Abstract
Small areas of the wetter parts of southeast Australia including Tasmania support high-biomass “wet” eucalypt forests, including “mixed” forests consisting of mature eucalypts up to 100 m high with a rainforest understorey. In Tasmania, mixed forests transition to lower biomass rainforests over time. In the scientific and public debate on ways to mitigate climate change, these forests have received attention for their ability to store large amounts of carbon (C), but the contribution of soil C stocks to the total C in these two ecosystems has not been systematically researched, and consequently, the potential of wet eucalypt forests to serve as long-term C sinks is uncertain. This study compared soil C stocks to 1 m depth at paired sites under rainforest and mixed forests and found that there was no detectable difference of mean total soil C between the two forest types, and on average, both contained about 200 Mg·ha−1of C. Some C in subsoil under rainforests is 3000 years old and retains a chemical signature of pyrogenic C, detectable in NMR spectra, indicating that soil C stocks are buffered against the effects of forest succession. The mean loss of C in biomass as mixed forests transition to rainforests is estimated to be about 260 Mg·ha−1over a c. 400-year period, so the mature mixed forest ecosystem emits about 0.65 Mg·ha−1·yr−1of C during its transition to rainforest. For this reason and because of the risk of forest fires, setting aside large areas of wet eucalypt forests as reserves in order to increase landscape C storage is not a sound strategy for long-term climate change mitigation. Maintaining a mosaic of managed native forests, including regenerating eucalypts, mixed forests, rainforests, and reserves, is likely to be the best strategy for maintaining landscape C stocks.
Highlights
Natural forest ecosystems contain about 1500 Pg of carbon (C) [1], and on a worldwide basis, forest soils contain about twice the C of the vegetation they support [2]
40–100 m high with a rainforest understorey; rainforest lacks standing eucalypts and the canopy is dominated by species such as Nothofagus cunninghamii, Atherosperma moschatum, and Phyllocladus aspleniifolius.) These forest pairs provided a framework for a soil C comparison
203 Mg·ha−1 in soils under rainforest and 199 Mg·ha−1 in soils under mixed forest. The latter figure is close to the mean value measured by Cotching ([50], p. 86) to 1 m depth for six soil orders under unspecified native forest in Tasmania
Summary
Natural forest ecosystems contain about 1500 Pg of carbon (C) [1], and on a worldwide basis, forest soils contain about twice the C of the vegetation they support [2]. The natural forests of Australia’s wet southeast include the tallest hardwoods in the world (up to 100 m high) (Figure 1), and according to some authors (a) (b). (e.g., Keith et al [6]), the aboveground C content of these ecosystems is over 2500 Mg·ha−1, which is over 50 times the mean Australian forest aboveground value (45 Mg·ha−1) and greatly exceeds mean values for other midlatitude forests (range 32–114 Mg·ha−1; [2]), the IPCC figure (96 Mg·ha−1; [8]) for biomass C in world temperate forests, and the mean aboveground C for mountain ash (Eucalyptus regnans) forests in Victoria (246 Mg·ha−1; [7]). 706 Mg·ha−1, but probably lower (438 Mg·ha−1) due to the loss of mass in decayed hollow trunks and limbs, and questioned the assertion that the tallest southeast Australian eucalypt forests are the most C-dense forests in the world, quoting measurements of North American redwood (Sequoia sempervirens) forests which contain more than
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