Colonisation of jarrah forest bauxite-mine rehabilitation areas by orchid mycorrhizal fungi

Abstract

Orchids require mycorrhizal fungi for germination of seed and growth of seedlings. The colonisation of bauxite-mine rehabilitation areas by orchids is therefore dependent on the availability of both seed and mycorrhizal fungi. Orchid mycorrhizal fungi baiting trials were carried out in rehabilitation areas that were 1, 10 and 26 years old (established in 2001, 1992 and 1976) and adjacent unmined jarrah forest areas at Jarrahdale, Western Australia. Fungal baits consisted of buried six-chambered nylon-mesh packets containing seed of six jarrah forest orchid taxa, Caladenia flava subsp. flava R.Br., Disa bracteata Sw., Microtis media subsp. media R.Br., Pterostylis recurva Benth., Pyrorchis nigricans (R.Br.) D.L.Jones & M.A.Clem. and Thelymitra crinita Lindl. Detection of orchid mycorrhizal fungi was infrequent, especially at the youngest rehabilitation sites where only mycorrhizal fungi associated with P. recurva were detected. Mycorrhizal fungi of the other orchid taxa were widespread but sparsely distributed in older rehabilitation and forest areas. Detection of mycorrhizal fungi varied between taxa and baiting sites for the two survey years (2002 and 2004). Caladenia flava subsp. flava and T. crinita mycorrhizal fungi were the most frequently detected. The presence of C. flava mycorrhizal fungi was correlated with leafy litter cover and maximum depth, and soil moisture at the vegetation type scale (50 × 5 m belt transects), as well as tree and litter cover at the microhabitat scale (1-m2 quadrats). The presence of T. crinita mycorrhizal fungi was positively correlated with soil moisture in rehabilitation areas and low shrub cover in forest. The frequency of detection of orchid mycorrhizal fungi both at rehabilitated sites (15–25% of baits) and in unmined forest (15–50% of baits) tended to increase with rehabilitation age as vegetation recovered. The failure of some orchid taxa to reinvade rehabilitation areas is unlikely to be entirely due to absence of the appropriate mycorrhizal fungi. However, since the infrequent detection of fungi suggests that they occur in isolated patches of soil, the majority of dispersed orchid seeds are likely to perish, especially in recently disturbed habitats.