Competition for soil water and nitrogen between Holcus lanatus L. and young Eucalyptus globulus Labill.

Competition for below-ground resources was investigated between young Eucalyptus globulus Labill. (Tasmanian blue gum) and Holcus lanatus L. (Yorkshire fog grass) in a plantationin SE Tasmania, Australia. The objectives of the study were (1) to measure the competitive effect of weeds on the growth of E. globulus under contrasting levels of soil water and nitrogen availability and (2) to determine the 'critical period' of competition for soil water and nitrogen in a developing stand of E. globulus. This information lead to improved understanding of the mechanisms of competition between H. lanatus and young E. globulus. A two hectare field experiment was established on a low rainfall (500 mm yr-1 ), ex-pasture site with a sandy soil of low to moderate fertility. Two levels of both irrigation and nitrogen (N) fertiliser (urea) were applied to provide contrasting soil water and nitrogen levels. A range of periods of weed presence and absence (following the Critical Period method) were used in specific combinations of water and nitrogen availability. Weedy plots were rapidly dominated by H. lanatus, a very competitive grass, that had up to 100% cover and grass biomass production of 9 - 14 t ha-1 one year after planting E. globulus. There were strong responses to nitrogen fertiliser in the weedy treatments, but there were only small responses to irrigation, regardless of weed status. Lateral root growth of E. globulus was severely restricted by the vigorous development of grass roots that completely dominated the surface soil horizons. After two years, E. globulus height and diameter growth was up to 3 and 4.8 times greater, respectively, in weed-free plots compared to trees growing with grass. Where grass was present, there was no tree growth response to level of irrigation, but diameter was 1.8 times greater due to N-fertilisation, which indicated more severe competition for N in the unfertilised treatment, even though grass growth was also less vigorous in this treatment. Growth recovery after the application of weed control was also slower in the unfertilised treatment, and was influenced by season, with faster tree recovery following spring compared to autumn weed removal. Although there was no significant growth response to irrigation, the response to nitrogen in weedy treatments was stronger with the higher level of irrigation indicating a water by nitrogen interaction in the surface soil. The critical period for grass control at this site was from planting to age 20 months to avoid growth reduction caused by grass competition. There was no early period after planting when weeds could be tolerated by the young eucalypts. Most of the growth suppression occurred during the first 12 months, while maximum diameter growth (80%) was approached after approximately one year of weed control. These effects on growth were still evident 4.4 years after planting. Grass growth reduced surface soil water content during the first year and mild to moderate tree water stress (˜ìv†max< - 0.55 MPa) occurred on a number of occasions during the first 18 months of growth. There was high availability of water for plant use in the top 1 m of soil for most of the period, reflecting the overriding influence of the shallow water table at the site. However, in one block on the drier part of the site, available water was reduced by up to 80% due to grass presence during the first year, and this area exhibited the greatest degree of water stress. Soil nitrogen was strongly affected by grass presence, with NH4 + concentrations higher under grass, while NO3- was lower. Rates of nitrification and leaching were high in the surface soil in weed-free conditions. The application of nitrogen fertiliser ( NHigh) caused rapid increases in the concentrations of NH4+ and NO3 - in soil solution especially in weed-free plots, with greater increases after spring compared to autumn application. Both NH4+ and NO3- concentrations decreased to low values (<0.10 mM) during the first winter after planting (age six to nine months) except in Nhigh weed free plots. Thereafter, mineral N concentrations were variable with wide fluctuations that were probably related mostly to season, temperature, moisture and grass presence. Concentrations of N in foliage reflected levels of soil available nitrogen. Trees grown with grass had half the foliar nitrogen concentration of trees growing under weed-free conditions early in the first year. Concentrations increased following the addition of nitrogen fertiliser, and were equivalent to those growing under weed-free conditions by the end of the first year. The increased nitrogen status did not compensate for the lost growth. Vector analysis demonstrated that the competitive effects were mediated by water and nitrogen. Evidence from this research suggests that competition between H. lanatus and young E. globulus at this site was mainly for nitrogen, and to a lesser extent soil water during the first 12 ‚ÄövÑvÆ 18 months of tree growth. Where water was non-limiting in surface soil, competition for nitrogen was high, while in the drier part of the site, both water and nitrogen were limiting in the presence of grass. The use of the critical period method has provided a definition of the period when grass should be controlled to avoid E. globulus growth losses and has indicated the potential importance of an interaction between nutrient and water availability to the expression of the response. This research indicates weed control requirements for E. globulus on ex-pasture sites can be determined on the basis of site fertility and moisture regime. In this way operations can be tailored on a soil and site basis. The trade-off will be increased management complexity, however, improved knowledge and understanding of these factors enhances more effective and efficient plantation weed management. The extension of this approach to other species (weeds and trees) is also possible.