Decreases in standing tree-based carbon stocks associated with repeated prescribed fires in a temperate mixed-species eucalypt forest

Prescribed fire is a common management practice in fire-tolerant forests, and one that has potential carbon costs. Previous assessments of the carbon costs of prescribed fire regimes in temperate Australia have been based on little empirical data, and have focused on direct fire effects (area burnt, fuel consumed) but have largely ignored potentially substantive indirect effects on tree mortality and growth. This study measures effects of four prescribed fire treatments on standing tree-based carbon stocks, and on individual tree growth and mortality, in a fire-tolerant eucalypt forest of south-eastern Australia. Prescribed fire treatments were as a factorial combination of two seasons (autumn or spring) and two frequencies (3-yearly ‘High’, or 10-yearly ‘Low’), were replicated over five study areas, and involved 2–7 low-intensity fires over 27 years. Total standing tree-based carbon stocks (live and dead) were significantly less in prescribed fire than control treatments. However, the mean carbon difference (25 Mg ha −1 ) had a wide 95% confidence interval (2–48 Mg ha −1 ), indicating a high degree of uncertainty about the magnitude of prescribed fire effects in these native forests. Overall decreases were consistent with detection of both direct and indirect effects of prescribed fire treatments. Direct combustion effects on bark were minimal (c. 0.2–0.4 Mg ha −1 ), but were also indicated by significantly less carbon in dead large stems in fire than control treatments despite evidence of marginally increased mortality of individual large stems in the former. Indirect effects of repeated prescribed fires were also detected as significantly decreased mean annual diameter increment of individual large Eucalyptus obliqua over 27 years (particularly of stems 20–50 cm diameter). With respect to prescribed fire type, small live stem densities and associated carbon stocks were greater in autumn than spring, and in Low than High frequency treatments, and carbon stocks in large dead stems were greater in High than Low frequency treatments. This suggested that c. 10-yearly fires in autumn provided the most scope for maintaining future capacity to fix carbon. Nonetheless, decreases in total standing tree-based carbon stocks were not significantly different among prescribed fire treatments, suggesting tree-based carbon stocks were more influenced by prescribed fire per se than by fire season or frequency.