Impacts of an extreme cyclone event on landscape-scale savanna fire, productivity and greenhouse gas emissions

Abstract

North Australian tropical savanna accounts for 12% of the world's total savanna land cover. Accordingly, understanding processes that govern carbon, water and energy exchange within this biome is critical to global carbon and water budgeting. Climate and disturbances drive ecosystem carbon dynamics. Savanna ecosystems of the coastal and sub-coastal of north Australia experience a unique combination of climatic extremes and are in a state of near constant disturbance from fire events (1 in 3 years), storms resulting in windthrow (1 in 5-10 years) and mega-cyclones (1 in 500-1000 years). Critically, these disturbances occur over large areas creating a spatial and temporal mosaic of carbon sources and sinks. We quantify the impact on gross primary productivity (GPP) and fire occurrence from a tropical mega-cyclone, tropical Cyclone Monica (TC Monica), which affected 10 400 km 2 of savanna across north Australia, resulting in the mortality and severe structural damage to ∼140 million trees. We estimate a net carbon equivalent emission of 43 Tg of CO 2 -e using the moderate resolution imaging spectroradiometer (MODIS) GPP (MOD17A2) to quantify spatial and temporal patterns pre-and post-TC Monica. GPP was suppressed for four years after the event, equivalent to a loss of GPP of 0.5 Tg C over this period. On-ground fuel loads were estimated to potentially release 51.2 Mt CO 2 -e, equivalent to ∼10% of Australia's accountable greenhouse gas emissions. We present a simple carbon balance to examine the relative importance of frequency versus impact for a number of key disturbance processes such as fire, termite consumption and intense but infrequent mega-cyclones. Our estimates suggested that fire and termite consumption had a larger impact on Net Biome Productivity than infrequent mega-cyclones. We demonstrate the importance of understanding how climate variability and disturbance impacts savanna dynamics in the context of the increasing interest in using savanna landscapes for enhanced carbon sinks in emission offset schemes.

The contribution of trees and grasses to productivity of an Australian tropical savanna

“…The Howard Springs site is described in detail by Hutley et al (2013), so only a summary is provided here. Longterm (1941Longterm ( -2014 mean annual rainfall for the Darwin Airport is 1732 (±44 SE) mm, (Australian Bureau of Meteorology (BoM), station ID: 014015, www.bom.gov.au), which is approximately 20 km from Howard Springs.…”

“…While fire is the most recurrent disturbance in these savannas, wind storms and cyclones common to this region also cause damage on longer timescales, altering tree-grass productivity (Staben and Evans, 2008;Hutley et al, 2013). Disturbance also arises from biomass grazing of feral buffalo and termites (Werner and Prior, 2007;Jamali et al, 2011), which feeds back into the productivity balance of Australian savannas.…”

Reviews and syntheses: Australian vegetation phenology: new insights from satellite remote sensing and digital repeat photography

“…Fire can affect phenology in the short term through its impact on canopy area (Cernusak et al, 2006;Beringer et al, 2007) and over longer timescales through feedbacks to plant demography Werner and Franklin, 2010;Werner and Prior, 2013). In addition to fire, cyclone activity can disturb Australia's savannas, with wind throw from severe tropical storms resulting in patches of defoliation (roughly every 5 years) and extreme cyclones causing up to complete destruction (once every 500-1000 years; Hutley et al, 2013). In Australian savannas, pronounced spatiotemporal variability exists in this phenology, which still requires further examination in fine detail so it can be more accurately understood (Ma et al, 2013).…”

Fire-Driven Decline of Endemic Allosyncarpia Monsoon Rainforests in Northern Australia

“…Immediate effects of severe storms on forest vegetation, exemplified by severe Cyclone Monica in 2006, include substantial tree death, wind-throw and defoliation, whereas longerterm effects include invasion of flammable grasses and increased fire susceptibility [84,85]. Although most cyclonic-scale wind events occur within 50-100 km of the coast, effects of Cyclone Monica were evident up to 130 km inland.…”