In south-east Queensland, spring burning is being recommended as a management practice for restoring native pastures that have become dominated by undesirable grass species such as Aristida ramosa. Spring burning in combination with strategic grazing management reduces the proportion of the undesirable A. ramosa and increases that of the desirable Heteropogon contortus. The studies outlined in this thesis were undertaken to identify the plant mechanisms responsible for the differential responses of H. contortus and A. ramosa to spring burning. Four laboratory, one glasshouse and twelve field experiments were conducted between 1991 and 1994 to identify the key mechanisms involved. Findings from this research will be incorporated with existing knowledge to provide graziers with management recommendations that ensure spring burning achieves desirable botanical composition changes.
To determine major research directions, an initial field experiment monitored the dynamics of populations of both H. contortus and A. ramosa following exposure to a range of defoliation strategies. After three years, annually burnt treatments contained the largest basal area of H. contortus. Mowing also increased the basal area of H. contortus, but to a lesser extent than burning. Both annual burning and mowing reduced the proportion of A. ramosa, although a large decline in its basal area occurred in the first year, irrespective of the treatment applied. This initial reduction appeared to be associated with ·the removal of cattle from the trial site prior to the experiment. Removing cattle would have allowed H. contortus to regain vigour and competitiveness which may have contributed to the reduction in A. ramosa.
Population changes observed in the field were due to differential effects on mature plants and seedling recruitment/establishment between the species. While burning did not affect the survival of mature plants of either species, it did have a significant influence on plant size. The basal area of H. contortus plants increased under an annual spring burning regime, whilst the opposite occurred with A. ramosa.
Furthermore, spring burning promoted seedling recruitment of H. contortus but was detrimental to both recruitment and establishment of A. ramosa. Consequently, in annually burnt treatments, the density of H. contortus increased markedly, while that of A. ramosa remained relatively unchanged.
A series of studies on the seed dynamics and seedling emergence of the two species were undertaken to identify why these differences in recruitment and establishment occurred.
A. ramosa is insensitive to photoperiod with plants flowering early in summer, and a large proportion of the annual seed production has fallen by mid autumn. The lack of any seed burial mechanism ensures that most of this seed (92%) remains on the soil surface. Furthermore, seed of A. ramosa overcomes innate dormancy in less than 3 months so that large scale recruitment can occur if favourable rain falls in late summer/autumn. In contrast, H. contortus flowers in late summer/early autumn in response to a short day photoperiod and seed fall occurs during mid/late autumn. H. contortus seed has a period of 6-8 months innate dormancy so that no seed becomes available to germinate under undisturbed field conditions until late spring/early summer. In addition, the hygroscopic awn of H. contortus seed ensures that by early spring, most seed (> 90%) has become buried under the soil surface. Consequently, in spring when burning occurs, a proportion of the previous season's A. ramosa seed production may have germinated, while the remaining ungerminated viable seed remains on the soil surface. In contrast, seed of H. contortus is buried in the top 1 cm of the soil and remains dormant. The temperatures reached on the soil surface even during low intensity fires are sufficient to kill most of the surface located A. ramosa seed, but not the buried H. contortus seed. Thus, burning in spring reduces the recruitment potential of A. ramosa through its deleterious effect on surface located seed, while the burial ability of H. contortus ensures that seed is available for recruitment over the following wet season.
Not only does most of the H. contortus seed survive, but fire also stimulates this previously dormant seed to germinate. In one field emergence study, 390 H. contortus seedlings emerged per m2 in burnt plots, 56 times more than the 7 seedlings per m2 recorded in unburnt treatments. Exposure to either high temperatures or plant-derived smoke increased the germinability of H. contortus seed. Seed heated to 80°C reached peak germinability after 360 s exposure, whilst germination of smoke treated seed was three times higher than that of untreated seed which had only 8% germination.
In the absence of burning, seed populations of H. contortus in south-east Queensland overcome dormancy in late spring/summer, becoming germinable for 1-2 months before entering into a secondary dormancy phase. Thus, the opportunity for recruitment under unburnt conditions is limited and dependent on suitable rainfall occurring during the short period when seed is germinable. Furthermore, germination at this time of the year means that seedlings have to compete against actively growing mature plants and seedlings of other species that may have germinated on previous rainfall events.
Seedlings of H. contortus also have a much better chance of surviving fire than those of A. ramosa. This is because (1) the field germination of A. ramosa occurs mainly in late summer/autumn so that these seedlings are still small when burning is conducted in spring; (2) seedlings of H. contortus germinate in the previous spring and are therefore usually 10 12 months old before being exposed to fire. When plots containing tagged seedlings of both grasses were burnt in· spring, 92% of 1 yr old H. contortus seedlings survived compared with only 8 % for 6 month old A. ramosa seedlings.
Two indirect effects of fire were also studied. Firstly, a competition study between the two grasses showed that H. contortus is much more competitive than A. ramosa. Thus, as the density of H. contortus increases in spring burnt pastures it is able to provide increasing suppression of the growth of A. ramosa. The second study showed that H. contortus has a low tolerance of shading, particularly when it is also heavily defoliated. This would often be the situation in degraded speargrass pastures, with grazed H. contortus plants su"ounded by ungrazed A. ramosa plants. Spring burning removes this shading effect, thereby increasing H. contortus tillering and subsequently plant size.
These studies have demonstrated that the success of spring burning as a management strategy for restoring degraded speargrass pastures can be attributed mainly to its effect on (1) the size of original plants and (2) on seedling recruitment and establishment. Under the ungrazed conditions of these studies spring burning increased the basal area of H. contortus plants while markedly reducing that of A. ramosa. Burning also increased the recruitment of H. contortus by stimulating the germination of dormant seeds. In contrast, burning has a deleterious effect on both seeds and seedlings of A. ramosa, resulting in little recruitment and even less establishment. For spring burning to be successful under grazed conditions it will be important that pre- and post- fire grazing management is adjusted to allow: (1) H. contortus the opportunity to set sufficient seed for recruitment to occur; (2) accumulation of dry matter to carry a fire, if a fire is deemed necessary and; (3) H. contortus to maintain vigour and growth following burning.