Forest succession where trees become smaller and wood carbon stocks reduce

Abstract

As Tasmania’s wet forests transition from mixed forest (eucalypt overstory with a rainforest understory) to rainforest they can be expected to lose more than half their total (live+dead, standing+downed) bole wood volume and biomass. On average rainforest sites contained 205MgCha−1, or 708m3ha−1 less wood (live+dead, standing+downed) than mixed forests. This occurs as smaller dimension rainforest trees replace the larger eucalypts. On the fertile study sites, the largest rainforest trees were 20m shorter and 113cm smaller in breast height diameter than the mature eucalypts in mixed forests. Tasmanian wet forests do not attain the highest site level C-stocks possible in late succession, as is expected for many other forests. Rather, the maxima is attained for a short period several centuries after disturbance and regeneration of a eucalypt cohort. Hence, the maintenance of the highest C-density stands in the landscape, and potentially that largest landscape level C-stocks among Tasmania’s wet forests ironically requires the periodic killing of all, or part, of the large C-dense eucalypt overstory by intense wildfire to allow these large eucalypts to regenerate and persist on these sites. This creates challenges when modeling forest carbon in Tasmanian wet forests, as not only is the area of forest that has progressed sufficiently toward rainforest to be emitting C is unknown, but the rate of C emission and the amount that will be lost over time across the various site fertility types is also unknown. Certainly, setting aside Tasmanian wet eucalypt forest to store C will not deliver the usual long term C accumulation benefits common to forests elsewhere. Maximizing landscape level C-stocks is likely to require periodic disturbance to maintain the C-dense eucalypts in the landscape.