When to mow a meadow? Ecological challenges in threatened grasslands

Canberra, in the Australian Capital Territory, is known as ‘the bush capital’. Relatively new by city standards, at just over 100 years old, strategic planning saw the protection of significant areas of natural vegetation within the bounds of the city.

Foresight by early town planners has preserved vegetation types in the Canberra region that are now nationally threatened. One of these is a Natural Temperate Grassland community that is relatively common in the Canberra region, so it is too easy to forget the real threats at a national scale. Grasses provide most food for both people and livestock. Natural grasslands around the world have often been highly altered through grazing or other land management practices. Also, grasslands are often preferentially utilised for broad-scale agriculture as there are usually few trees to remove.

Grasslands and the Molonglo River corridor provide rich habitats for a large numbers of animals. These include larger animals such as echidnas, which feed on termites.

Many birds feed in grasslands and nest in denser vegetation, or in trees along the river.

A large array of spiders and even the occasional scorpion can be found making webs among the grass or under rocks.

Grasshoppers are common, often blending very well into their surroundings. If you are lucky, you might spot the endangered Peranga Grasshopper, with a distinctive 'X' on its back (last two images below).

Beetles are found everywhere, and grasslands are no exception.

Butterflies and moths are common too, starting out as caterpillars and often feeding on very specific plant hosts (see Bond et al. 2016).

Bees and bee-flies can be very important for the pollination of native flowers.

Many other insects can be found, the preying mantis showing quite an attitude if disturbed. The rare Mungarda can also be found in old spider burrows (last photo).

A number of lizard species are found in grasslands or along the river, including the rare Pink-tailed Worm Lizard, which has no legs and spends its time in ant burrows. It is most often found under rocks. Rocks should not be disturbed as these small lizards are easily injured, or their small homes destroyed when rocks are moved.

Maintaining good condition for these threatened grasslands is however a more serious matter. With a long history of grazing and weed invasion, combined with close proximity to housing and subsequent high fire risks, current management practices can be complex. It is well understood that reduced fire fuel loads close to houses significantly increase the ability to control fires in emergency situations, the best way to control fuel loads in natural vegetation, and especially a threatened community, is less certain.

Grasslands are commonly shaped by disturbance events such as fire and grazing, so excluding these natural processes can lead to changes in structure and composition, especially an increase in biomass (the physical quantity of vegetation).

Grazing by kangaroos, wallabies and wombats can influence the density of natural species, but grazing levels are rarely high enough to reduce the level of introduced species that often grow more rapidly than natives. Grazing with cattle and even goats is still practiced in some areas, but has its limitations.

As an alternative to fire and grazing, biomass levels can be reduced by mowing. However, many land managers are reluctant to use mowing due to concerns over weed invasion and changes to species responses. We used extensive mown firebreaks in Natural Temperate Grasslands in the Canberra region to examine the effects of long-term mowing on both native and introduced species.

High natural variability in Natural Temperate Grassland species diversity is reflected by the relatively few species that re-occurred in sufficient frequency across the 200 sample sites to allow statistical analyses to be undertaken. The tedious nature of documenting the presence and abundance of every species in 200 plots limits the number of sites that can be scored in a year by a pair of botanists (important for consistency). It would be interesting to see if larger-scale sampling or measurements across multiple years enabled the analysis of a larger proportion of species.

We found increased richness for 16 out of 24 functional groups (differing in various ecological responses), and no reduction in species richness. In most cases, mowing had a positive effect on natives species, and had little or no influence on introduced species. Overall, consistent mowing increased both the cover of native perennial grasses and naturally bare earth (important for new seedling recruitment), corresponding to a reduction in introduced perennial grasses.

Clear positive effects of mowing were demonstrated for six native species, and species complexity was maintained. Mowing may lower the abundance of some species, including the highly invasive Chilean Needlegrass (Nassella neesiana).

We conclude that mowing can be an effective tool for the management of Natural Temperate Grasslands, provided high standards of vehicle hygiene are maintained. The best management results are likely to be gained from a variable mowing regime, combined with targeted management of invasive weeds (e.g. direct herbicide application).

With ongoing management, the Natural Temperate Grasslands around Canberra should continue to delight visitors for generations to come.

THE PLANT PRESS

Botany, Natural History and more...

by Dr Russell Barrett

When to mow a meadow? Ecological challenges in threatened grasslands