In the Southern Province of Zambia and in the southeast Queensland region of Australia, the botanical composition of the native pastures has changed to varying degrees through grazing and other related land use practices in the past 150 years. In both regions, a major change has been a decline in the palatable perennial tall tussock grasses and a proliferation of less palatable grazing-resistant annual and perennial short tussock grasses, forbs and herbaceous weeds. In the Crows Nest district in southeast Queensland, many native pastures contain patches dominated by tall tussock grasses, short tussock grasses or rhizomatous grasses.
It is desirable that the decline of the perennial tussock grasses be reversed or minimized, and encouraging seedling regeneration from soil seed banks would be the most economical way to do so. To know whether the soil seed banks of the pastures in these regions are able to do this, their ecology and regeneration attributes should be understood.
This study investigated how site, season, patch type, mowing and fertilization affected the size and species composition of the soil seed banks of the grazed native pastures in southeast Queensland. The botanical composition of the above-ground vegetation was also studied and related to the soil seed banks. The study was conducted in the Crows Nest district of southeast Queensland from January 2000 to March 2001. Soil samples were taken with a steel corer (5 cm diameter and 5 cm deep). They were taken to the University of Queensland, St Lucia glasshouse where the size, numbers of and species composition of the seed banks were measured by subjecting the samples to suitable germination conditions. Then the seedlings emerging during two, approximately 6 week long, wetting cycles were identified and counted.
The size and species diversity of the soil seed banks was measured in patches (dominated by tall tussock species, short tussock species or rhizomatous grass lawns dominated by Cynodon dactylon) in pastures at four sites. Soil samples were taken in October 2000.
Total seed numbers in the seed banks had a mean value of 7,500 seeds m-2 with a range from 5,500 to 9,600 seed m-2, There were seeds of 49 different species in the soil seed bank, including 25 non-legume-forbs, 6 tall grasses, 5 short and medium grasses, 5 rhizomatous grasses, 5 sedges and 3 legume-forbs. There were no significant differences in total seed number among the sites but seed numbers differed significantly among the patch types. The lawn patches had larger seed numbers than other patch types due to the high seed numbers of short and medium grasses, nonlegume- forbs and sedges. The tall grasses had low numbers of seeds overall. Seed numbers of this group were highest in the tall tussock patches where these species dominated the sward.
Seasonal variation in the size and composition of the soil seed banks was investigated in the same three patch types (tall tussock, short tussock and lawn) in one pasture. Samples were collected in mid summer, late autumn, early spring and early summer. Total seed numbers ranged from 2,420 (early summer) to 9,100 seeds m-2 (mid summer) with a mean value of 5,500 seeds m-2. There were 45 species of which 26 were non-legume-forbs, 6 were tall grasses, 4 were rhizomatous grasses, 4 were sedges, 3 were short and medium grasses, 2 were legume-forbs and 1 was a rush. As in the first experiment, the tall grasses had low seed numbers in all patch types with highest values in the tall tussock patches.
The botanical composition of the above-ground vegetation was estimated in April 2000 using the dry-weight-rank method of the BOT ANAL package. The tall tussock patches were dominated by Bothriochloa decipiens, the short tussock patches by Eremochloa bimaculata, while the lawn patches were dominated by Cynodon dactylon. The major species germinating from the soil samples collected from the tall tussock and short tussock patches was Gamochaeta spp. In the samples collected from the lawn patches, the main species was Paronychia brasiliana. There was no significant relationship between the species in the sward and the species in the soil seed bank in the tall or short tussock patches, and only a weak relationship in the lawn patches.
The effect of mowing and fertilizing on the seed banks in a pasture originally dominated by tall tussock species was studied at one site. The mean total seed number was 2,550 seeds m-2. There were 40 species in the soil seed bank: 20 were non-legume-forbs, 6 were short and medium grasses, 5 were tall grasses, 4 were sedges, 3 were rhizomatous grasses and 2 were legume-forbs. There were more seeds in the lightly mown than in the heavily mown plots (P<0.001). Fertilization affected the seed numbers of rhizomatous grasses and forbs with plots with high soil fertility having higher seed numbers of rhizomatous grasses and lower numbers of forb seeds.
In conclusion, the most important species, the perennial tall grasses which are sensitive to grazing, have low seed numbers in the soil seed banks of these pastures. This shows their vulnerability to extinction if the existing plants are lost. To maintain the grasses in the pastures, management should aim to minimize excessive mortality of the existing perennial plants as seedling recruitment is likely to be limited by the small number of seeds in the soil seed banks. The practical implications of the results and their relevance to Zambia are discussed.