Orchid losers and winners after fire in West Australian urban bushland: A response continuum deeply integrated with other traits
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Context
The Southwest Australia Floristic Region (SWAFR) is a global hotspot for plant taxonomic and functional diversity. This region has many orchids with highly specialised associations with pollinators and mycorrhizal fungi that also face increasing threats from habitat loss, grazing, weeds, fire and climate change.
Aims
To identify short and long-term fire outcomes for 17 orchids with diverse ecological strategies in an isolated urban banksia and eucalypt woodland, and to develop fire response monitoring methods and criteria for management of species.
Methods
A 72-year fire record was overlain on 982 orchid locations with 7-11 years of baseline data and 4.5 km of transects to determine fire history effects on orchid abundance and diversity. Key traits such as plant size, flowering, pollination, tuber depth, clonality and population dynamics were also measured.
Key results
Fire history was very complex with 58 large overlapping major fires over 5 decades, averaging 8.7% of the 63-ha area annually. Correlating fire history with orchid occurrences revealed species usually lost to fire (5 spp.), or with substantial fire impacts (6). Others benefited from fire (6), including strongly enhanced flowering (3), but even the latter could be killed by unseasonal autumn fires. Pollination varied from moderate decreases to substantial increases post-fire, including three orchids producing seed primarily post-fire. Overall, impacts of fire greatly out way benefits, as most orchids preferred long-unburnt areas, and five species were confined to them. Paradoxically, Pyrorchis nigricans requires fire to flower, but was most productive in long-unburnt areas and primarily reproduced clonally.
Conclusions
Fire history maps revealed a spectrum of outcomes from catastrophic to beneficial for orchid species summarised by fire response indexes (FRI) and fire age safe thresholds (FAST) lasting decades. This response continuum was highly correlated with trait such as tuber depth, seed vs clonal reproduction, dispersion and lifespan. Fire was deeply integrated into the biology and ecology of SWAFR orchids which are adapted for long fire intervals which maximise their diversity and abundance.
Implications
Research in an urban nature reserve provided essential tools for sustainable management of orchids that are also relevant to rare species. These tools include fire history maps, FRI and FAST. Many SWAFR orchids were most productive in long unburnt areas or intolerant to fire, and even fire tolerant orchids can be harmed by autumn fires. Thus, fire must be carefully managed in their habitats, which are most productive two or more decades after fire.
