Summary 1The relative roles of climate and management for driving changes in woody cover in savannas over the past century are the subject of active debate. Perspectives arising from short-term, small-scale, local experiments are rarely tested over larger scales and longer time frames. 2 Regression analysis and aerial photography were used to assess the relative importance of land-use history (fire and grazing), rainfall and initial woody cover (woody cover at the beginning of a sample period relative to the range of woody cover expressed within a land type) in accounting for rates of change in overstorey and understorey cover between the 1940s and 1990s in central Queensland, Australia. Analyses included 279 site-period combinations representing five semi-arid eucalypt savanna land-types within a 125 755 km 2 region. 3 Fire and grazing variables provided no explanatory power. In general, relative rainfall (rainfall for a given period standardized against mean annual rainfall) was positively related and initial woody cover negatively related to rates of change in both the overstorey and the understorey. The interaction between rainfall and initial woody cover was significant, reflecting the fact that increases in cover coincided with low initial cover when rainfall is higher than average, whereas decreases in cover typically occurred with high initial cover, regardless of rainfall. 4 On average, overstorey and understorey cover increased over the second half of the 20th century. This pattern is consistent with the first half of the 20th century having more intense droughts and being drier overall than the relatively wet second half. 5 The findings highlight the primary importance of interactions between rainfall fluctuations and density dependence as determinants of large-scale, long-term woody plant cover dynamics in savannas subject to large rainfall excess and deficit over multiyear time-scales.
Increasing densities of woody plants in savannas has been attributed to both elevated atmospheric CO 2 and reduced burning with grazing management, such that the biome could represent a substantial carbon sink. However, we show that extreme droughts (less than two-thirds expected rainfall over 3 years) occur in the drier half of the savanna biome and can cause substantial tree death. An Australian case study reveals that a net increase in tree cover over five decades of above-average rainfall was offset by sudden tree death during drought. The relationship between woody cover change and rainfall is moderated by competition with growth being facilitated by low woody cover and drought-induced death more likely as the woody component of savanna increases. The results are not supportive of a sustained increase in the woody component of xeric savannas resulting from CO 2 fertilization or land management. Extensive tree death in savanna regions will become a stark consequence of climate change if predictions of increasing severity and frequency of drought are realized.
Focusing on woody vegetation in Queensland, Australia, the study aimed to establish whether the relationship between Advanced Land Observing Satellite (ALOS) Phased Array L-band SAR (PALSAR) HH and HV backscattering coefficients and above ground biomass (AGB) was consistent within and between structural formations (forests, woodlands and open woodlands, including scrub). Across these formations, 2781 plot-based measurements (from 1139 sites) of tree diameters by species were collated, from which AGB was estimated using generic allometric equations. For Queensland, PALSAR fine beam dual (FBD) 50 m strip data for 2007 were provided through the Japanese Space Exploration Agency's (JAXA) Kyoto and Carbon (K&C) Initiative, with up to 3 acquisitions available for each Reference System for Planning (RSP) paths. When individual strips acquired over Queensland were combined, 'banding' was evident within the resulting mosaics, with this attributed to enhanced L-band backscatter following rainfall events in some areas. Reference to Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data indicated that strips with enhanced L-band backscatter corresponded to areas with increased effective vegetation water Manuscript
Seventy-three per cent of dry rainforest in Forty Mile Scrub National Park and large areas in adjacent savanna woodland have more than 5000 individuals per ha of lantana {Lantana camara L.). Transect studies in dry rainforest and savanna woodland across varying intensities of lantana infestation show a negative correlation between the density of lantana and tree cover in rainforest. The density of pig rooting is very high in areas of the dry rainforest on deep soil that was not heavily infested with lantana. It is suggested that the digging activities of these animals may cause tree death and subsequent increased light penetration, which favours lantana. The species richness ofthe dry rainforest declines as the density of lantana increases. However, the saplings and seedlings and the soil seed bank of dry rainforest and savanna woodland tree species have comparable densities in heavy and light lantana infestations. The proliferation of lantana results in the build up of heavy fuel loads across the boundary of dry rainforest and savanna woodland. Recent fires have killed the canopy trees in a large area of dry rainforest within the Park. Active management of Forty Mile Scrub National Park is urgent and some initiatives are suggested.
Summary The effects of a fire on plant community structure were examined in a remnant Gidgee (Acacia cambagei) and Brigalow (Acacia harpophylla) woodland infested with the naturalized exotic, Buffel Grass (Cenchrus ciliaris) in central Queensland. Fifteen months after the fire, burnt areas had about half the basal area of living trees and more than twice the cover of Buffel Grass and Parthenium Weed (Parthenium hysterophorus) as unburnt areas. This is consistent with the idea that Buffel Grass invasion, which increases ground fuel loads in Acacia woodlands, is facilitated by burning, producing positive feedback between Buffel Grass and fire. The result is accelerating remnant degradation, making the interaction between Buffel Grass and fire an important target for management measures. Fire‐breaks and fuel reduction strategies including periodic intense grazing and canopy enhancement have potential to reduce the impact of Buffel Grass invasion, but long‐term community survival will probably require effective control of Buffel Grass. The impact of weed invasion within remnant vegetation is clearly a complex issue but the simple ‘case study’ approach employed here can both increase and communicate understanding.
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