|Cooper Street grassland, dominated by the C4 grass Themeda triandra|
One of the fundamental challenges to grassland management in
is the role of biomass accumulation and the perceived necessity for disturbance regimes that reduce this biomass. In the absence of disturbance (to the vegetation, not the soil), biomass accumulates and smothers intertussock flora, preventing seedling recruitment by native species. In extreme cases, biomass accumulation smothers dominant tussock grasses, leading to their decline and replacement with annual exotic species. For some fauna, dense litter reduces favoured habitat states. Victoria
The rate of biomass accumulation, however, varies across the geographic range of native grasslands in southern
. Undisturbed Themeda grasslands in mesic regions have relatively high productivity and accumulate large quantities of dead grass, which decomposes very slowly and accounts for the majority of accumulated litter. By contrast, dead grass does not appear to accumulate over long periods in drier grasslands (dominated mainly by Rytidosperma spp., Austrostipa spp.), but instead appears to decay (or blow away) relatively quickly. Thus, low levels of accumulated biomass in xeric grasslands reflect high decomposition rates as well as low productivity rates. Australia
The differences that occur in litter build-up underpin the notion that different types of native grassland need disturbance regimes that differ in their frequency. The frequency of disturbance might be timed to coincide with increasing litter levels that compromise native plant and animal diversity but developing ‘absolute’ disturbance regimes for native grasslands has proved difficult, chiefly because grasslands vary in their rate of litter accumulation. What is needed is a simple, effective assessment tool to quantify levels of biomass accumulation at individual sites upon which management actions can then be implemented.
Since 2006, La Trobe University, in conjunction with Parks Victoria, have been developing a grassy ecosystem biomass monitoring protocol to enable rapid field assessment. The basis premise of this assessment was to use a surrogate measure that would (a) indicate something about absolute levels of biomass in a grassland and (b) the assessment outcomes could be linked to management actions necessary to maintain diversity. This idea builds upon the habitat condition assessment method that identifies the optimum range of grassland structures necessary to maintain the Plains Wanderer, an endangered bird that prefers open-structured grasslands on the riverine plain.
Nick Shultz developed a pilot project to examine the potential value of using surrogates to estimate grassland 'condition'. Briefly, a 1 x 1 m quadrat is located in uniform grassland vegetation and 18 golf balls are dropped into the vegetation (one at a time) from a height of approximately 1.3 m. A photograph is taken of the plot and ‘golf ball visibility’ assessed either from the photo or in the field, and a biomass sample (0.25 m2) is harvested from the plot. The aim of the study was to determine whether the biomass of a grassland could be estimated using the surrogate of ‘golf ball visibility’. The rationale is simple: the fewer golf balls that are observed, the denser the vegetation and the higher its biomass. If the surrogate measure was found to be robust, managers could use the golf ball visibility method to estimate their grassland’s current condition, and plan management interventions based on this information. Essentially, if grassland biomass drives species interactions (negative interactions occur between grassland biomass and plant species diversity, as well as being poor habitat for some grassland fauna), giving managers a tool to estimate biomass is an important step to managing for biodiversity conservation.
| Grazed grassland on the Northern Plains. |
How many of the 18 golf balls can you see?
What does this tell us about biomass and the need for biomass reduction?
(Photo: John Morgan)
The initial work was promising but several issues were identified for further study before it could be adopted, including the quantification of the variability in the scoring of the number of golf balls between observers to determine whether this is significant.
While it is an imprecise method of estimaing biomass in grasslands, the golf ball visibility method does seem useful to inform grassland managers about the existence of grassland 'states' (i.e. low biomass, moderate biomass, high biomass) and the dynamic nature of these states (i.e. how quickly they change from year to year), particularly in response to rainfall. When linked to biodiversity values (such as the existence of Plains Wanderers), the surrogate measure does seem to help managers make decisions about their grassland 'state' and when management actions might be desirable.
|The Plains Wanderer|