Biocrusts, ecological indicators in the Australian rangelands

Abstract

The Australian rangelands that cover around 70% of the country (~6 million km2) are inhabited by some of the most extensive and diverse biocrusts globally. These regions are predominately managed as natural grazing lands. Across northern Australia where the climate is influenced by the monsoon (wet season) biocrusts dominated by cyanobacteria and liverworts occupy the interspaces between grass plants as a coherent layer that binds the upper millimetres of soil and forms a living cover of photoautotrophic (cyanobacteria, algae, lichens and bryophytes), and heterotrophic (bacteria, fungi and archaea) organisms. During the dry season biocrusts are inactive, then recover at the onset of the wet season, actively participating in nitrogen and carbon fixation and accumulation. To identify the role of biocrusts as ecological indicators, we modelled their distribution, diversity and function across microhabitats. We also determined how biocrust community dynamics had been influenced by long-term fire and grazing management regimes.

At Victoria River Research Station (Kidman Springs, NT), after 30 years of fire research we compared managed burning practices at 2, 4 or 6-yearly intervals on two soil types (calcarosol, vertosol). Biocrusts were resilient and recovered rapidly from fire, where diversity and genetic function altered seasonally, between soil types, and fire regimes. Post-fire, after a wet season (cattle excluded), biocrusts recovered with significantly more carbon and nitrogen in the surface soils of cooler fire treatments every four years.

At Wambiana Cattle Station (QLD), grazing trials have been established for 25 years. When paddocks were rested from cattle grazing every second year, biocrusts in duplex soils recovered to levels comparable to ungrazed natural sites. In red-yellow earths, there were biocrust hotspots that provided protection to the soil surfaces when there was a loss of grass cover during drought. Biocrust diversity and function differed between soil type and management (moderate and heavy stocking).

This research demonstrates that the top centimetre of biocrust-rich soil is central to ground cover integrity and is essential to soil nutrient cycling and fertility. Biocrusts are an important indicator for overall vegetative recovery post fire, grazing and drought disturbances. The principles of land cover condition based on landscape function should include biocrust presence as a metric that describes landscape resilience, as opposed to bare unprotected ground. Importantly, land managers can apply this research to grazing practices that show that giving the land rest periods timed to coincide with the wet season allows time for biocrust organisms to recover from disturbance.