Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

Abstract

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis-Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.

Effects of inoculated Microcoleus vaginatus on the structure and function of biological soil crusts of desert

“…Our results that the accompanying microalgal taxa enhanced the BSC structure and function also support conclusions from previous studies that the species diversity and species interactions might play an important role in maintaining the ecosystem function and stability (GarciaPichel et al 2003;Nagy et al 2005;Gundlapally and Garcia-Pichel 2006;O'Bryan et al 2009;Bates and GarciaPichel 2009). The effect of species diversity in maintaining ecosystem functions, such as productivity, had been reported for forest (Bunker et al 2005;Wills et al 2006), cropland (Huang et al 2005), and grassland Loreau et al 2001) ecosystems.…”

Management legacies shape decadal‐scale responses of plant diversity to experimental disturbance regimes in fragmented grassy woodlands

“…Biennial fires similarly promoted the highest richness and cover of soil crust flora at Monteagle (O'Bryan et al . ). However, significant mortality of Themeda tussocks occurred in 2‐year burn plots (in interaction with drought, Prober, Thiele & Lunt ), and forbs had not declined substantially by 6 years after fire (at average rainfall), supporting a moderate fire interval at this site (Prober, Thiele & Lunt ).…”

Experimental changes in disturbance type do not induce short‐term shifts in plant community structure in three semi‐arid grasslands of the Victorian Riverine Plain managed for nature conservation

“…Our results show a neutral response under both a deferred and ‘status quo’ grazing disturbance, which may be maintaining these systems in a stable, grazing‐derived condition. The exposure of these systems to fire disturbance events, however, caused an increase in soil crust cover, suggesting that the function and composition of this important grassland component may be differentially affected by alternate disturbance regimes, as seen elsewhere in south‐eastern Australia (O’Bryan et al. 2009).…”

“…The recovery of these grasslands may be reliant on an increase in the level and complexity of soil crust components (Eldridge & Koen 1998; Belnap & Eldridge 2001; Römermann et al. 2005; O’Bryan et al. 2009) as these are a significant driver of stability, function and recovery of semi‐arid ecosystems.…”