The effect of climate change on New Zealand's planted forests: impacts, risks and opportunities

Authors: Watt, M S; Kirschbaum, MUF; Paul TSH,; Tait, A; Pearce, H G; Brockerhoff, E G; Moore, J R; Bulman, L S; Kriticos, D J

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Executive summary

Objectives

The objectives of this report were to:

  1. Synthesise current knowledge on likely climatic changes in New Zealand over the course of this century (Chapter 1).
  2. Summarise current knowledge of the likely direct climate-change impacts on plantation productivity (Chapter 2).
  3. Summarise the current knowledge of likely climate-change effects on abiotic (wind and fire risks) and biotic (insects, diseases, weeds) factors, and how changes in these secondary factors are likely to influence plantation productivity (Chapter 3).
  4. Develop an interdisciplinary framework which can be used to describe the direct and indirect effects of climate change on plantation productivity. For this framework, define the linkages and feedbacks between direct and indirect effects on plantation productivity. Using this framework, identify key knowledge gaps and uncertainties, and provide a comprehensive research plan to address these uncertainties (Chapter 4).

Major results

Climate change

The mean annual temperature for all of New Zealand is projected to increase by around 2°C from 1990 to 2090. Annual rainfall projections indicate a likely difference in trends, with the western side of both islands trending towards increasing rainfall while the eastern side trends towards decreasing rainfall. The projected increase in temperature is likely to lead to a significant reduction of frost days, an increase in hot days and an increase in the intensity of heavy rainfall events. Drought risk is likely to increase in currently drought-prone areas where reductions in rainfall and increases in evaporation are anticipated. There are indications that the annual mean westerly wind component may increase by about 10% by 2090, and that high winds (wind speeds exceeded 1% of the time) may increase by around on average 2–3%.

Climate change and tree growth

Climate change is likely to have a significant impact on the future growth of trees in planted forests because tree growth responds directly to changes in CO2 concentration, temperature and nutrient and water availability. There are direct responses to these drivers and indirect interactions and feedback processes.

Increasing CO2 concentration is generally beneficial for the growth of plants, but the magnitude of the response varies with species and growth stage and is affected by interactions with other environmental factors that can limit (e.g. lack of essential nutrients) or enhance (e.g. warmer and drier conditions) the ‘CO2 fertilisation effect'. CO2 fertilisation is likely to be most beneficial in the drier parts of the country, such as Otago, Canterbury, Hawke's Bay and East Cape. Nutrient limitations (especially nitrogen) are unlikely to limit the CO2 response provided that the nutrient levels, which are currently sufficient in most of New Zealand's plantations, can be maintained into the future. Increasing temperatures can also stimulate decomposition of soil organic matter and mineralise more nitrogen to further boost the nutritional status of trees.

Increases in air temperature expected with climate change are likely to have a predominantly positive effect on plantation growth, as increasing temperature is likely to lengthen the growing season.

While radiata pine is New Zealand's most important plantation species, there has not yet been a comprehensive modelling study to investigate the likely growth response of radiata pine to the combination of likely climatic changes over the short or longer term. Comprehensive models that include these processes exist and are available, but they need to be tested in New Zealand before being used to quantify the forest growth response across the wide edaphic and climatic range over which plantations are grown in New Zealand.

Climate change is also likely to affect many abiotic and biotic factors, which may in turn affect plantation growth and productivity. The main factors which we have considered here include weeds, insects, pathogens and the risks from wind and fire. All of these factors currently cause significant economic losses in planted forests.

Changes in risk from fire and wind

Climate change is likely to increase fire risk and the incidence of damaging winds. Previous research has shown that fire danger is likely to increase significantly in most areas of New Zealand, and that the length of the fire season will probably increase. Increases in fire risk are likely to result in an increase in the incidence of fires and plantation area burned.

It is also predicted that the westerly wind speed component will increase during the winter and spring periods, leading to an increase in the mean and extreme wind speeds for many regions of New Zealand. For the central and upper regions of the North Island, which contain a large proportion of the plantation estate, extreme wind speeds are predicted to decrease. However, the upper and eastern parts of the North Island may be subject to more severe extra-tropical cyclones. In those regions where there is a predicted increase in the severe wind climate, it is likely that the annual frequency of winds sufficient in magnitude to cause widespread damage to forests could increase substantially. Previous research and simulations indicate that the impacts from these projected increases in extreme wind speeds will vary widely, both within and between regions due to differences in the underlying vulnerability of forests in these regions. These impacts are likely to range from little or no change, to a significant increase in the risk of wind damage to planted forests.

Changes in risk from exotic pests

Biotic factors are also likely to be strongly influenced by climate change. As the distribution of weeds, insects and pathogens is strongly determined by climatic conditions, changes in these conditions are likely to result in shifts in the geographic range of many species. Under climate change, global regions from which future invasions might occur are also likely to shift. Simulations suggest that the global areas that pose an invasion risk for New Zealand could expand, and this report highlights the new areas that might currently harbour potential future invaders.

Plant pests - Climate change is likely to result in range expansion within New Zealand of a number of native Australian tree species, such as Eucalyptus and Acacia spp. and the proliferation of currently ornamental and potentially invasive weed species such as Melaleuca quinquenervia (broad-leaved paperbark) and Pueraria montana (kudzu). Climate change is also likely to affect growth rates of weeds through changes in CO2 concentration, root-zone water storage, temperature and changing length of the growing season. These changes to weed composition and growth rates resulting from climate change are likely to have a detrimental effect on tree growth.

Insect pests - Because of our limited knowledge of climate effects on forest insects in New Zealand, it is difficult to make any generalised assessment about likely climate-change effects on their abundance and distribution. However, it is likely that climate change will increase the risk of establishment of new species from warmtemperate or subtropical regions and generally result in greater abundance of insect pests due to their better survival over winter. Without further research it is difficult to determine how changes in insect abundance and distribution will influence plantation productivity.

Pathogens - Climate change is unlikely to greatly influence the distribution of the main needle-cast diseases of radiata pine in New Zealand as these are already found throughout the country. Climate change is, however, likely to change the regional incidence and severity of some fungal diseases. It is likely that Dothistroma needle blight will remain rare in dry east coast regions, which are projected to become even more arid under climate change. The severity of Swiss needle cast, caused by Phaeocryptopus gaeumannii, which is the most widespread disease of Douglas-fir is likely to increase with climate change throughout the country as pathogen abundance is strongly correlated with winter air temperature. For the major diseases of radiata pine, changes to pathogen distribution and abundance will need to be quantified before the influence of these on plantation productivity can be determined.

Under climate change, there are also a number of interactions between abiotic and biotic factors, which need to be considered in order to gain a full understanding of likely climate change effects on productivity. These are described fully in the report.

Framework

The numerous interactions between plantation productivity, climatic variables and secondary effects described in this report demonstrate the complexity of the system and the necessity to model the effects of climate change on plantations in an integrated manner. We present such a framework which could be used to determine the effects of climate change on plantation productivity. At the core of the framework is a growth model which can account for the ecophysiological effects of climatic change on plantation productivity. This core model is linked to sub-models accounting for the influence of additional biotic (weeds, pests, diseases) and abiotic (wind, fire) factors on plantation productivity. The key results and the proposed framework described above have identified a possible approach to quantify the overall effects of climate change on New Zealand's production forests in an integrative way. Key knowledge gaps have also been identified, and a comprehensive research plan to address these uncertainties is presented at the end of section four.

Contact for Enquiries

Sustainable Land Management and Climate Change
MAF
Pastoral House
25 The Terrace
PO Box 2526, Wellington
Tel: 0800 CLIMATE (254 628)
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