Modelling the disappearance of coarse woody debris, following a land clearing event

Background Land clearing generates coarse woody debris (CWD), much of which ultimately becomes atmospheric CO2. Schemes for greenhouse gas accounting must consider the contribution from land clearing, but the timing of the contribution will have large uncertainty, due to a paucity of knowledge about the rate of CWD disappearance. To better understand above-ground CWD disappearance following a land clearing event-through the actions of microorganisms, invertebrates, wildfire, or deliberate burning-we combined statistical modelling with an archive of semi-quantitative observations (units of CWD %), made within Queensland, Australia. Results Using a generalised additive mixed-effects model (median absolute error = 14.7%), we found that CWD disappearance was strongly influenced by the: (i) number of years elapsed since clearing; (ii) clearing method; (iii) bioregion (effectively a climate-by-tree species interaction); and (iv) the number of times burned. Years-since-clearing had a strongly non-linear effect on the rate of CWD disappearance. The data suggested that disappearance was reverse-sigmoidal, with little change in CWD apparent for the first three years after clearing. In typical conditions for Queensland, the model predicted that it will take 38 years for 95% of CWD to disappear, following a land clearing event; however, accounting for uncertainty in the data and model, this value could be as few as 5 years, or > 100 years. In contrast, due to an assumption about the propensity of land managers to burn CWD, the official method used to assess Australia's greenhouse gas emissions predicted that 95% of CWD will disappear in < 1 year. Conclusions In Queensland, the CWD generated by land clearing typically takes 38 years to disappear. This ultimately implies that a key assumption of Australia's official greenhouse gas reporting-i.e. that 98% of CWD is burned soon after a clearing event-does not adequately account for delayed CO2 emissions.

Automated summary

The study found that various factors, such as years since clearing, clearing method, bioregion, and number of times burned, strongly influence the disappearance of CWD following a land clearing event in Queensland. In typical conditions, it was predicted to take 38 years for 95% of CWD to disappear, but uncertainty in the data and model could lead to a range of 5 to >100 years for CWD disappearance. Contrary to official greenhouse gas reporting in Australia, it was found that a significant amount of CWD may not be burned soon after clearing, leading to delayed CO2 emissions.