Prior to rehabilitation, plant cover on quartz gravel
overburden at Wangaloa coal mine was highly patchy. Stabilisation techniques
have since considerably altered the condition of the main overburden yet
deposits in the east and west remain largely intact. On the eastern overburden,
three distinct zones of revegetation have naturally developed: Zone 1 represents
a dense plant cover, Zone 2 a patchy cover and Zone 3 a sparse cover of isolated
plants. Aerial photographs show much of the eastern overburden was deposited
between 1940 and 1960s. Yet we found that the temporal scale of deposition did
not wholly account for the differences observed in vegetation development. To
investigate other influential factors, we set up 12 study plots using four
replicates in each vegetation zone. The diversity and density of plants in each
plot were measured to quantify the degree of vegetation development. Samples
from the substrate in each plot were collected using soil cores to a depth of 10
cm. Cores were analysed for indicators of nutrient availability, metal content
and physical composition. We found that substrate chemistry varied between
individual study plots, but showed little difference overall along the
vegetation gradient. Substrate physical differences were much greater. The ratio
of coarse quartz pebbles: fines was considerably higher in Zone 3 than other
zones, and within zones where plants were not growing. Bulk density also
increased successively from Zone 1 - 3. We found clear evidence for the reversed
stratigraphy of excavation superimposed over the time scale of deposition, and
the corresponding nature of fine particles. The rate of revegetation was
therefore highly dependent upon the substrate composition of the last surface
deposit from the dumping truck. Our data suggests an optimum ratio exists
between inert coarse material and a fine matrix for progressive natural
vegetation development on mine substrates.