Soil moisture results

  • Soil  samples taken in April show that the average soil moisture content at the  surface was 1.4% and at 20cm depth it was 3.4% - so pretty dry already and  reflective of the early end to the wet season.
  • There  was no evidence to suggest any difference in soil moisture between treatments based  on whether the plots were burnt or not.

Tagged plants results

At the start of the experiment, we tagged 289 plants and re-  measured them over time. Included in the 289 plants were 144 feathertop plants,  58 weeping Mitchell grass plants and 87 curly bluegrass plants which were  fairly equally spread throughout the 16 experimental plots.

Measurements on these plants included the maximum width of  the base of each plant, the width perpendicular to that, and estimated  percentage of the base that was moribund (dead). From these measurements the  area of living basal area for each plant was calculated. The number of seed  tillers present on each plant at each visit was recorded as well as whether any  plants had died.

The results showed that only three of the 289 tagged plants died. All three  were feathertop plants that were burnt in July.

According to Table 1, burning strongly stimulated seed tillering in  curly bluegrass but did not seem to have any impact on tillering in weeping  Mitchell grass or a strong influence on seed tillering in feathertop.

Table 1: Average number of seed tillers per plant

 

At Start (July17)

At End (Apr18)

Unburnt weeping Mitchell

0.4

10

Unburnt feathertop

19

24

Unburnt curly bluegrass

4

41

   

July burnt weeping Mitchell

0.9

7

July burnt feathertop

20

19

July burnt curly bluegrass

1

119

Sept burnt weeping Mitchell

0.4

6

Sept burnt feathertop

20

21

Sept burnt curly bluegrass

5

137

The tagged plants had more living basal area at the start of the  experiment in July 2017 than at the end in April 2018 and this was true even  for plots that were never burnt (see graph below). Given that the plants were  protected from grazing for the entire experiment, this suggests that the  2017/18 Wet Season was harder on all species compared to the year before.

Although all plants (both burnt and unburnt) decreased in basal area,  it is apparent that the impact was much higher on those plants that were burnt  in July (orange bars) and September (grey bars):


Graph 3: The average living basal area per plant for  each plot

Table 2 shows the change in average living basal area per plant broken  down by species and burning treatment. The only plants that increased in  basal area between the start and end of the experiment were feathertop  wiregrass plants that weren’t burnt. It would appear that the September burn  had a bigger impact on basal area than the July burn but more statistical  analysis is required.

Table 2: Change in average living basal area per plant

 

Never burnt

Burnt in July

Burnt in Sept

Weeping Mitchell

-36%

-58%

-57%

Feathertop

+3.6%

-21%

-26%

Curly bluegrass

-9%

-34%

-47%

Research conducted in western Queensland showed that high mortality  rates of feathertop can be achieved when a moderately hot fire is applied to  feathertop in July or August. This timing maximises the period that the  shallow-rooted feathertop plants are drought-stressed. Success appears to be  highly dependent on having very low or no soil moisture before and after  burning – if there is soil moisture present at the time of burning, or within  six weeks after burning, the burnt plants can re-sprout and mortality rates can  be as low as 10-30%.We are currently considering repeating the experiment in  August, as historical records indicate that it is not uncommon for Newcastle  Waters to receive rain in November, to see whether the early end to the wet  might help us kill more feathertop this year. This timing should ensure the  soil moisture is low enough to burn, but also reduces the chance of receiving  rain within six weeks after burning.

We still have a large amount of biomass, ground cover and species  composition data to analyse which may provide further insights into the impact  of the burns on both the feathertop and also the desirable 3P species. Stay tuned!

Last updated: 06 December 2018