Abstract
BackgroundLand 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.ResultsUsing 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.ConclusionsIn 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.
Highlights
Land clearing generates coarse woody debris (CWD), much of which becomes atmospheric CO2
We evaluate the assumptions of the national greenhouse gas inventory (NGGI) in regard to land clearing, by using a previously unpublished archive of semi-quantitative field observations of CWD disappearance
Many studies have previously found that temperature and tree species are important factors that determine the variability in CWD disappearance [17, 25, 28], so it is not surprising that we found bioregion—an explanatory variable that effectively describes a climate-by-tree species interaction—to be important
Summary
Land clearing generates coarse woody debris (CWD), much of which 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. The ultimate product of much CWD disappearance is CO2 emission into the atmosphere, which makes the study of CWD interesting from the perspective of climate change, and greenhouse gas inventory. Mackensen et al [17] found that temperature, rather than moisture, is the key influence on the rate of CWD disappearance, a finding that has been echoed by studies since [25,26,27]. CWD disappearance is influenced by tree species [28, 29], wood diameter [22], fire [19, 30], and soil factors such as clay type [31]
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