I reckon four key insights have been revealed by this work, with each insight critically interacting with the others to inform our understanding of vegetation dynamics:
1) land use history affects vegetation composition and species diversity. Importantly, by quantifying vegetation structure and composition using land use comparisons (i.e. grazed grasslands versus burned but ungrazed grasslands), it is apparent that native species richness and functional trait diversity is highest when disturbed by fire rather than by other agents such as slashing or stock grazing, and that frequent fire maintains higher alpha diversity than infrequent fire.
2) when frequent burning of grasslands is relaxed, species can go locally extinct. This has been well documented in recent times by Nick Williams & others as socio-economic drivers of burning change.
3) many intertussock species are poor competitors for light and space (and these species can be predicted by their traits). Just as there is a war going on in woodlands between trees and grasses, the war extends to grasslands where grasses often outcompete forbs because they are taller, accumulate more biomass (and hence, sequester more of the light and nutrient resources) and they grow faster.
4) many grassland forbs have transient soil seed banks (i.e. seed does not accumulate in large numbers in the soil). This has important consequences for regeneration. Once grassland plants are lost from grasslands (because of grazing or lack of fire), they are unlikely to re-populate from dormant seed stored in the soil.
The core findings all lead back to the key role that fire plays in the dynamics of the system.
This has long been recognised. But curiously, despite the importance of fire in the function of grasslands, we still know relatively little about (a) fire history (particularly as it relates to the high quality grasslands that sustain much of the species diversity of this ecosystem), nor (b) how fire behaviour varies in grasslands.
Recent research is shedding new light on both these questions. In this Blog, I'll tackle the first question - if frequently burned temperate grasslands are the "gold standard" for grasslands, just when did this regime start?
Sarah Dickson-Hoyle (University of Melbourne) recently completed a Master of Forest Ecosystem Science thesis with the core question being: when did Country Fire Authority (CFA) burning by rural brigades in western Victoria start, and why? She focused her attention on the 3-chain wide roads around Dunkeld-Cavendish where there has been settlement since the mid-1800s. Until now, I'm not aware of when this burning started nor why it started in the first place.
Sarah interviewed current CFA brigade captains in the area about why they burn, how often they burn, and whether they knew about the grasslands and their ecological values. She also examined brigade Minutes to try and pinpoint when this burning began.
In this district, she found that burning for fire protection began in the period 1941 to 1947. This is before much of the land was re-settled by soldiers returning from the Second World War. Burning was implemented on 2- and 3-chain wide roadsides as frequently as possible to maintain low fuel levels.
Interestingly, according to the Minutes of CFAs, burning began largely because of the introduction of exotic pasture grasses such as Phalaris (as superphosphate use increased) and the perception that they created an elevated fire risk due to their high fuel loads and early curing. Burning of roadsides started because local communities wanted risk reduction strategies implemented. It was thought that 1-2 year intervals between fires would provide the greatest protection to the community, with little regard for the ecological effects of such regimes.
I think the dating of the start of frequent fires in these systems in the 1940s is at least 10-20 years earlier than most grassland botanists had estimated.
|The original survey of the Karabeal Plains from 1865 showing 2 and 3 chain wide roadsides that later became|
strategic fire breaks and, fortuitously, some of the best temperate grassland remnants in Australia.
In Part 2 on fire, I'll return to the question: what do we know about fire behaviour in grasslands? We've been quantifying fire this summer, and getting some surprising results.
Dickson-Hoyle, S. (2013) Risk, remnants and roadsides: understanding fire and conservation management along a rural road, western Victoria. Master of Forest Ecosystem Science thesis, University of Melbourne.