School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffects of adding nutrients on soil chemistry and tree growth in native Eucalyptus forests of south-eastern Australia( 2007)The decreasing area available for timber extraction in south-eastern Australia, due largely to social pressure to reserve greater areas of forest, has led to the consideration of fertiliser-application to increase wood output from the remaining available forest. Potentially deleterious effects of fertilising on water quality must be assessed before implementation on a wide scale. This is in accordance with relevant forest management policies. This study examined the effects of applying fertilisers containing nitrogen and phosphorus, on soil and soil-water chemistry in two pole-sized stands of mixed Eucalyptus spp in the Wombat Forest, in the Midlands Forest Management Area, Victoria, Australia. The findings are synthesised and discussed in relation to management of regenerating mixed-eucalypt forests in south-eastern Australia. Fertiliser treatments were none (R); 400 kg N ha-1 as ammonium-sulphate (N); or 400 kg ha-1 plus 202 kg P ha-1 as triple superphosphate coated with 10% sulphur (NP). It was calculated that incidental additions of S were 1371 kg ha -1 (N treatments), and 1696 kg ha-1 (NP treatments). It was expected that P would be principally adsorbed on soil surfaces; N immobilised in the soil organic pool and that metallic cations would enter the soil solution to varying degrees. Fertiliser-addition increased both plot-basal-area (BA) growth and the rate of stand self-thinning. In 3.8 years, BA in reference (R) plots at two sites increased by 7.3% and 23.4%. Where N alone was added, BA increased by 14.2% and 27.1%, while in NP plots BA increased by 17.1% and 42.7% respectively. Mortality was 9% in untreated plots compared to 14% in NP plots. Estimated increases in biomass growth equated to additional above-ground nutrient accumulation of 0.4 to 1.5 kg ha-1 of P, and 5.5 to 20.8 kg ha-1 of N. This represented only 0.2 to 0.7% of added P, and 1.4 to 5.2% of added N. Soil solution was extracted from 10 and 50 cm with porous-ceramic-cup tension-lysimeters (-0.6 kPa). Concentrations of P and N were low both before and after adding fertiliser. Across all treatments the maximum median PO43- concentration in soil-water at 50 cm was 0.12 ppm (mean 0.28 ppm). Typically PO43- concentrations were not higher than 0.03 ppm. The 400 kg ha-1 of added N was rapidly immobilised in the soil organic pool. The greatest mean NH4' concentration from a single sampling occasion was 1.1 ppm. The mean NO3 concentration at 50 cm was never higher than 0.26 ppm. After adding N in fertiliser the proportion of NO3- relative to NH4* in soil-water increased and was correlated with decreasing soil-water pH. Less than 1% of added P and N was recovered from soil solution at 50 cm. The largest pool of added P recovered was PO43- adsorbed to soil between 0 and 20 cm, due to the soil adsorption capacity being well in excess of the applied 202 kg P ha-1. Phosphate desorption using sequential extractions with a mild acid extractant (0.3M NH4F, 0.1M HCI) recovered between 25% and 116% of added P. Differences were attributed to both the amount of P added and the effect of time since treatment at different sites. Soil disturbance during sampler installation was found to be more likely to raise soil-water P concentrations at 50 cm than would adding up to 202 kg P ha-1. Among the ions in solution. SO42- and CI' were the dominant anions while Cat+ dominated the cation chemistry. In untreated forest 5042- in soil-water ranged from 7.7 to 16.0 ppm at 10 cm and 7.9 to 12.2 ppm at 50 cm. In fertilised plots up to 100.5 ppm SO42 was measured in soil-water at 50 cm depth. In the N treatment at 50 cm, SO42- in soil-water accounted for 9.4 % of applied S. compared to 14.0 % in NP. In untreated forest, soil-water Cl- and SO42- accounted for over 98% of the total soil-water anions, in roughly equal proportions at 10 cm, and CI- slightly higher at 50 cm. Following fertiliser-application soil-water pH at 10 cm fell from 6.3 in R to as low as 4.81 (N) and 4.45 (NP). At 50 cm pH never dropped below 6 and there were no visible departures from reference concentrations. Relative activities of K+ and Mg2+ in solution increased with decreasing pH, indicating increased leaching potential. Sulphate in soil-water increased total anion charge further in NP than in N. Total charge (cmolc L-1) for cations followed anions. A slight deficit in anion charge was likely due to the unquantified contribution of organic anions. These results confirm that despite the quantity of fertilisers added in this trial being double likely operational quantities, the forest and associated soils had the capacity to retain these nutrients through a variety of processes. The study validates the environmental sustainability of proposed intensive management practices including fertiliser-application in this forest type. It also emphasises the importance of understanding fundamental forest nutrient cycling processes when aiming to carry out intensive forest management practices in an environmentally sensitive manner.
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ItemPhotosynthetic responses to light, nitrogen, phosphorus and pruning of Eucalyptus in south-eastern Australia( 2005)Eucalypts frequently grow faster after additions of fertiliser, but more slowly in the shade or following `green pruning'. The coupling of rates of growth to environmental factors is at least partly due to acclimation of photosynthetic processes. Photosynthesis rarely proceeds at maximum rates in natural environments as photosynthetic processes and the supply of basic requirements of photosynthesis (CO2, H20, light, phosphorus and nitrogen) vary at both short (minutes to hours) and longer (days to months) time scales. Currently we lack mechanistic explanations for how these variables, alone and in combination underpin changed growth rates in Eucalyptus. This study examined growth and photosynthetic characteristics in glasshouse-grown seedlings and field-grown trees of Eucalyptus species that are commonly planted for forestry and revegetation purposes in central Victoria. Acclimation to light (among seedlings and within canopies), nutrient availability (phosphorus and nitrogen) and increased sink-strength for photosynthates were the primary foci of the study. In each instance I examined distribution of leaf nutrients within a canopy and allocation of N to Rubisco and chlorophyll to assess the degree to which nutrients limit photosynthesis in Eucalyptus. A novel technique was introduced to quantify the allocation of inorganic phosphorus within cells (cytoplasm versus vacuole), followed by an assessment of inorganic phosphorus allocation in response to a long-term reduction in phosphorus supply. In all circumstances, rates of growth were responsive to environmental conditions. Growth responses were underpinned by altered patterns of biomass partitioning and changed leaf morphology more than by rates of photosynthesis per se. There was little difference in adaptive strategies implemented by seedlings and trees: both were oriented towards the accumulation of nutrients rather than increasing rates of photosynthesis. Photosynthesis was reduced by shading (among different plants and within the canopy of a tree) and reduced phosphorus supply whereas N had little effect on photosynthesis. Analysis of pools of inorganic P revealed that adequate supplies were maintained for photosynthetic processes regardless of P supply, therefore reduced photosynthesis follows, rather than leads, a more general leaf-level response to reduced P. Similarly, changed partitioning of nitrogen between Rubisco and chlorophyll was unnecessary as leaf nitrogen concentrations were consistently maintained at well above published minimum levels. Hence, photosynthesis was not up-regulated following increased nitrogen or phosphorus supply; instead excess nutrients were accumulated and used to support increased biomass. One exception was after defoliation, when up-regulation of photosynthesis was observed, presumably to ensure the demand for photosynthates could be met by a reduced leaf area. Sensitivity analyses consistently revealed variation in photosynthetic rates owed more to altered biochemical activity (e.g. Jmax and Vcmax) rather than stomatal conductance regardless of growth condition (glasshouse versus field). Hence, whilst Eucalyptus has considerable photosynthetic potential, faster rates of carbon fixation are only exhibited in the short-term. In part, this is due to the multiplicity of factors involved in `optimisation' of photosynthesis and their individual and collective responses to environmental conditions. In the long term however, increased canopy photosynthetic capacity follows only an increased photosynthetic area.
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ItemEcology and management of Vulpia spp. G.C. Gmelin in perennial pastures of southern Australia( 2004)Vulpia species G.C. Gmel. cost the Australian wool industry around $AUS 30 million annually in lost production (Sloane et al, 1998). Vulpia provides poor quality forage and replaces other desirable pasture species, thus reducing stock carrying capacity (Code, 1996). In addition, vulpia seed causes vegetable fault of wool, hides and carcasses, and vulpia litter can impede the germination and growth of desirable species because of allelopathic effects (Code, 1996). This weed is prevalent in pastures and cropping regions throughout southern Australia. Presently vulpia is controlled by different methods, including combinations of herbicide application, grazing management, fertiliser application, oversowing competitive pasture species and mechanical defoliation (Michalk & Dowling, 1996; Matthews et al., 1998; Taylor & Sindel, 2000; Dunsford & Morris, 2001). However, greater understanding is required of vulpia ecology and how it spreads, to develop cost effective, sustainable control strategies in perennial pastures of southern Australia. In particular, key questions that need to be addressed are: 1) how do management strategies influence vulpia populations in perennial pastures; 2) what is the effect of neighbour competition on vulpia growth; and 3) how much space is required to sustain a vulpia population? Thus vulpia management studies incorporating varying levels of disturbance and competition in different environmental conditions were combined with canopy gap and competition studies in field and controlled conditions, to investigate factors that influence pasture botanical composition and vulpia population dynamics. The effect of disturbance and competition management treatments on pasture botanical composition was investigated in phalaris (Phalaris aquatica L.)-based pastures in a 575 mm rainfall zone (Ararat) and a 625 mm rainfall zone (Vasey) in western Victoria, between 1999 and 2002. At Ararat, different fertiliser, herbicide (SpraySeed and simazine) and pasture rest treatments were applied. A combination of all treatments was most effective in enhancing the perennial content and in reducing vulpia content, tiller density and seed production. At Vasey, the effect of simazine and ryegrass competition on vulpia content, tiller density and seed production was investigated in set-stocked, strategic and four-paddock rotationally grazed pastures. Although vulpia increased over the duration of the study in all grazing systems, the rate of increase was least in the four-paddock rotation. Simazine initially reduced vulpia content and tiller density, but vulpia rapidly increased and simazine was unable to provide effective control of vulpia. Oversowing ryegrass in simazine treated swards improved vulpia control. These results demonstrate that competition from perennial pasture species is a key factor in controlling the growth of vulpia populations. The dynamics of competition between perennial plants and vulpia were investigated further in a series of field and controlled experiments. Barley grass (Hordeum murinum L.) was included in these studies to compare and contrast the responses of invasive annual grass weed species. In the first of the experiments, the influence of canopy gap characteristics on vulpia growth and survival was investigated in rotationally grazed and set stocked pastures at Vasey. Vulpia and barley grass growth, survival and panicle production were lower under rotational grazing than set-stocking irrespective of gap size, due to greater perennial competition and pasture height, and reduced photosynthetically active radiation in canopy gaps. The estimated critical gap size below which vulpia and barley grass seed production was prevented was approximately 2 cm diameter in both grazing systems. Timing of gap appearance influenced vulpia and barley grass establishment, but not tiller production, dry weight, survival or panicle production. Fewer vulpia and barley grass seedlings established when annual grasses were sown in June compared to April. The linear effect of perennial competition on vulpia growth, survival and seed production was investigated under controlled and field conditions by sowing vulpia seed at a range of distances from established perennial grass plants. While perennial neighbour competition reduced vulpia growth and panicle production, it did not prevent vulpia from producing seed when sown directly adjacent to a row of phalaris or cocksfoot neighbours (Dactylis glomerata L.). Cocksfoot plants suppressed vulpia growth and reproduction to a greater extent than phalaris plants, particularly in close proximity to the perennial neighbour. This was most likely because of cocksfoot plants being more compact, resulting in greater light competition when annual grasses grew in close proximity to cocksfoot neighbours. In addition, vulpia survival was much less under field than controlled conditions, showing that factors other than competition influenced annual grass survival in the field. Based on these results, it is unrealistic to expect that vulpia can be eliminated from pastures in southern Australia because the normal pattern of dry summers depletes pasture density and creates space that vulpia can exploit when soil moisture levels increase in autumn/winter. The critical gap size required to prevent vulpia seed production is small and vulpia populations are predicted to increase even when seed emerges immediately adjacent to perennial neighbours. However, management practices such as pasture rest, rotational grazing, fertiliser application and herbicide application, which increase perennial ground cover and reduce disturbance of perennial species, do allow some control of this weed. In conclusion, vulpia can best be managed in perennial pastures by a combination of increasing fertility levels in rotationally grazed pastures or pastures rested from grazing over summer, applying herbicide as a winter-cleaning treatment, and over-sowing ryegrass or other competitive species to fill in canopy gaps.