CASE STUDY 16.2  Nutrient response in Eucalyptus grandis

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Robin Cromer

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Figure 1  A stand of Eucalyptus grandis ages 4.5 years at Toolara, near Gympie in Queensland. The trees in (a) received regular applications of N and P fertiliser for three years after planting and when the photograph was taken had a standing stemwood volume of 160 m3 ha-1. By comparison, the stand which received no fertiliser (b) had a volume of only 44 m3 ha-1 at the same age. (Photograph courtesy R.N. Cromer)

Eucalyptus grandis (flooded gum or rose gum) is native to high-rainfall and nutrient-rich sites in warm–temperate and sub-tropical zones along the east coast of Australia. This species has evolved in situations where fast growth conferred a selective advantage and that adaptative feature of this species has been used in commercial plantation forests around the world.

Data reported here represent part of a broader project to examine the feasibility of growing commercial plantations of E. grandis in Queensland for production of pulpwood. Experiments were established to determine the maximum production of stemwood that could be achieved where limitations due to nutrients and water were eliminated. Maintenance of a highly favourable environment for tree growth also provides ideal conditions for prolific growth of competing weeds, so early canopy closure to facilitate ‘capture’ of the site by the tree crop and thus suppress weeds was a major objective.

A range of nutrient treatments was applied at regular intervals during the first three years after planting to determine optimum rates for tree growth. In this case study, however, only results showing the highest rate of fertiliser application, with and without supplemental irrigation, are highlighted. In addition to a basal dressing containing K, S, Cu, Zn and B, a total of 1540 kg ha–1 of N with 460 kg ha–1 of P was applied in the heaviest treatment. This treatment was used to ensure nutrients did not limit growth, recognising that such high rates were far beyond economic levels of nutrient application. Irrigation was applied so that total natural precipitation plus irrigation was approximately equal to Class A pan evaporation.

Figure 1(a), (b) shows a stand of E. grandis aged 4.5 years at Toolara, near Gympie in Queensland, which received regular applications of N and P fertiliser (+F) during the first three years after the trees were planted. When the photograph was taken, the standing volume of stemwood was 160 m3 ha–1, compared with only 44 m3 ha–1 (see Figure 1b) for a nearby stand of the same age which received no fertiliser (–F).

Leaf area index (LAI) in +F stands increased rapidly to 4.5 during the first year after planting, and then became relatively stable at about 5 (Figure 2a). By comparison, LAI in –F stands increased quite slowly for about 18 months and then stabilised around 1.

These marked differences in LAI were nutrient driven and had a major impact on accumulation of stemwood volume during the first three years of tree growth (Figure 2b; Cromer et al. 1993). Trees did not experience water stress during the first 2.5 years after planting so there was no response to irrigation and data for irrigation treatments have not been presented. A dry period after that time resulted in some reduction of LAI in +F plots (to about 4, Figure 2a) but there was no significant response to irrigation until three or more years of age.

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Figure 2 Development of leaf area index (a) and stemwood volume (b) over time in Eucalyptus Grandis with (●) or without (Ο) fertiliser. Enhanced nutrient status enabled planted seedlings to accumulate leaf area rapidly and close canopy within 12 months after planting. The ability to capture a high proportion of available photosynthetically active radiation enabled trees to gain carbon and build stem volume rapidly. (Based on Cromer 1995 and Cromer et al. 1993)

Substantial differences in stemwood volume between +F and –F stands developed over the first three years after planting. However, large early differences in LAI were a major determinant of subsequent growth, and were clearly established well before the first measurement at 0.66 years (Figure 2a). Studies with seedlings and saplings (Section 6.2) indicate that changes in specific leaf area (SLA) due to nutrition are accompanied by greater photosynthetic capacity. Both features are important determinants of growth rate in seedlings and small trees but their influence declines rapidly as trees approach canopy closure.

Related observations on canopy extent and plantation biomass showed a strong relationship between net primary production (NPP) and mean LAI in each year of this three-year study. The relationship between NPP and LAI was independent of nutrient treatment, and demonstrated that area of foliage available to intercept radiant energy rather than canopy nutrient status was the more important factor driving productivity (see Section 12.4.2 for further discussion on underlying principles). In effect, application of fertiliser increased LAI substantially thus enabling useful interception of a high proportion of incident sunlight. After canopy closure, high radiation levels in this subtropical environment enabled +F stands to intercept in excess of 4 GJ m–2 year–1 of photosynthetically active radiation, driving NPP at rates of about 30 t ha–1 year–1. Such levels of wood production rank among the highest values ever reported worldwide (Miller 1989) and are particularly impressive for such a young plantation (trees less than three years old).

Positive interactions between genotype and nutrient supply have been observed in plantation foresty where trees from families characterised by inherently faster growth have showed a stronger response to added nutrients. To ensure that full expression of growth responses would be recorded in these present trials, seedlings were sourced from four different seedlots of E. grandis. A significant interaction between seedlot and fertiliser treatment was observed. Seedlots selected from plan-tations within the general climatic zone of the experiments (Coffs Harbour and Pomona) generally performed well and responded more strongly to fertiliser, whereas seed collected from native forest stands further south (Bulahdelah) or further north (Atherton) performed relatively poorly and did not respond so well to higher nutrient levels. By implication, superior genetic material could be selected for superior per-formance under well-nourished plantation conditions. As an extra bonus, the Pomona seedlot also responded to irrigation in combination with fertiliser after three years of age, such that this combination produced outstanding growth with a mean annual increment in stemwood volume of 50 m3 ha–1 year–1 once trees were five years old (Cromer 1995).

References

Cromer, R.N. (1995). ‘Fast-growing sub-tropical eucalypt plantations’, Onwood, 11 (Summer 1995–96), 4–5.

Cromer, R.N., Cameron, D.M., Rance, S.J., Ryan, P. and Brown, M. (1993). ‘Response to nutrients in Eucalyptus grandis: I Biomass accumulation’, Forest Ecology and Management,
62, 211–230.

Miller, H.G. (1989). ‘Internal and external cycling of nutrients in forest stands’, in Biomass Production by Fast-Growing Trees, eds J.S. Pereira and J.J. Landsberg, 73–80, Kluwer Academic Publications: Dordrecht.

 

 

 

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