CLIMATE AND NET CARBON AVAILABILITY DETERMINE TEMPORAL PATTERNS OF SEED PRODUCTION BY NOTHOFAGUS

We analyzed seed production of mountain beech (Nothofagus solandri var. cliffortioides) forest along an elevational gradient in New Zealand from 1020 to 1370 m (treeline) for the years 1973-2002. We used seed production data from nine elevations and a site- and species-specific net carbon (C) availability model from two elevations (1050 m and 1340 m) to examine how three variables (temperature, soil moisture, and net C avail- ability) during three key periods (resource priming, flowering primordia development, and flowering) influence seed production. These three strongly interrelated variables have all been considered determinants of seed production but have not previously been tested to- gether in a single analysis. Seed production increased over the 30-year period, with the greatest increases at high elevations; this increase was driven by a greater frequency of intermediate-sized seeding years. We then determined how temperature, soil moisture, and net C availability determined seeding, and examined whether temporal trends in the seeding data could be linked to similar temporal trends in temperature, soil moisture, or net C availability. High seed production was related to cool summers with high soil moisture during resource priming, warm summers during flower primordia development, and low net C availability during flowering. Positive temporal trends in temperatures during the period of flower primordia development accounted for the increase in seed production, suggesting that increasing temperatures are promoting more frequent seed production at high elevations.

Influence of nitrogen and phosphorus supply on foliage growth and internal recycling of nitrogen in conifer seedlings (Prumnopitys ferruginea)

The dynamics of internal cycling of nitrogen were studied in the southern hemisphere conifer miro [Prumnopitys ferruginea (G. Benn. ex D. Don) de Laub.], which has an indeterminate growth habit. In a 2-year experiment, P. ferruginea seedlings were supplied with nutrient solutions consisting of two different concentrations of nitrogen (5 and 0.5 mM) and phosphorus (1.33 and 0.133 mM) in the first year, and two concentrations (5 and 0.5mM) of a 15N-labelled nitrogen solution in the second year. Growth and nitrogen content of new foliage were shown to be largely dependent on seedling nitrogen status at the end of the first year, and only weakly dependent on nutrient supply in the second. An average of 70% of total nitrogen in new foliage was remobilised from storage in the first 63 d after flushing began. The remainder of new-foliage nitrogen was derived by root uptake from the nutrient supply in the second year. There was some response of nitrogen uptake to high nitrogen supply in the second year where seedlings had been nitrogen deficient at the end of the first year. However, it was concluded that the indeterminate growth habit of P. ferruginea did not distinguish its pattern of nitrogen storage and remobilisation from that of determinate conifers.

Functional Traits Reveal Processes Driving Natural Afforestation at Large Spatial Scales

An understanding of the processes governing natural afforestation over large spatial scales is vital for enhancing forest carbon sequestration. Models of tree species occurrence probability in non-forest vegetation could potentially identify the primary variables determining natural afforestation. However, inferring processes governing afforestation using tree species occurrence is potentially problematic, since it is impossible to know whether observed occurrences are due to recruitment or persistence of existing trees following disturbance. Plant functional traits have the potential to reveal the processes by which key environmental and land cover variables influence afforestation. We used 10,061 survey plots to identify the primary environmental and land cover variables influencing tree occurrence probability in non-forest vegetation in New Zealand. We also examined how these variables influenced diversity of functional traits linked to plant ecological strategy and dispersal ability. Mean annual temperature was the most important environmental predictor of tree occurrence. Local woody cover and distance to forest were the most important land cover variables. Relationships between these variables and ecological strategy traits revealed a trade-off between ability to compete for light and colonize sites that were marginal for tree occurrence. Biotically dispersed species occurred less frequently with declining temperature and local woody cover, suggesting that abiotic stress limited their establishment and that biotic dispersal did not increase ability to colonize non-woody vegetation. Functional diversity for ecological strategy traits declined with declining temperature and woody cover and increasing distance to forest. Functional diversity for dispersal traits showed the opposite trend. This suggests that low temperatures and woody cover and high distance to forest may limit tree species establishment through filtering on ecological strategy traits, but not on dispersal traits. This study shows that ‘snapshot’ survey plot data, combined with functional trait data, may reveal the processes driving tree species establishment in non-forest vegetation over large spatial scales.

Opportunities for restoring indigenous forest in New Zealand

Abstract We assessed the opportunities for restoring indigenous forest from grassland. Using a map of basic ecosystems at 2008, we identified grassland areas with a high ratio of biodiversity benefit (after conversion to indigenous forest) over loss of agricultural production. With approximately 2 mha of grassland with a benefit/cost ratio of 0.2 or more, there are many opportunities for restoration. We also assessed the opportunities to protect shrublands already regenerating to indigenous forest. There are presently about 0.7 mha of shrublands with a high carbon sequestration over stock-carrying capacity ratio (>20). National models of other ecosystem services such as erosion control, provision of fresh water, water regulation, and tourism and recreation, may be used in a second-tier selection process. The methods developed here could be used in a formalised system for calculating ecosystem service offsets.

Moving beyond “mitigation and adaptation”: examining climate change responses in New Zealand

Despite the apparent failure of international negotiations and renewed criticism of the accuracy of climate science, responses to climate change continue in households, cities, fields, and meeting rooms. Notions of “doing something about”, or “taking action on” or “mitigating and adapting” to climate change inform practices of carbon trading, restoring native forests, constructing wind turbines, insulating houses, using energy efficient light bulbs, and lobbying politicians for more or less of these actions. These expressions of agency in relation to climate change provide the focus of our enquiry. We found that relationships or social networks linked through local government are building capabilities to respond to climate change. However, the framework of “mitigation–adaptation” will need to be supplemented by a more diverse suite of mental models for making sense of climate change. Use of appropriate languages, cultural reference points, and metaphors embedded in diverse histories of climates and change will assist actors in their networked climate change responses.