Survival of transplanted terrestrial orchid seedlings in urban bushland habitats with high or low weed cover

The conservation of wild orchid populations may depend on the establishment of propagated orchids to field sites to help sustain depleted populations if natural recruitment is not successful. However, very little is known about biotic factors which influence the establishment of terrestrial orchid seedlings in natural habitats. An experiment was established to measure the survival of six orchid species during their first growing season following transplantation to a West Australian urban bushland with a Banksia and Eucalyptus canopy and understorey dominated either by weeds or native vegetation. Symbiotically germinated orchid seedlings raised in the laboratory for 5 months before planting were established in adjacent field sites with high or low weed cover. There was a gradual mortality of seedlings at field sites throughout the growing season, primarily owing to insect grazing, and this was not affected by the enclosure of seedlings by wire mesh or shade cloth. Overall rates of survival varied from 49% for Microtis media R. Br., a species capable of growing in disturbed habitats, to 21% for Caladenia arenicola Hopper & A. P. Brown, the most common native orchid at these sites. However, not all surviving seedlings produced a tuber, so their expected rate of survival after the next dry season was reduced further. The factors having the greatest impact on seedling survival were site aspect (slope and canopy cover), weed cover and orchid species respectively. Orchid seedling survival was not well correlated with the presence of existing orchids of the same species at the same sites or the presence of compatible fungi in soil at these sites (simultaneously measured by orchid seed baiting).