Projected impacts of climate changes on forestry

Over the next 30 to 70 years (one to two plantation rotations) a number of climatic variables are predicted to change. The potential impacts of these changes on forests are not clearly understood, especially for Australian tree species under typical Australian conditions.

Based on international research carried out over the past 20 years, a certain amount is understood about the direct effects of increasing carbon dioxide concentration on plant growth and function. Doubling of carbon dioxide concentration generally leads to increases in plant growth of 10%–25% (Nowak et al. 2004; Luo et al. 2005b; Norby et al. 2005). Most of the research has been on Northern Hemisphere species under environmental conditions different to those typical of Australia. Furthermore, most research does not consider the feedbacks at the ecosystem level that need to be factored in when predicting effects on whole forests from results measured on individual trees/saplings.

Importantly, plant physiologists and modellers alike now recognise that the effects of elevated carbon dioxide measured in experimental settings and implemented in models may overestimate actual field responses, due to many limiting factors, such as pests, weeds, competition for resources, soil water and air quality, which are neither well understood on large scales, nor well implemented in leading models (Korner et al. 2005; Ainsworth & Long 2005; Tubiello & Ewert 2002; Karonsky 2003; Fuhrer 2003). A handful of experiments are now in progress to directly measure forest ecosystem-level responses to increased carbon dioxide; however, these FACE (free air carbon dioxide enrichment) experiments are all being conducted on temperate species in the Northern Hemisphere.

The lack of field-based forest experimentation in Australia makes it difficult to predict the effect of climate change on Australian native forests and plantations. The Hawkesbury Forest Experiment, in which NSW DPI is a collaborator, is beginning to redress this deficiency (See section 10 of Climate change research priorities for primary industries - discussion paper).

Increased fire incidence may negate productivity gains achieved through longer growing seasons.

Increased fire incidence may negate productivity gains achieved through longer growing seasons.

Where trees are not water-limited, climate warming is likely to expand the growing season in southern Australia; however, increased fire incidence and pest damage may negate some productivity gains. Productivity of exotic softwood and native hardwood plantations is likely to be increased by carbon dioxide fertilisation effects, although the amount of increase will be limited by feedbacks such as nutrient cycling (Kirschbaum 1999). Elevated atmospheric carbon dioxide is predicted to increase water use efficiency, which may offset negative impacts on growth in those areas in which rainfall is predicted to decline. Extreme heat may limit forest growth in summer, as observed in European forests by Angert et al. (2005) and Ciais et al. (2005). Carbon stock in soil organic matter and surface litter may decline due to faster decay in a warmer environment, though this could be offset by a higher rate of input through CO2 fertilisation and reduced biological activity if soil moisture declines. Thus, the interacting effects of temperature and CO2 on growth and heterotrophic respiration will determine the impact of climate change on the net carbon balance of forest systems. Better understanding of these interacting factors is required, so that forest managers can plan species selection, silvicultural management and breeding programs to cope with predicted changes in climate.

The actual climate tolerance of many Australian tree species is wider than the climatic envelope that they currently occupy; furthermore, natural distributions rarely extend to fill the current climate envelope (Jovanovic & Booth 2002). Increasing carbon dioxide concentrations, which change photosynthetic rates and water use efficiency, and which may affect the temperature response (Curtis 1996), will modify species climatic envelopes; so, although climate change will move climate envelopes geographically, it is not at all clear what effect this will have on species distributions. The relative effectivenes of the various seed disperal mechansims employed by different species will influence their ability to migrate as climate changes.

Australia’s south-east is recognised as one of the most fire-prone areas in the world, and fire management agencies have identified climate change as one of the most important strategic issues confronting fire managers in Australia (Bushfire CRC 2006). The danger posed by wildfire is dependent on the probability of a fire starting, its subsequent rate of spread, intensity and ease of suppression. Suppression is affected by the air temperature, relative humidity, wind speed, the properties and arrangement of the available fuel, and prior rainfall.

Climate change is likely to impact on wildfire risk, largely through its impact on climate extremes, rather than gradual changes in average temperature and rainfall occurring over decades. Internationally, there is great uncertainty associated with studies on the impact of climate change on forest fires (Lemmen & Warren 2004; Shugart et al. 2003). Current projections suggest that, in south-eastern Australia, the frequency of very high and extreme fire danger may increase by 4%–25% by 2020, and by 15%–70% by 2050, with greater changes predicted for the inland than for the coast (Hennessy et al. 2005). Lightning strikes are predicted to increase in tropical northern Australia, but the impact of climate change on their incidence in the south is currently uncertain.

In the longer term, changes in the distribution of flora as a result of climate change will also impact fire risk in native forests; for example, replacement of cool temperate rainforest with sclerophyllous forest would increase flammability (Bushfire CRC, 2006). Fire danger is predicted to increase in spring, summer and autumn, so periods suitable for prescribed burning are likely to be restricted (Hennessy et al. 2005).

Forest fires emit large quantities of carbon dioxide, as well as small but significant quantities of the greenhouse gases methane and nitrous oxide, and the greenhouse gas precursors CO and NOx and NMVOC (IPCC 2005). Excluding carbon dioxide , forest fires (controlled fire and wildfire) contributed 4.4 Mt CO2-e in 2004, and 1.2 Mt in 2005 (AGO 2007c). Thus, management to reduce the incidence of fire could assist in the mitigation of greenhouse gas emissions. Indeed, some have advocated the inclusion of fire management as an eligible offset activity in the proposed National Emissions Trading Scheme (NETT 2006; see also Section 9.10); however, accounting for the impacts of fire management on greenhouse gas emissions will be complex, in that:

  • if prescribed burning is used to reduce the risk of high-intensity wildfire, the emissions from prescribed burns must be balanced against predicted emissions from avoided wildfire
  • carbon dioxide emissions during fire are assumed to be balanced by sequestration during regrowth, so carbon dioxide emissions are excluded from reporting. However, if fire incidence increases in extent or intensity due to management or climate change, average carbon stocks will be affected, and this impact must be included
  • the formation of charcoal should be included in the estimation of greenhouse impacts of fire: approximately 4%–5% of the carbon consumed by forest fires remains on site as black carbon, which has a turnover time of thousands of years (Forbes et al. 2006).

Climate change is likely to affect the incidence and severity of pest and disease outbreaks in native forests and plantations. Firstly, changes in average or extreme values of climate variables can affect the life cycles of pest populations and the severity of disease. Increased summer temperatures are likely to accelerate the development rate and reproductive potential of insect pests, while warmer winters will increase over-winter survival (Old & Stone 2005). For example, the devastating mountain pine beetle (Dendroctonus ponderosae) infestation in the Canadian province of British Columbia, which currently affects 8.7 million ha of forest and is predicted to kill over 800 million cubic metres of pine by 2013, is attributed in part to recent mild winters, which contribute to the high survival of beetle populations over winter (Eng et al. 2006). In southern Australia, increased frequency of extreme wet and dry periods may increase incidence of the root rot pathogen Phytophthora cinnamomi. Trees weakened by P. cinnamoni have a reduced capacity to survive periods of drought.

Secondly, climate change may extend the geographic distribution of pests and pathogens, affecting forest communities not previously at risk (Cannon 1998).

Thirdly, effects of climate change on the host plant may increase its susceptibility to insect pests and diseases, or its ability to tolerate and recover from herbivory. For example, elevated CO2 concentration affects the nutritional quality of foliage, largely due to a decline in leaf N concentration (Ainsworth & Long 2005). The resultant change in C:N ratio may result in increased foliage consumption by some species tolerant of low N availability, while others will be inhibited (Old & Stone 2005). It is, therefore, difficult to predict the impact of climate change on defoliation.

In their review of the likely impacts of climate change on pests and pathogens of Australian forests, Old & Stone (2005) concluded that the diversity of Australia’s native forests gives them a strong resistance to pests and pathogens, but that climatic variability due to climate change – particularly an increase in drought frequency – could increase the impact of pest and pathogen attack, thereby compromising the health, and hence the carbon stocks, of Australia’s forests.

The interaction between climate change and the impact of insect pests or fungal pathogens is strongly mediated by the condition of the host tree. Potential increases in crown growth rates, due to increased carbon dioxide or length of growing season, may offset the impact of defoliation; however, slow-growing, stressed trees are less able to recover from defoliation events, and are more vulnerable to secondary damaging agents, such as stem borers. The potential impact of climate change on plantations depends, therefore, on the direct effects of climate change on the population dynamics of the damaging agents, as well as the resultant condition and vigour of the trees. Overall, an increase in extreme weather events is likely to exacerbate the impact of insect pests and fungal pathogens on plantations.