Trends for climate change impacts in Australia have been reviewed (e.g. Mansergh and Cheal, 2007; Williams et al., 2009) and general conclusions are: 1) increased temperature may increase decomposition rates and hence C efflux; 2) more frequent fires; 3) changing vegetation will affect SOC; 4) ultimate control over SOC stocks may be through fire and land-use change management; 5) some trees will be stressed
55 due to temperature increases and sporadic water deficit whereas others may benefit from reduced frosts; 6) disease, insect attacks, weeds, higher storm intensity, increasingly episodic rainfall and increased fire frequency may deplete stressed forests, lowering their productivity; 7) there will be increased pressure on survival of some species and ecological communities; and 8) more CO2 may accelerate plant
growth or decrease growth by disrupting ecophysiology but the change in temperature and rainfall patterns is expected to dominate (hence the focus when modelling ΔSOC).
The analysis of ΔSOC with climate change presented here generally indicated efflux, with the highest percentage change being for SWA (which follows the already- evident drop in rainfall), followed by that for the Tasmania rainforest, and the lowest change being for Tasmanian TOF. However, there was high variability between climate models for the latter forests. Climate-change-induced drought has already caused decline of trees in central Tasmania (Calder and Kirkpatrick, 2008),
indicating a net C efflux. The highest magnitude of emission from SOC will be from rainforests, this being similar to the long-term emissions from primary-forest logging planned for public forests in Tasmania.
For karri TOFs the higher fire intensities and frequencies with climate change, suggests a shift from karri to karri-marri and from karri-marri to jarrah-marri forest, i.e. decreasing biomass. Higher fire frequency and intensity in Tasmania shifts ecosystem types from rainforest to mixed-forest to wet-sclerophyll to dry-sclerophyll (Gilbert, 1959; Cremer, 1960), which incurs an average decrease in C stocks (Table 2-1). Other types of ecosystem change can occur only with more severe change in fire regime, e.g. woodland-to-open savannah, and increased grass/tree ratio (e.g. Cloudsley-Thompson, 1975), thereby inducing higher C emissions. Where more- frequent fire changes ecosystems to those with vegetation that survives fire, then the new vegetation is usually more flammable and conducive to fire propagation
56 Carbon emissions from non-stand-replacing fires of moderate intensity can be in the order of 34% of biomass C, and post-fire sequestration is limited more by tree
mortality than by survivor productivity (Irvine et al., 2007), and significant emissions come from decaying roots after tree mortality. Emissions from more-intense fires are up to three times as high as those from moderate fires (Irvine et al., 2007).
Frequent fires can reduce site-quality, and consequently long-term C storage, and may increase erosion, providing positive feedback to climate change (e.g. Gough et al., 2007a; Williams et al., 2009). Soil nutrient losses are higher with more intense or higher frequency fire, and feedback leads to a more fire-prone ecosystem with reduced soil nutrients (McIntosh et al., 2005). Over several millennia the process can dramatically alter the soil type, for example by reducing clay content by eluviation (McIntosh et al., 2005). Examples of other positive feedbacks to climate change are: reduced forest biomass and reduction in locally-induced rainfall (Li et al., 2000); a higher proportion of more labile SOC (Gill, 2007); and increased drought severity or frequency with climate change, causing emissions from biomass (Warszawski et al., 2013).
The forecast reduction in SOC is thus due to drought stress and higher fire frequency affecting vegetation type, and thus in turn, the soil. Earlier increased growth due to higher atmospheric CO2 concentration (e.g. Berry and Roderick, 2006)
(corresponding to net sequestration) would be counteracted by the drop in SOC. Climate modelling, more detailed than the present work, has confirmed the forecast an increased fire danger index forecast for Tasmania this century, especially for the southern region which contains the most carbon-dense TOFs and largest trees (Fox- Hughes et al., 2014). Prescribed burning has been proposed to reduce readily combustible material (so-called ‘fuel’3) in forests (and thereby increase climate change resilience) but modelling and evidence suggests that it would not reduce
3 The terminology which comes from firefighting refers mainly to dead, dry aboveground biomass but
can be subjective, and has been adopted in applied science and industry. From a physical science perspective anything combustible under appropriate conditions is fuel, e.g. all living and dead biomass.
57 carbon emissions (Bradstock et al., 2012) and that ‘fuel’ and fire management close to human infrastructure would be more fruitful (Enright and Fontaine, 2013).
Limiting forest fragmentation and general anthropogenic disturbance is likely to help maintain existing mesic micro-environments that protect ‘fuel’ against drying-out and thus fire, in mixed-forests. The importance of primary forests in climate change policy (including those of TOF) has been re-iterated, with policy initiatives broached for their conservation (Mackey et al., 2015).
In line with the longer half-life of SOC than biomass C it must be made clear that, although the declining SOC stock was modelled by correlation with climate changes occurring by 2100, SOC was only a proxy for biomass C (section 7.5), with the decline being based on response to a changing climate, and that change in biomass C will occur long before corresponding ∆SOC, at a timescale an order of magnitude different (100 years compared with 1,000 years). The direct effect of warmer temperature on SOC itself is a different matter however, with possibly a more spontaneous reaction.
Exotic plant species can out-compete native species, accrue high fuel loads, change fire frequency, increase the area burnt, increase C emissions and reduce aboveground biomass (e.g. Litton et al., 2006). Venevsky et al. (2002) showed a logarithmic increase in fire frequency with human population density. The more sparsely- populated regions of Australian are presently at the most responsive part of that curve. Under climate change, the effects of exotic plants and fire-induced change, may synergistically impact on biodiversity and C stock.
In summary, climate change is likely to cause net emission by converting forest types and through positive feedback. Human population increase will also inflict attrition on forests. The function of primary-forest forests under climate change will thus be influenced by people’s impacts upon them today.
58