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by W Vogler, S Navie, S Adkins and C Setter
November 2006
RIRDC Publication No 06/130 RIRDC Project No QDN-7A
Executive Summary
What the report is about
This publication examines
the effect of burning on parthenium and other species in native pastures
in Central Queensland. It also examines the effect of smoke produced from
burning native grasses on the germination of parthenium.
Background
Parthenium weed (Parthenium
hysterophorus L.; Asteraceae) is a major pest plant in Queensland.
It causes serious economic, health and environmental problems in rural
areas of Queensland.
Parthenium has the potential to spread further in Queensland and throughout much of Australia, with small infestations currently present in New South Wales, Victoria and the Northern Territory.
Fire is an important part of the ecology of the Australian landscape and is commonly used by land managers in northern Australia for pasture management and woody weed control. Currently, there are no data on the effects of fire on parthenium with most advice based on anecdotal evidence.
Present management recommendations generally discourage burning of parthenium infested areas.
An experiment aimed at improving the understanding of the impact of fire on parthenium was conducted from July 1999 until June 2002 at Albinia National Park near Rolleston and at ‘Morbridge’ near Clermont in central Queensland.
Objectives
An improved understanding
of the impact of fire on parthenium in native pastures is needed to develop
best management practices for parthenium.
The main objectives were to:
Methods and Results
Fire
A field experiment was conducted
at Albinia National Park to study the effects of fire on parthenium plant
numbers and soil seed banks. Fire, mow (vegetation removal) and control
treatments were applied in spring and autumn. The mow treatment was used
to isolate the effects of vegetation removal from those of heat and smoke
of the fire. Soil seed banks were measured pre and post treatment application,
with parthenium and other pasture species frequency assessed annually in
autumn. A similar, less intensive study was conducted at ‘Morbridge’, a
commercial grazing property near Clermont, to test the results on a commercial
scale.
Fire did not directly affect the size of the germinable parthenium soil seed banks. The greatest reduction in the size of the germinable parthenium seed bank occurred when above ground plant material was removed during spring, either through burning or mowing which promoted parthenium germination. A similar trend occurred with the autumn burn and mow treatments, although the reduction was generally slightly less than that which occurred for spring treatments.
There was an increase in parthenium frequency after the initial spring and autumn burns, with a subsequent large decline in future years even though follow-up burns were conducted. In the ungrazed control treatments parthenium frequency declined in the first year and remained at a low level until reaching zero in 2002. In general parthenium frequency declined in all treatments with some one-off increases stimulated by the removal of pasture cover as a result of mowing or fire.
The mean number of flowering parthenium plants per plot in each treatment followed a similar pattern to the parthenium frequency. Significant numbers of flowering parthenium plants only occurred following the first application of fire or mowing which removed the pasture cover.
Subsequent removal of pasture cover by fire or mowing did not result in significant numbers of flowering parthenium plants.
In all treatments the germinable seed bank of herbaceous plants showed no general change but increased and decreased throughout the study with time of sampling. Almost all of these species are annuals/ephemerals appearing when conditions are suitable and being absent when conditions are unsuitable with subsequent fluctuations in seed production and germinable soil seed banks. The legume and sedge germinable soil seed banks did not change substantially in any treatment.
Generally the desirable grass germinable seed bank remained relatively stable during the study with some increases and decreases according to seasonal seed production and germination events. This trend was evident in all treatments. In contrast the germinable soil seed bank of all undesirable grass species in all treatments declined to zero or almost zero during the study.
The pasture was dominated by Dichanthium sericeum, Aristida leptopoda, Dichanthium queenslandicum and Bothriochloa erianthoides. Other significant species were Eriochloa crebra, Panicum queenslandicum, Heteropogon contortus and legumes with the remaining plant species making minor contributions to the pasture sward. Overall there appears to be little change in the species biodiversity of the pastures regardless of the treatment applied although some change in the proportion of each species in the pasture occurred. B. erianthoides, D. queenslandicum and D. sericeum increased in frequency in all treatments with significant increases occurring in burn and mow treatments regardless of season. In contrast the frequency of A. leptopoda, an undesirable grass for grazing, decreased significantly in burn and mow treatments regardless of season.
The results obtained from ‘Morbridge’ were similar to those obtained at Albinia National Park.
Parthenium frequency declined to zero at ‘Morbridge’ over the study period. The pasture at ‘Morbridge’ was dominated by D. sericeum, Aristida spp., Digitaria brownii and Enneapogon virens. Other significant species were D. queenslandicum, Panicum decompositum, P. queenslandicum and Glycine latifolia.
There was no significant change in the frequency of D. sericeum, Aristida spp., P. decompositum, Digitaria brownii and E. virens in any treatment during the study. There was a general increase in the frequency of D. queenslandicum in the burn treatments while Glycine latifolia decreased in all treatments particularly those where fire was present.
The common feature of this study at both sites is the absence of grazing or only light grazing. This allows pasture plants to exert maximum competition on weeds in contrast to heavily grazed pastures, which are less competitive and vulnerable to invasion by weeds such as parthenium. Even though mowing or burning removed vegetative matter, in a similar manner to ‘crash’ grazing, parthenium declined to minimal levels during the study at Albinia National Park and ‘Morbridge’. This indicates that grazing management to maintain native pastures in a healthy competitive condition is critical for the effective management of parthenium.
Therefore burning of native pastures to control woody weeds or for pasture management reasons should not substantially increase the presence of parthenium as long as post-fire pasture management maintains pasture competitiveness. Often spelling the pasture so that fuel can accumulate to carry a fire is beneficial by itself in terms of pasture composition and cover. The findings also reinforce current management guidelines that encourage pastures to be managed so that high levels of competitiveness are maintained.
Smoke
In April 2000 soil seed
bank samples were collected at Albinia National Park from a site several
kilometres away from that used for the fire study. The herbaceous vegetation
at this site was dominated by C. ciliaris with Aristida sp.
and parthenium also relatively common. The composition of the entire seed
bank was investigated in a pot experiment to characterise the effects of
smoke on different components of the seed bank.
There were six treatments: a control, heat only, smoke only (mild), smoke (mild) and heat, smoke only (intense), and smoke (intense) and heat. Heat along with aerosol smoke derived from grass obtained from a native pasture dominated by H. contortus was applied to the soil. The germinable soil seed bank of these soil samples was then measured.
Parthenium was by far the most abundant species present and accounted for between 79 and 88% of the seed bank depending on the germination treatment. Grasses were also common in the seed bank (9-17% of the total), particularly C. ciliaris, which represented the vast majority of the germinable grass seed bank.
The seed bank of all other species, apart from parthenium, was stimulated by the intense smoke treatment, and even more-so when in conjunction with a heat treatment. As this portion of the seed bank mainly consisted of grasses, it is probably due to smoke and heat breaking their dormancy mechanisms. These findings suggest that smoke may promote the germination of pasture grasses and hence decrease the competitiveness of parthenium.
Conclusions
This study has shown that
fire does not significantly reduce the germinable soil seed bank of parthenium
(at least on black cracking clay soils in central Queensland), nor does
smoke from such fires stimulate parthenium seed germination. Fire did however
result in one-off increases in parthenium densities, which after subsequent
fires rapidly declined. This result was similar for the mow treatments
the effects of which are somewhat similar to crash grazing (the removal
of standing vegetative matter over a short duration).
The study has also demonstrated that removal of grazing animals for extended periods (as with the control treatments) delivers similar reductions in the presence of parthenium to that of burning or mowing the pasture. This indicates that managing grazing to maintain cover/biomass and desirable pasture composition is potentially the most important factor influencing the amount of parthenium present in native pastures.
Therefore landholders should not refrain from using fire when necessary as a cost effective management tool for pasture species manipulation or woody weed control. Rather, it appears that post fire pasture/grazing management is a more significant factor in determining the level of subsequent parthenium infestations than the use of fire.
 
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