Using a demographic model to project the long‐term effects of fire management on tree biomass in Australian savannas

Abstract

Tropical savannas are characterized by high primary productivity and high fire frequency, such that much of the carbon captured by vegetation is rapidly returned to the atmosphere. Hence, there have been suggestions that management‐driven reductions in savanna fire frequency and/or severity could significantly reduce greenhouse gas emissions and sequester carbon in tree biomass. However, a key knowledge gap is the extent to which savanna tree biomass will respond to modest shifts in fire regimes due to plausible, large‐scale management interventions. Here, we: (1) characterize relationships between the frequency and severity of fires and key demographic rates of savanna trees, based on long‐term observations in vegetation monitoring plots across northern Australia; (2) use these relationships to develop a process‐explicit demographic model describing the effects of fire on savanna tree populations; and (3) use the demographic model to address the question: to what extent is it feasible, through the strategic application of prescribed burning, to increase tree biomass in Australian tropical savannas? Our long‐term tree monitoring dataset included observations of 12,344 tagged trees in 236 plots, monitored for between 3 and 24 years. Analysis of this dataset showed that frequent high‐severity fires significantly reduced savanna tree recruitment, survival, and growth. Our demographic model suggested that: (1) despite the negative effects of frequent high‐severity fires on demographic rates, savanna tree biomass appears to be suppressed by only a relatively small amount by contemporary fire regimes, characterized by a mix of low‐ to high‐severity fires; and (2) plausible, management‐driven reductions in the frequency of high‐severity fires are likely to lead to increases in tree biomass of about 11.0 t DM ha−1 (95% CI: −1.2–20.8) over a century. Accounting for this increase in carbon storage could generate significant carbon credits, worth, on average, three times those generated annually by current greenhouse gas (methane and nitrous oxide) abatement projects, and has the potential to significantly increase the economic viability of fire/carbon projects, thereby promoting ecologically sustainable management of tropical savannas in Australia and elsewhere. This growing industry has the potential to bring much‐needed economic activity to savanna landscapes, without compromising important natural and cultural values.

A national accounting framework for fire and carbon dynamics in Australian savannas

“…Consistent with the other recent studies (Levick et al 2019;Werner and Peacock 2019;Murphy et al 2023), our results indicate that sequestration of carbon in live biomass is a key driver of abatement following savanna fire management, noting that predictions are highly sensitive to the assumed upper limit of AGB, or the M input layer. S4-S8.…”

“…Lonsdale and Braithwaite 1991;Prior et al 2009;Cook et al 2016Cook et al , 2020. There is a paucity of data to indicate how fire-and non-fire-related mortality interact, but an assumption was made that fire-related mortality would be between ~50 and 75% of these estimates of total mortality (Cook et al 2020), but declining to a negligible contribution of total mortality under conditions of low fire intensities (Murphy et al 2023). Based on these estimates, when calibrating typical rates at which AGB declines (and subsequently regrows) owing to non-fire-related mortality, values of 2.24% year −1 (or 2.70% year −1 ) for categories of vegetation from high (or low) rainfall zones were assumed to be the upper limit, and were implemented within FullCAM as percentage transfers from the aboveand below-ground standing biomass pools to the standing dead pool.…”

“…This entails igniting cool and patchy fires in the early dry season (EDS) to decrease prevalence of large high intensity late dry season (LDS) wildfires. This management decreases greenhouse gas emissions (Russell-Smith et al 2013) and may also lead to increased storage of carbon in pools of dead (Cook et al 2016(Cook et al , 2020 and live (Ryan and Williams 2011;Murphy et al 2023) biomass.…”

Framework for a savanna burning emissions abatement methodology applicable to fire-prone miombo woodlands in southern Africa

“…Building on extensive Australian experience with the undertaking of Kyoto-compliant, commercial 'savanna burning' greenhouse gas (GHG) emissions abatement projects, particularly by reducing the extent and severity of late dry season (LDS) fires through prescribed early dry season (EDS) management (Russell-Smith et al 2013a;Edwards et al 2021), it has been suggested that a similar approach could provide tangible environmental and societal benefits in comparable savanna landscape settings elsewhere (ISFMI (International Savanna Fire Management Initiative) 2015; Lipsett-Moore et al 2018;Moura et al 2019;Russell-Smith et al 2013bTear et al 2021). A complementary approach accounting for fire regime effects on woody biomass sequestration in Australian savannas is under advanced development (Murphy et al 2023;Paul and Roxburgh 2024).…”

Fire, Rain and CO2: Potential Drivers of Tropical Savanna Vegetation Change, with Implications for Carbon Crediting

“…It has been estimated [39] that the unmeasured annual sequestration benefit from Australia's program could be 3.5 times the amount currently being credited. A recent study [40] used a demographic model derived from observed changes in field plots to predict increases in tree biomass, with an estimated crediting value three times that produced under the current method. Land-based crediting methodologies often use a baseline and credit approach [41].…”