An unprecedented study of the impact of bushfires within southern farming systems has delivered some surprising results and new knowledge to inform future post-fire management strategies.
In 2020, GRDC invested in a unique research program that aimed to provide grain growers with greater understanding of the effect fire has on the cropping landscape, with a particular focus on pest and beneficial invertebrate species.
The study was conducted by AgXtra on fire grounds on the lower Yorke Peninsula and Kangaroo Island in South Australia, where the 2019-20 bushfires burned about 10,400 hectares of cropping land.
GRDC crop protection manager (south) Ruth Peek says the SA fires provided a valuable opportunity to examine the implications of such events for cropping environments over a 12-month period.
“We were keen to assess the impact of bushfires on invertebrates and to discover if they re-establish in the landscape post-fire and, if they do, just how quickly they return and to what extent,” Ms Peek says.
AgXtra personnel, led at the time by senior research officer Julianne Farrell and technical officer David Traeger, trapped for aerial and ground-dwelling invertebrates in burned and comparative unburned paddocks and green bridges on seven properties over the two regions.
AgXtra deployed 4050 traps (changed monthly) and recorded more than 880,000 specimens representing 32 orders and 190 families.
A visual inspection of the study sites at the beginning of the project indicated substantial changes in the macro and micro-habitats of paddocks, and it was hypothesised that these changes would have had a significant impact on terrestrial invertebrate communities.
Fast-recovering communities
However, Mrs Farrell says data from the study indicated the bushfires did not have the severe effects on populations and diversity that were expected.
“Data analysis has shown that while there were negative impacts on invertebrate populations in the months after the fires, these communities recovered quickly and, within 12 months of the fires passing through, invertebrate populations and diversity were very similar in burned and unburned trial sites,” she says.
The only exception to this outcome were snails, whose populations in burned areas were heavily impacted.
A high proportion of monthly trap catches were beneficial insects, a result that was unexpected in a predominantly monoculture environment.
These beneficials – including parasitoid wasps, honey bees, hover flies, spiders, moths, carabid and ladybird beetles – are important predators and parasites of significant pests such as mites, aphids, caterpillars and earwigs.
“The presence of a diverse range of beneficial invertebrate families indicates that the South Australian cropping environments are being managed well by growers,” Mrs Farrell says.
Refuge areas
She says the study indicated that refuge areas such as shelter belts and native trees and shrubs planted along fencelines, roadside verges and farm roads – where practical for machinery movement – had the potential to enhance the survival of beneficials and their recolonisation capacities after future fires.
Mouse populations on the Yorke Peninsula continued to build during the study period in line with the emerging plague across eastern Australia. Mice were more prevalent in unburned areas of the trial sites; however, active burrows were still common in burned areas. Live-capture trapping was also conducted in timbered areas near the trial sites for the critically endangered Kangaroo Island dunnart, but none were caught.
The study also involved soil sampling to assess the presence of soil fauna, soil-borne pathogens and nutrient levels. Crop disease assessments were conducted soon after the fires and again prior to harvest.
Soil tests produced mixed results. For example, in some paddocks on Kangaroo Island, elevated levels of take-all and root lesion nematode were recorded in burned cereal stubbles post-fire (compared to non-burned stubbles), but levels fell considerably post-harvest.
In contrast, the results were reversed on Yorke Peninsula, where unburned stubbles recorded higher levels of root lesion nematode. This could be due to the fires burning plants and stubble to ground level and into the root zone, where many root lesion nematodes were likely to be found.
In six of the 10 paddocks investigated, counts of Rhyzobium groups E and F were significantly higher in burned ground at the post-fire assessment and remained higher at the post-harvest assessment.
Insect damage
July crop disease and damage assessments returned statistically significant insect damage in three paddocks on Yorke Peninsula in lentils sown into burned cereal stubble and in barley sown into wheat stubble. One Yorke Peninsula site returned statistically significant chemical damage in barley sown into burned wheat stubble during the same assessment period.
Based on anecdotal grower observations, Mrs Farrell says, fires appear to have had a negative impact on crop yields in the first post-fire season. “Growers and agronomists need to be aware of fire-induced changes in soil health and the impact on yields, possibly for several years after the fires,” she says.
Overall, the data collected in the project will serve as a valuable benchmark for future studies elsewhere in Australia’s grain growing regions.
Seven grower families collaborated with the study: Jason and Jenny Berends; Bill and Natasha Giles; Neil and Linley Hancock; Michael and Tracey Mills; Ben and Sarah Pontifex; Will and Jenny Stanton; and Reece and Ian Warren.
More information: Julianne Farrell, 0411 257 831, juliannefarrell17@gmail.com; Ruth Peek, 0455 534 040, ruth.peek@grdc.com.au