11.4.4 Forestry

In Australia, the value of wood and wood products in 2001-2002 was US$5 billion/yr. About 164 million ha are classified as forest, with 1% as plantation forests and 7% available for timber production in state-managed, multiple-use native forests (BRS, 2003). New Zealand’s indigenous forests cover 6.4 million ha, with 1.7 million ha of planted production exotic forests, the latter providing substantial export income (MAF, 2001). Research since the TAR confirms that climate change is likely to have both positive and negative impacts on forestry in both countries. Productivity of exotic softwood and native hardwood plantations is likely to be increased by CO₂ fertilisation effects, although the amount of increase will be limited by projected increases in temperature, reductions in rainfall and by feedbacks such as nutrient cycling (Howden et al., 1999c; Kirschbaum, 1999a, b).

Where trees are not water-limited, warming expands the growing season in southern Australia, but pest damage is likely to negate some gains (see Section 5.4.5). Reduction in average runoff in some regions (see Section 11.4.1) and increased fire risk (see Section 11.3.1) are very likely to reduce productivity, whilst increased rainfall intensity is likely to exacerbate soil erosion problems and pollution of streams during forestry operations (Howden et al., 1999c). In Pinus radiata and Eucalyptus plantations, fertile sites are likely to have increased productivity for moderate warming, whereas infertile sites are likely to have decreased production (Howden et al., 1999c).

In New Zealand, the growth rates for plantation forestry (mainly P. radiata) are likely to increase in response to elevated CO₂ and wetter conditions in the south and west. Studies of pine seedlings confirm that the growth and wood density of P. radiata are enhanced during the first two years of artificial CO₂ fertilisation (Atwell et al., 2003). Tree growth reductions are likely for the east of the North Island due to projected rainfall decreases and increased fire risk (see Section 11.3.1). However, uncertainties remain regarding increased water-use efficiency with elevated CO₂ (MfE, 2001), and whether warmer and drier conditions could increase the frequency of upper mid-crown yellowing and winter fungal diseases (MfE, 2001).