Below-ground in grassy woodland: smoke, heat and fire effects on the soil seedbank of remnant Cumberland Plain Woodland in Western Sydney

Abstract

Cumberland Plain Woodland (CPW) is the original native vegetation of the shale soils in Western Sydney but now so reduced in extent that it has been declared a Critically Endangered Ecological Community. While the species composition of above-ground CPW is well documented, this is not the case for the below-ground seedbank. An investigation of the below-ground seedbank was undertaken for two sites of remnant CPW to determine species presence, growth form and numbers, and whether germination treatments of smoke, heat or fire affected species composition. Soil blocks taken in June 2001 were subjected to the following treatments: untreated controls, aerosol smoke, heat, smoke and heat, or fire. Seedlings were counted over July – November and left to grow on to allow identification where possible. A total of 10,436 seedlings appeared, of which 7,596 from 60 taxa were identified. The study showed that natives were present in the seedbank, were responsive to germination cues and outnumbered exotics both in species richness and number of seedlings.

Of the identified seedlings, 43 were native species and 17 were exotics. Native forbs, shrubs and grasses each had 10 or more species. Exotics were largely forbs and some grasses. 39% of the CPW groundcover and shrub indicator species appeared in the experiment.  Germination treatments significantly increased total natives species richness; smoke alone gave a significant increase over the control, and heat or heat plus smoke gave further significant increases. Species richness of exotics did not respond to any of the germination treatments.

Seedlings numbers were dominated by natives, with 6,503 counted (5,011 identified as Themeda triandra). Exotics numbered 1,073 seedlings. Germination treatments significantly increased numbers of seedlings for native shrubs, and approached significance for total natives and for grasses.  Numbers of exotic seedlings, while highest in the controls, did not vary significantly with germination treatments. Seedlings from 8 individual species responded to germination treatments in the same way over both sites; 4 were natives in which numbers of seedlings increased, and in a further 4 species (1 native, 3 exotic) numbers of seedlings decreased.

Species composition was significantly affected by heat and by site; but smoke effect only approached significance. Heat and smoke effects were consistent across the two sites. Plotting the nMDS analysis grouped the controls with the smoke treatment, which were separate from the treatments with a heat component. Analysis of species contributing to cumulative similarity showed that the number of native species was lowest in the controls, but this progressively increased over the smoke, heat, smoke and heat and fire treatments.