A Preliminary Assessment of the Environmental Impacts Of
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A preliminary assessment of the environmental impacts of Cyclone Larry on the forest landscapes of northeast Queensland, with reference to responses to natural resource management issues in the aftermath Dr Stephen M. Turton1 and Dr Allan Dale2 1 Professor and Director JCU/CSIRO Tropical Landscapes Joint Venture, Australian Tropical Forest Institute, James Cook University, PO Box 6811, Cairns 4870, Australia 2 Chief Executive Officer Far North Queensland Natural Resource Management Ltd 45 Rankin Street, Innisfail PO Box 1756, Innisfail QLD 4860, Australia Report submitted to the Bureau of Meteorology (March 2007) Introduction Disturbances, both natural and anthropogenic, shape forest ecosystems by controlling their species composition, structure and functional processes. The tropical forests of the Wet Tropics of northeast Australia have been moulded by their natural disturbance and land-use history and over many millennia but particularly over the past 100 years. Forests of the region are subjected to a range of natural disturbances, including bush fires, droughts, floods, occasional landslides and tropical cyclones. All these natural disturbances interact in complex ways with anthropogenic disturbances across the landscape, such as land-use change resulting from forest conversion to agricultural systems, indigenous fire management regimes, earlier logging practices, urban and peri-urban development and infrastructure projects, such as major roads, powerline corridors and dams. Tropical cyclones are significant natural disturbance phenomena for forest ecosystems in the Wet Tropics of northeast Australia, especially for forests near the coast. For this reason, even continuous forests in the region have been described as hyper-disturbed ecosystems with patches of damaged forest constantly recovering from previous cyclonic events, often in concert with floods, droughts and fires. Cyclones are part of the ecosystem dynamics of these forested landscapes and recovery of canopy cover following such events is often remarkably rapid, although forest structure and composition may take many decades to recover. The same cannot be said for fragmented forests in the Wet Tropics, located within either an agricultural, urban, linear infrastructure or grassland matrix. These forest fragments are particularly vulnerable to impacts of tropical cyclones and their associated strong winds, largely due to their high forest edge to area ratios. Severe cyclones cause widespread defoliation of rainforest canopy trees, removal of vines and epiphytes, along with the breakage of crown stems and associated tree falls. These catastrophic impacts typically result in significant changes in forest microclimates in the understorey and canopy, and complex vegetation and faunal responses to newly created light, temperature and humidity regimes. Cyclonic disturbance has also been shown to accelerate invasion by exotic tree, vine and grass species leading to a decline in biodiversity of native plant species in some forest regions. Given the relatively high frequency of tropical cyclones, there is a general consensus that they contribute to the structure and function of tropical rainforests in cyclone-prone areas, with ecosystem impacts and recovery processes occurring at several spatial and temporal scales. Impacts of tropical cyclones on forests at the landscape-scale (>10km) are the result of the complex interaction of anthropogenic, meteorological, topographical and biotic factors. Three main factors control forest damage at this scale: 1) wind velocity gradients resulting from cyclone size, speed of forward movement, cyclone intensity and proximity to the storm track, complicated by local convective-scale effects; 2) variations in site exposure and other effects of local topography (e.g. severe lee waves or leeward acceleration, windward exposure, topographic shading); and 3) differential response of individual ecosystems to wind disturbance as a function of species composition and forest structure. Severe Tropical Cyclone Larry, with maximum wind gusts were near 280 km h 1, crossed the coast near Innisfail (Fig. 1) as a borderline Category 4/5 storm on 20 March 2006 causing extensive damage to human communities, primary industries, infrastructure and ecosystems across a 100 km strip of coastal lowlands and adjacent uplands. The system was moving at 25 km h-1 when it crossed the coast and was still Category 3 status when it crossed over the Atherton Tablelands some 60 km from the coast (Fig. 1). Cyclone Larry produced numerous tornado-type features within the system’s eyewall and feeder bands that have been linked to patches of catastrophic forest damage. Larry was a very ‘compact’ system with its radius of maximum winds extending only 20-30 km from the centre (BOM, this report). 2 We examine the landscape-scale impacts of Cyclone “Larry” on the ecosystems of far north Queensland and provide a comparison with previous cyclones that have impacted along the wet tropical coast of Queensland over the period 1858-2006. We also discuss responses to the natural resource management issues facing the affected region in the aftermath. Figure 1: The track of Cyclone Larry with respect to continuous and fragmented forest areas (Map prepared by Caroline Bruce, CSIRO, Atherton). 3 Aerial surveys of forest damage following Cyclone Larry Two weeks after Cyclone Larry helicopter aerial surveys were conducted to examine landscape-scale impacts from what has been described as a severe but ‘compact’ cyclonic system (BOM, this report). Aerial surveys covered the main impact area, from Cairns to Tully and as far west as Herberton and Ravenshoe (Fig. 1). Cloud cover prevented surveys of higher elevation forests above 1000 m as, but such forests constitute less than 5 per cent of the study area (Fig. 1). Adopting a rapid assessment methodology, developed by Unwin et al1 following Cyclone Winifred, four classes of initial forest damage were identified from the aerial surveys: 1) severe and extensive, 2) severe and localised, 3) moderate canopy disturbance and 4) slight canopy disturbance (see Table 2 for classes). GPS readings were also taken to allow for identification of damaged areas of forest for subsequent mapping purposes. Cyclone impacts on the Wet Tropics region Table 1 provides a summary of notable severe cyclones (Categories 3-5) that have impacted on the Wet Tropics region from 1858 - 20062. Cyclone Larry was the second strongest cyclone to impact along the wet tropical coast since records began in 1858, with only the un-named 1918 cyclone, that crossed in a similar location, considered to be more powerful. It would therefore be reasonable to consider Cyclone Larry to be about a 1 in 75 year event for the Wet Tropics region. Table 1: Notable east to west moving tropical cyclones by code-name that have impacted along the wet tropical coast (Cooktown-Ingham) over the period 1858-2006 (after Callaghan, BOM). Estimated lowest Code Name Date Area most impacted barometric Pressure (hPa) Un-named 8 March 1878 Cairns not available Un-named 16 March 1911 Port Douglas not available Un-named 10 March 1918 Innisfail region 910 Un-named 3 February 1920 Port Douglas 962 Un-named 9 February 1927 Cairns-Port Douglas 971 Un-named 12 March 1934 Cape Tribulation 978 Un-named 18 February 1940 Cardwell 965 Agnes 6 March 1956 Cairns-Townsville 961 Winifred 1 February 1986 Cairns-Tully 958 Rona 11 February 1999 Daintree-Cape Tribulation 970 Larry 20 March 2006 Innisfail region 915* * Subject to verification 1 Unwin, G. L., Applegate G. B., Stocker G. C. & Nicholson D. I. (1988): Initial effects of tropical cyclone Winifred on forests in north Queensland. Proceedings of Ecological Society of Australia 15, 283-296. 2 Callaghan J. (2004): Tropical cyclone impacts along the Australian east coast from November to April (1858 to 2004). Bureau of Meteorology, Brisbane Regional Office, australiasevereweather.com/cyclones/impacts-eastcoast.pdf 4 Landscape scale impacts of Cyclone Larry on forests across the Wet Tropics Table 2 summarises the results of the aerial surveys of the area impacted by Cyclone Larry according to the four damage classes described above. Figs. 2-5 provide photographic examples of the four classes of forest damage described in Table 2. Figs. 2 and 3 provide examples of windward and leeward forest damage (both Class 2), respectively. Table 2: Initial effects of Severe Cyclone Larry on forests in the Wet Tropics of northeast Queensland derived from aerial surveys. Forest damage classes adopted from Unwin et al. (1988: Initial effects of tropical cyclone Winifred on forests in north Queensland. Proceedings of Ecological Society of Australia 15, 283-296). Forest Damage Definition Occurrence in Occurrence in Classes (areal (by essential rainforests open forests extent of damage) characteristics) 1.Severe and extensive Boles or crowns of most Extensive among coastal Rarely evidenced (mostly within 30 km trees broken, smashed or forests, riparian forests, from centre of cyclone) wind-thrown. Impact forest remnants and multi-directional. foothills in area along track of the cyclone’s core, including foothills as far west as Millaa Millaa. 2. Severe and localised Severe canopy disruption Defoliation, tree falls and As above, (transitional (mostly 30-50 km from (as above) restricted to stem breakage common understoreys, defoliated centre of cyclone) windward aspects. on windward aspects as for rainforests) Direction of destructive only. Leeward forest winds clearly identifiable. areas