Pasture management in semi-arid tropical woodlands: effects on tree regrowth

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1 Tropical Grasslands (2006) Volume 40, Pasture management in semi-arid tropical woodlands: effects on tree regrowth JOHN G. McIVOR CSIRO Sustainable Ecosystems, St Lucia, Queensland, Australia Abstract The impact of pasture management on tree regrowth (from seedlings and/or root suckers) was measured from 1982 to 1992 at Hillgrove and Cardigan, near Charters Towers, north-east Queensland. Oversowing introduced legumes and grasses, applying superphosphate and cultivating the soil before sowing had little impact. In contrast, on plots where the trees were killed, the number of stems, height and leaf area indices of regrowth were much greater than on plots with live trees. Heavy stocking rates (1 steer/ha) also greatly reduced stem number and leaf area index of regrowth but these stocking rates were not sustainable. Clearing and killing trees can give large herbage growth responses but the regrowth problems reported here, and other potential negative impacts, necessitate a cautious approach to clearing and careful post-clearing management. Introduction Woodlands in the semi-arid tropics of northern Australia have an open tree layer (usually of Eucalyptus and Corymbia species) and an herbaceous layer dominated by tall, perennial tussock grasses. Extensive beef cattle grazing is the main land use in the region. The major limitation to animal production in these woodlands is the low quantity and/or quality of the herbage. The environment is characterised by variable rainfall both between and within years. Between-year variability leads to large annual variations in herbage production, while within-year variability produces Correspondence: Dr J.G. McIvor, CSIRO Sustainable Ecosystems, Queensland Bioscience Precinct, 306 Carmody Road, St Lucia, Qld 4067, Australia. john.mcivor@csiro.au variation in both herbage growth rates and quality. A number of options are available to overcome the limitations imposed by low quantity/quality of the herbage including tree clearing or killing, sowing introduced legumes and grasses, applying superphosphate and feeding nutritional supplements. The tree population in these woodlands can contain many small plants, either seedlings or root suckers (Scanlan 1988), which may greatly outnumber the larger and mature trees (e.g. McIvor and Gardener 1991). There is continuing debate about woodland thickening in Queensland ( Burrows 2002) with concern about increases in the number and size of trees and shrubs. Burrows et al. (2002) concluded that such increases were due to a combination of factors including growing conditions, grazing and fire. This raises the following question: What impacts do management options designed to improve pasture production and quality have on the populations of tree seedlings and suckers? The ECOSSAT (Ecological Studies in the Semi-arid Tropics) project was established in 1981 to examine a number of management options in pasture systems ranging from native woodland to fully developed pastures on 2 soil types in the Charters Towers district. Impacts of the pasture treatments on the population of tree seedlings and suckers were measured from 1982 to 1992 and the results are presented in this paper. Materials and methods Sites The experimental sites were at Hillgrove (80 km north-west of Charters Towers) and Cardigan (40 km south-east of Charters Towers). Detailed descriptions of the sites are given in McIvor and Gardener (1991) and McIvor et al. (1991). Both sites have a subhumid tropical climate with an average growing season of 14 weeks. The

2 Pasture management in the semi-arid tropics 15 vegetation at both sites is an open woodland with Eucalyptus crebra (Hillgrove), E. drepanophylla (Cardigan) and Corymbia erythrophloia (both sites) the dominant trees. The herbaceous layer is dominated by perennial tussock grasses; the most important species are Bothriochloa ewartiana, Heteropogon contortus and Chrysopogon fallax. The soil at Hillgrove is a euchrozem (Gn3.12, Northcote 1979; eutrophic red ferrosol, Isbell 1996) derived from basalt and the soil at Cardigan is a neutral red duplex (Dr 2.12, eutrophic red chromosol) derived from granodiorite. Tree (i.e., stems > 2m tall) numbers were 64 per ha at Hillgrove and 127 per ha at Cardigan when the experiment commenced. Treatments At each site, the experimental plots consisted of 9 unreplicated pasture systems each grazed at 4 stocking rates (McIvor and Gardener 1995). Eight of the pasture systems (Systems 1 8) formed a factorial of pastures, timber treatment and fertiliser application (Table 1). The ninth system was a fertilised sown pasture established after clearing the trees and cultivating the soil before sowing. Table 1. Pasture treatments and stocking rates on the 36 experimental plots at each site., no treatment; +, treatment applied. System 9 was cleared of trees and the soil cultivated before sowing. Pasture System Sown species Timber treatment Fertiliser Stocking rates: 0.1, 0.2, 0.33, 0.5 steers/ha Stocking rates: 0.2, 0.33, 0.5, 1.0 steers/ha Pastures. The native pastures consisted of the existing native and naturalised species with no intentionally sown species. The oversown pastures were sown with a mixture of 4 legumes: Macroptilium atropurpureum cv. Siratro, Stylosanthes guianensis cv. Graham, S. hamata cv. Verano and S. scabra cv. Seca (2 kg/ha of each); and 4 grasses: Bothriochloa insculpta cv. Hatch, Chloris gayana cv. Callide, Cenchrus ciliaris cv. American and Urochloa mosambicensis CPI (1 kg/ha of each). Seed was spread on the soil surface at the start of the growing season after the existing herbage had been burnt; Hillgrove was sown in December 1981 and Cardigan in January Additional seed was spread at the start of the following growing season. Timber treatment. Trees (i.e. all mature trees and any younger trees that were visible above the grass layer) on the killed plots were killed by stem injection with arboricide. The trees were treated at Hillgrove in April 1982 and at Cardigan in January Trees on the live plots were not treated. Fertiliser application. The nil plots received no fertiliser. The superphosphate plots received 100 kg/ha of superphosphate at sowing and again annually during the late dry season. Tree clearing and cultivation. The trees were removed mechanically and the soil cultivated with 2 passes of offset tandem discs before sowing. This produced a reasonable seedbed at Cardigan but the seedbed was poor at Hillgrove where surface rock often prevented the discs entering the soil. Stocking rates. The stocking rates were 0.1, 0.2, 0.33 and 0.5 steers/ha on the native pastures ( Systems 1 4) and 0.2, 0.33, 0.5 and 1.0 steers/ha on the sown pastures (Systems 5 9). This range would be considered light to very heavy in commercial practice. Stocking rates were applied by varying plot sizes. Areas of plots grazed at 0.1, 0.2, 0.33, 0.5 and 1.0 steers/ha were 2.5, 1.25, 0.75, 0.5 and 0.5 ha, respectively. The plots were grazed by single steers (1 to 2-year-old, kg) in a similar manner to studies by Jones et al. (1980) and McIvor (1985), except for plots grazed at 1 steer/ha where 2 steers were used. Steers were moved between plots twiceweekly so that each plot was grazed for 25% of the time. Plot management Grazing commenced at Hillgrove in June 1984 and at Cardigan in September After this, each plot was grazed throughout the experimental period except when it carried insufficient feed

3 16 John G. McIvor to support an animal. When this occurred, the plot was omitted from the rotation until herbage regrew sufficiently to recommence grazing. Animals removed from the plots were maintained on laneways and adjacent spare land. At Hillgrove in April 1988, 20 plots had insufficient feed and the site was destocked until October The lightly grazed (0.1 and 0.2 steers/ha) native pasture plots were burnt at the start of the growing season in 1986, 1987 and 1989 at Hillgrove and in 1990 at Cardigan. Fuel loads on all burnt plots exceeded 2400 kg/ha. Such burning is consistent with commercial practice where burning to remove accumulated dry herbage is common when seasonal conditions and stocking rates permit such accumulation. Measurements Twelve permanent quadrats (each a 10 m diameter circle) were established and marked in each plot in 1982 at Hillgrove and in 1985 at Cardigan. Any tree stems < 2 m tall were considered to be seedlings or suckers and the numbers of these in each quadrat were counted. Similar counts were made during the dry season (June October) in each subsequent year. Initially all seedlings and suckers were < 2 m tall but in later years some exceeded this height. No attempt was made to distinguish between seedlings and suckers, or to determine when they first appeared. On plots where the trees were killed, it is likely that many of the seedlings and suckers were present at the start of the experiment but the plants were too small to be seen and killed by the arboricide treatment. In 1988, the size distribution of seedlings and suckers was determined in 8 plots (consisting of a factorial combination of pastures, tree killing and stocking rate [0.2 and 0.5 steers/ha]) at Hillgrove by counting the number of stems in the height categories, < 50, , and > 200 cm, in each permanent quadrat. The leaf area of tree seedlings and suckers growing in the permanent quadrats was estimated using the methods of Carbon et al. (1979) from 1990 to 1992 at Hillgrove and in 1991 and 1992 at Cardigan. The leaf areas of all seedlings and suckers in a quadrat were estimated on an arbitrary rating scale. Estimates were also made for a number (10 12) of calibration seedlings/suckers that covered the range in plant size encountered during the sampling. These were then harvested, the leaves plucked and their area determined by planimeter. A regression was calculated between the estimated ratings and the measured leaf areas of the calibration standards. This regression was then used to convert the ratings made on each plot to leaf areas. The leaf area index for a plot was calculated by dividing this leaf area by the ground area of the quadrats. Estimates of the leaf area indices of adult trees in plots where the trees were not killed were also made in 1990 at Hillgrove and in 1991 at Cardigan. Statistical analyses At both sites, there was considerable variation between plots in the first year, which was not related to treatment and appeared to be random with some plots having more seedlings and suckers than others at the plot scale (possibly due to some past unknown events). This variation masked any subsequent treatment effects so the results were analysed on a relative basis by expressing the stem number for a plot in subsequent years as a ratio of the stem number of that plot in the first year (1982 at Hillgrove and 1985 at Cardigan). The relative stem numbers and leaf area indices for Systems 1 8 for each year were analysed using factorial analysis of variance. All main effects (pastures, fertiliser, timber treatment and stocking rate) and 2-factor interactions were fitted. All 3-way interactions and the 4-way interaction were combined to form the error sum of squares. Residuals were calculated, plotted using stem-leaf plots and normal quantile-quantile plots, and tested using the Shapiro-Wilks statistic (Royston 1982) to check the normality and constancy of variance. Results over more than one year were analysed using a multivariate repeated measures approach (Crowder and Hand 1990). This approach produces a between-plots analysis, and a within-plots analysis. The between-plots analysis is equivalent to an analysis of the results for each plot meaned over years. This analysis produces univariate F tests for the effects concerned. The within-plots analysis tests the year main effect and all interactions with year based on the multivariate test statistic, Wilks Lambda. Pasture Systems 8 and 9 differed only in treatment before sowing on System 9 plots, trees were cleared mechanically and the soil cultivated whereas on System 8 plots, trees were killed

4 Pasture management in the semi-arid tropics 17 with arboricide and there was no cultivation. The results from the 2 systems were compared to assess the effects of clearing and cultivation. The data were analysed by analysis of variance using the pasture system stocking rate interaction as the error term. Results Rainfall and growing conditions Over the long term, January March is the wettest quarter. However, during the experiment, rainfall during this quarter was below average in 9 of the 11 years at Hillgrove, and 7 of the 8 years at Cardigan (Table 2). Mean rainfalls during the other quarters were similar to or above the longterm average values. The variable rainfall produced widely varying growing conditions with the length of the growing seasons varying from 4 17 weeks at Hillgrove and from 4 25 weeks at Cardigan (estimated using the methods of McCown 1973). Treatment effects on stem numbers of seedlings and suckers Mean initial stem numbers of seedlings and suckers were 264/ha at Hillgrove in 1982 and 993/ha at Cardigan in Subsequently, there were no significant effects of pasture, fertiliser application or cultivation on stem numbers but there were significantly more stems on plots where the trees were killed than on plots with live trees, particularly at Hillgrove. The stocking rate main effect was significant at both sites (Table 3). There were significant interactions of timber treatment with year of sampling and these are shown in Figure 1. At Hillgrove, there was no change in stem numbers from 1982 to 1984 with either live or killed trees but from 1985 to 1988 the effects of timber treatment were marked. On plots with live trees, stem numbers fluctuated widely between years but remained at a similar overall level (mean relative stem number during of 0.90 compared with 1.01 during ). In contrast, on plots where trees Table 2. Quarterly and annual rainfall during the experimental period at Hillgrove and Cardigan. Oct Dec Jan Mar Apr Jun Jul Sep Year Hillgrove Mean Long-term Cardigan Mean Long-term Long-term values for Charters Towers.

5 18 John G. McIvor Table 3. The effect of management on stem numbers relative to numbers in 1982 at Hillgrove and 1985 at Cardigan: means over treatments and years. Hillgrove Cardigan Year P< P< Pasture 2 P = P = Native Oversown Trees P = P = Live Killed Fertiliser P = P = Nil Superphosphate Cultivation P = P = System System Stocking rate (steers/ha) P = P = Native Oversown Native Oversown Significance (probability) of the main effect. 2 The pasture values are for plots stocked at 0.2, 0.33 and 0.5 steers/ha. were killed, stem numbers increased from 1985 to Stem numbers declined from 1989 to 1991 and then increased slightly in 1992 with both live and killed trees. At Cardigan, stem numbers were constant from 1985 to 1987, declined in 1988 with greater declines on plots with live trees than on plots where trees were killed, and then were relatively constant with both live and killed trees until There were also significant interactions of stocking rate with year of sampling. The interaction arose mainly from the differences between the oversown plots stocked at 1 steer/ha and the other plots (Figure 1). From 1985 at Hillgrove and 1988 at Cardigan, stem numbers were much lower on the heavily grazed oversown plots, and by 1992 the relative stem numbers on those plots were 0.02 and 0.12 compared with 1.75 and 0.88 for the other plots at Hillgrove and Cardigan, respectively. Burning plots at Hillgrove in 1986, 1987 and 1989 and at Cardigan in 1990 appeared to have little impact on stem numbers. On burnt plots, the average stem numbers in the year after the fire were 6% higher than on those plots the year before the fire; on the rest of the plots, the numbers were 1% lower. Size distribution of seedlings and suckers in 1988 at Hillgrove Neither pasture type nor stocking rate had a significant effect on height distribution of regrowth, but seedlings and suckers were much taller on plots where the trees were killed than on plots with live trees (Figure 2). Only 6% of seedlings and suckers were > 1 m tall on plots with live trees compared with 38% on plots where trees were killed.

6 Pasture management in the semi-arid tropics 19 Figure 1. Changes in relative stem numbers (mean and standard error) with time at Hillgrove [(a) and (c)] and Cardigan [(b) and (d)]. Graphs (a) and (b) compare plots with live trees and plots with killed trees. Graphs (c) and (d) compare native pasture plots, oversown plots grazed at low stocking rates (0.2, 0.33 and 0.5 steers/ ha), and oversown plots grazed at high stocking rate (1 steer/ha).

7 20 John G. McIvor 100 Proportion of total stems (%) Live Killed 0 < > 200 Height class (cm) Figure 2. The effect of tree killing on the height distribution of tree seedlings and suckers 6 years after treatment at Hillgrove. Values are means and standard errors over pasture type and stocking rate (n = 4). Leaf area indices of seedlings and suckers The leaf area indices of seedlings and suckers, measured during the final 3 years, increased with time at both sites but there were no significant effects of pasture type, fertiliser application or cultivation treatments (Table 4). Leaf area indices were significantly higher on plots where the trees were killed than on plots with live trees. The stocking rate main effect was not significant but leaf area indices were very low on plots grazed at 1 steer/ha. The largest leaf area index values for seedlings and suckers on individual plots in 1992 were 0.34 at Hillgrove and 0.20 at Cardigan. In both cases, these were plots with killed trees. The leaf area indices of the adult trees in the woodlands were 0.29 ± (mean and standard error) at Hillgrove and 0.48 ± at Cardigan. Discussion This study has verified that clearing trees in north-eastern Australia can lead to more stems of seedlings and suckers than occur in woodlands that have not been cleared. Clearing removes the negative impacts of mature trees, and in this study resulted in either a marked increase (at Hillgrove), or a smaller decrease in stem numbers of seedlings and suckers (at Cardigan) compared with uncleared woodland. These measurements quantify the changes in vegetation that occur after clearing and are consistent with the experiences of producers who have cleared country and then had dense stands of regrowth develop. Changes in stem numbers are complex and are the net result of recruitment and death. Both of these are affected by management (e.g. fire, tree clearing, grazing) and environmental (e.g. soil type, vegetation type, rainfall) factors, so results are likely to vary from site to site and on different occasions. This complexity also makes it difficult to explain some of the changes in stem numbers. For example, stem numbers increased in 1987 at Hillgrove on plots where the trees were killed, but decreased on plots with live trees. Rainfall was close to average during the 1986/87 season and it is possible these changes represent small but important improvements in the moisture supply where the trees were killed that allowed more seedlings to establish and/or more established plants to survive than occurred in the woodland plots. It is likely there is a threshold of numbers and sizes of stems that a particular system can support at a point in time. At Cardigan, the original stem numbers seemed to be near this threshold. However, at Hillgrove, the system could support more as evidenced by the marked increase in numbers after clearing. There were approximately twice as many stems of seedlings and suckers at Cardigan as at Hillgrove but the leaf area indices

8 Pasture management in the semi-arid tropics 21 Table 4. The effect of management on leaf area index of tree seedlings and root suckers: means over treatments and years. Hillgrove Cardigan Year P < P < Pasture 2 P = P = Native Oversown Trees P < P < Live Killed Fertiliser P = P = Nil Superphosphate Cultivation P = P = System System Stocking rate (steers/ha) P = P = Native Oversown Native Oversown Significance (probability) of the main effect. 2 The pasture values are for plots stocked at 0.2, 0.33 and 0.5 steers/ha. in 1991 and 1992 were larger at Hillgrove. These larger leaf area indices would be mainly due to the longer period of growth after tree killing at Hillgrove (10 11 years compared with 6 7 years at Cardigan), but also mirror the differences in adult trees, which are also fewer in number but larger than those at Cardigan. There was little change in stem numbers at both sites during the first two years after clearing, major changes in the next 2 to 4 years, and then relatively small changes after that, although the leaf area indices of the seedlings and suckers continued to increase. Although it was not done in this study, the early period with little change in stem numbers provides an opportunity to use fire to suppress the development of regrowth while it is still small. Plant size affects response to fire; for example, Williams et al. (1999) showed survival increased sharply with size over the range 2 20 cm diameter at breast height. Some plots were burned in later years but those fires had little impact on stem numbers. It is likely mortality would have been greater following fire in the early years when many of the seedlings and suckers would presumably have been smaller. In contrast to tree clearing, oversowing introduced legumes and grasses, applying superphosphate and cultivating the soil before sowing had little impact on the growth of seedlings and suckers. Although grazing by cattle can control tree regrowth (Tothill 1971), stocking rate had no effect on development of regrowth except at very high rates. Such high rates are not sustainable with frequent feed shortages and lower cover levels increasing erosion risks (McIvor 2002). High stocking rates would also prevent the accumulation of fuel and use of fire. There have been many reports of increased pasture growth following tree clearing and/or killing (Walker et al. 1972, 1986; Gillard 1979; Tothill 1983; Winter et al. 1989; Scanlan and Burrows 1990) and clearing has been widely practised, particularly in southern Queensland. In this experiment, the native pastures in the undisturbed woodlands had average yields of approximately 2000 kg/ha at both sites; where the trees were killed, these yields increased by 100% at Hillgrove and 25% at Cardigan (McIvor and Gardener 1995). While such responses can have a positive impact on animal production, the regrowth problems described here can negate these benefits. By the end of this experiment, pasture yield responses to tree clearing were reduced (McIvor and Gardener 1995) and managing cattle

9 22 John G. McIvor in the dense regrowth with a low leaf canopy would be more difficult than in the original more open woodland where the leaf canopy was more elevated. In addition, there are a number of other potential or actual negative impacts of tree clearing dryland salinity (Williams et al. 1997), increased susceptibility to weed invasion, and loss of wildlife habitat (Scanlan et al. 1992). Thus, although tree clearing can give large herbage responses, when the negative impacts are also considered, a cautious approach to tree clearing should be adopted and risks carefully evaluated. Practical implications Changes to legislation (e.g. the Vegetation Management Act 1999 and Vegetation Management and Other Legislation Amendment Act 2004 in Queensland) are likely to increasingly restrict the amount of tree clearing. However, these results remain relevant for any areas that are cleared in the future and also for areas that have been cleared in the past. Tree clearing and killing can give large herbage growth responses but these gains need to be balanced against possible negative impacts of dryland salinity, weed invasion and loss of wildlife habitat. When trees are cleared, regrowth should be expected and planned for by incorporating fire in the years immediately after clearing. This will mean low grazing pressures which will restrict the financial benefits of clearing but will ensure there is sufficient fuel for fires and also sufficient ground cover to lower erosion risks. Acknowledgements I thank: Messrs T.H. Mann and R. Porter for the use of land; Incitec Ltd for supplying superphosphate; Dr C.J. Gardener for contributions to experimental design; Dr M.R. Thomas for statistical advice; and Messrs P.E.J. Allen, W.A. Beyer and L.V. Whiteman for technical assistance. References BURROWS, W.H. (2002) Seeing the wood(land) for the trees An individual perspective of Queensland woodland studies ( ). 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10 Pasture management in the semi-arid tropics 23 WALKER, J., ROBERTSON, J.A., PENRIDGE, L.K. and SHARPE, P.J.H. (1986) Herbage response to tree thinning in a Eucalyptus crebra woodland. Australian Journal of Ecology, 11, WILLIAMS, J., BUI, E.N., GARDNER, E.A., LITTLEBOY, M. and PROBERT, M.E. (1997) Tree clearing and dryland salinity hazard in the upper Burdekin catchment of north Queensland. Australian Journal of Soil Research, 35, WILLIAMS, R.J., COOK, G.D., GILL, A.M. and MOORE, P.H.R. (1999) Fire regime, fire intensity and tree survival in a tropical savanna in northern Australia. Australian Journal of Ecology, 24, WINTER, W.H., MOTT, J.J. and MCLEAN, R.W. (1989) Evaluation of management options for increasing the productivity of tropical savanna pastures 3. Trees. Australian Journal of Experimental Agriculture, 29, (Received for publication July 22, 2004; accepted April 11, 2005)