Relative,but not absolute,thinning responses decline with increasing thinning age in a Eucalyptus nitens plantation

Growth rates of trees and stand structure change as stands age, and therefore absolute and relative thinning responses may also vary with thinning age. The study examined whether thinning age influenced the relative and absolute thinning responses in Eucalyptus nitens plantations near Carrajung, Victoria, Australia, and whether this effect was influenced by nitrogen (N) fertiliser application. Two levels of thinning and fertiliser application were applied in a factorial design replicated three times in a randomised block layout in each of two trials established in September 2006 in adjacent plantations aged 3.2 y and 13.2 y respectively and from the same seed source. Treatments included: unthinned or thinned non-commercially from below to 300 trees ha^–1; fertiliser treatments were nil or 300 kg ha^–1 N as urea in the younger trial and nil or mixed fertiliser supplying 256 kg ha^–1 N, 80 kg ha^–1 P and 100 kg ha^–1 K in the older trial. Five years after thinning, basal areas of the largest 200 sawlog crop trees ha^–1 (SCTs) were about 4.1 m² ha^–1 greater in thinned than in unthinned stands, regardless of thinning age, and volumes of SCTs were 30.1 m³ ha^–1 greater in thinned stands. As a result of the greater standing basal area and volumes in the older trial, relative thinning responses of SCTs were greater in the younger trial, such that thinning increased the basal area of the SCT₂₀₀ by about 42% (and volume by 32%) in the younger trial, but by only 21% (and volume by 17%) in the older trial. Fertiliser application also increased absolute thinning responses. Smaller relative thinning responses in the older stand were associated with age-related changes in stand structure of unthinned stands, including increasing skewness and decreasing kurtosis of diameter distributions as stands aged. The absence of a thinning-age effect on absolute responses suggests that there is some flexibility in the thinning age in E. nitens plantations. However, it is important to note that while the absolute thinning response may be slow to decline, the size of the SCTs will be smaller after later-age thinning owing to the extended period of competition they experience prior to thinning.     

HIGHER FORESTRY EDUCATION IN AUSTRALIA

The results of three field trials which were established in 1976 to determine the feasibility of increasing wood production of Pinus radiata by fertilisation of thinned stands are described after four years of growth following the fertilisation treatments. The stands of about 20 years of age were producing an average of 14 m³ ha^?1 stem-wood per year.

The extra volume of wood growth resulting from fertilisation averaged 4.2 m³ ha^?1 a¹ for all sites, and 5.9 m³ ha^?1 a^?1 for the two most responsive sites, the latter being 37% over that obtained from unfertilised stands. Fifty per cent of the extra volume was added to the largest one-third of trees in the stands. A dominant tree of diameter (bhob) 27.0 cm in 1976 became 30.7 cm in 1980 if unfertilised and 32.8 cm if fertilised, an increase of 2.1 cm.

The responses obtained were inversely proportional to both the relative growth rates of unfertilised stands following thinning, and to the nitrogen concentration in the foliage of unfertilised trees measured in the year following fertilisation. Some soil/plant relationships have been examined for nitrogen, phosphorus, potassium and sulphur.

Fertilisation at this age appeared to at least ‘break-even’ if costs were amortised at an interest rate of 6%. However, fertilisation may also be a useful means of increasing yields at relatively short notice and allow increased flexibility in harvesting and other forest operations.     

Thinning Pinus radiata plantation by operator selection: 1. Effects on the residual forest

Field trials were undertaken in Pinus radiata plantation, in order to compare thinning where the harvesting machine operator selects the trees to be thinned without prior tree marking, with conventional tree-marked thinning, in terms of stand and tree parameters such as residual stocking and spacing, basal area, mean tree diameter, and frequency of damaged trees. These trials included first and second thinning on high and low site quality and two trials at operational scale. On the whole, operators thinned to the same stand density and tree diameter distribution as tree-marked thinning in comparable stands. The removal of undesirable trees was generally as good as in tree-marked thinning. In two trials operator selected thinning resulted in a better quality residual stand than tree-marked thinning. Trials at the operational level also indicate practicalities of two types of guidelines for operator selection thinning: a diameter limit; and spacing of residual trees.     

Effects of fertiliser applications after thinning of Pinus radiata on a high-elevation site in south-eastern Australia

A trial to assess the effects of fertiliser applied after thinning was established in a ten-year-old Pinus radiata stand growing on a high elevation site in south-eastern Australia. The trial had two components. The first studied the effects of fertilisers applied in a factorial design with two levels of phosphorus (0 and 50 kg P ha^–1) and three of nitrogen (0, 150 and 300 kg N ha^–1) plus a mixed nutrient treatment to areas thinned to a routine intensity. In the second part, levels of thinning intensity were assessed with and without a fertiliser (N₁P₁) application. Over the seven years of the study, growth was affected by below average rainfall in four of the years. Significant responses to fertiliser were achieved with all treatments; the best was 150 kg N ha^–1 + 50 kg P ha^–1 plus trace elements. The volume increment was increased by 28%, however this was not significantly different from the N₁P₁ alone. The N₁P₁ fertiliser increased productivity at all thinning levels including the unthinned treatment. Models predicting relative growth response based on foliage nutrient levels gave reasonable predictions, that is, 29% increase in volume periodic increment and this was slightly higher than that achieved. On such low rainfall sites, rather than having multiple thinnings, one option is a relatively heavy thinning at a young age with fertiliser application and then leaving the stand until final harvest. Estate optimisation models provide forest managers with the means to evaluate fertiliser-thinning treatment options against objectives such as maximising the present value of expected future cash flow within production and budget constraints.     

Merchantability of Pinus radiata in relation to previous land use

An integrated thinning operation was carried out in 21-year-old radiata pine stands in the central tablelands of New South Wales to examine the effect of previous land use and associated problems of poor tree form on recovery of merchantable timber. The areas thinned were contiguous sections of plantation which had different types of land use prior to plantation establishment; native forest, cleared grazing land (pasture) and cleared land initially grazed and subsequently cultivated and cropped (cultivated pasture). Total volumes of harvested trees (to 8 cm small end diameter) were estimated (assuming no poor tree form) from regressions relating volume to stem diameter. This estimate was compared with the actual volume of saw logs and pulp wood recovered on site and with merchantable yields predicted using a locally derived Forestry Commission equation.

On-site yields of saw logs plus pulp wood were 143 m³ ha^?1 from the ex-native forest site, 191 m³ ha^?1 from the ex-cultivated pasture site and 210 m³ ha^?1 from the ex-pasture site. This represented a total recovery of more than 90% of the total stem volume to 8 cm small end diameter from ex-native forest and ex-pasture trees compared with 73% from the ex-cultivated pasture trees.

Of the trees harvested, 45% from the ex-native forest site had stem defects compared with 65% from the ex-pasture site and 88% from the ex-cultivated pasture site. These defects reduced the average saw log volume per tree by at least 50% in trees greater than 35 cm DBHOB. In contrast, the yield of pulpwood was similar for deformed and non-deformed trees because some logs which did not meet saw log specifications were down-graded to pulp logs.     

Species shade tolerance affects tree basal area growth behaviour in two eucalypt species in thinned and unthinned even-aged monoculture

Individual tree and stand basal area growth were studied in a thinning experiment established over two sites in subtropical eastern Australia. The experiment compared the less shade tolerant species Eucalyptus dunnii with the more shade tolerant Corymbia citriodora var. variegata, both grown in plantation monoculture. Growth was compared over a 2½?3 year period that immediately followed thinning at 5½?7½ years of age. Towards the end of the growth period, tree crown depths and leaf mass per unit area were measured at one site. Crown depth and leaf mass per area of C. citriodora were greater than of E. dunnii, characteristics consistent with a more shade tolerant species. Gross stand basal area growth of E. dunnii was faster than that of C. citriodora. Individual-tree basal area growth behaviour was consistent with inter-tree competition involving asymmetric competition for light. Opening the stand by thinning reduced inter-tree competition to near zero. In unthinned control stands, smaller, more shaded trees of C. citriodora were better able to retain their basal area growth rates than those of E. dunnii in the face of competition. In consequence, the more shade tolerant C. citriodora developed a more evenly spread frequency distribution of tree diameters than less shade tolerant E. dunnii. This would have consequences for the assortment of log sizes that would be available ultimately from monocultures of the two species.     

AUSTRALIAN NEWSPRINT

A summary of the main conclusions resulting from twenty years of thinning research in slash and loblolly pines is given, and important principles are illustrated.

Competition among the components of a stand commences at a very early age.

Thinning on a commercial basis to 350 stems per acre at 12–15 years is recommended for 8 by 8 ft. plantings, and to 450 per acre at 13–15 years in stands planted at 7 by 7 ft. The reason for the difference is explained. A second thinning to 240 stems per acre is required five or six years later.

If merchantable thinning is likely to be delayed, early non-commercial thinning is desirable. Thinning to 400–450 stems per acre at 3–4 years is advocated.

No plantations are old enough to have been thinned more than twice, except experimentally. Subsequent thinnings are discussed, and it is probable that they will be designed to maintain the basal area of the stand within the range of 100–140 sq. ft. per acre; crop stems may be allowed to develop beyond this figure before the final cut.     

SOME ASPECTS OF RINGBARKING IN ALPINE ASH FORESTS

The two methods for removing useless overhead cover following final fellings in eucalypt forests, namely felling and ringbarking, are compared, with reasons for and against each method given.

Some estimates are made of the durability of ringbarked trees in the alpine ash forest.

Ringbarked trees and their influence on the surrounding stand are discussed.

The influence of old seed trees and veterans on surrounding young stands is given, with actual examples from Bago S.F. 560, N.S.W.     

THE PROPORTIONS OF STAND MERCHANTABLE VOLUME BY LOG SIZE CLASSES IN PINUS RADIATA (D. Don)

Warren has developed a relationship for even aged stands of P. radiata in plantations in New Zealand whereby from the stand mean diameter over bark at breast height there can be derived the percentage of stand volume (up to a diameter limit of 4 or 6 in.) within various classes of log diameter (small end under bark).

The author discusses, for certain even aged plantation stands of P. radiata in Australia, the development and application of a similar relationship for thinned stands and thinnings, provided the distribution of the trees by diameter over bark at breast height approximates a normal distribution.     

Managing wood production from small grower acacia hybrid plantations on eroded soils in central Vietnam

An experiment in a second rotation acacia hybrid (Acacia mangium × A. auriculiformis) clonal plantation in central Vietnam examined factors determining total wood production and its apportionment to sawlog and pulpwood. Growth and stand characteristics were compared with those in nearby plantations owned by small growers. The experimental site was on sloping land damaged and eroded by war and previous land use. The soil was shallow, with 20–30% laterised stones by volume and had lost part of the A horizon. Mean annual increment (MAI, in standing merchantable volume over bark) of the first rotation at age 8.8 years was 17 m³ ha^?1 y^?1. In the second rotation at age 7.6 years it was 20 m³ ha^?1 y^?1. Application of P fertiliser at three rates ranging from 14 to 86 kg ha^?1 increased stem diameter over the second rotation although stand volume response to P was not significant by rotation end. Potassium (14 kg ha^?1) gave no growth response. Growth rate was similar under weed control by herbicide and manual means. MAI increased progressively from 16.5 m³ ha^?1 y^?1 at the hilltop to 25.0 m³ ha^?1 y^?1 at the lowest part of the experiment; slope position influenced growth more than management practices. Key soil properties, pH, N, Bray-1 P and organic C were maintained from the first to the second rotation. With simple management practices including conservation of site organic matter and early stand management, appropriate for local small growers, the second rotation yielded 46% of harvested volume as sawlogs and the balance as pulpwood. Although small growers achieved similar growth rates, their current management regime does not yield sawlogs. They can influence the log categories produced and stand value through changes to stocking rate, stand management and rotation age.     

AN ESTIMATE OF THE TIMBER RESOURCES OF NEW BRITAIN

Mechanical thinning of mountain ash regrowth stands in southern Australia offers the potential for harvesting substantial amounts of pulpwood for industry. In 1988/89 a trial operation using feller-buncher, processor, and forwarder demonstrated the general feasibility of this. The present study examined the productivity of an improved operation developed from the initial trial and conducted during 1990/91, with subsequent site follow-up. Two stands of regrowth ash species (Eucalyptus delegatensis and E. regnans) were thinned using a feller-processor and forwarder under rules prescribed by the managing forestry agency. The stands were 18–19 years of age and located near Toolangi (Victoria). The study derived considerable information on rates of production, damage, and cost. It showed there were relatively few environmental effects associated with the felling and processing, but that forwarding could impose a considerable number of machine passes on the soil. Major problems in the operation were the amount of residual hardwood debris from past logging, and keeping damage to retained stems below acceptable levels. Changes in the operating rules have the potential to reduce costs from $38 m^?3 to $27 m^?3 These involve allowing the feller processor to work during wet weather, allowing accumulations of wood stored within the forest and at landings, and concentrating both forwarding and trucking at times of good weather. These changes reflect the low impact of the felling-processing and the high productivity of the forwarders. The study showed that it was not economic to take material of less than 150 mm dbh, and if such trees are to be felled on silvicultural grounds it is more economic to thin to waste.     

Long-term growth responses to phosphatic fertilisers in a Pinus radiata plantation

The effects of phosphatic fertiliser on growth and nutrients in trials at different stages of stand development have been studied in second-rotation Pinus radiata plantation sites of Lidsdale State Forest, New South Wales. The sites were on phosphorus-deficient clay soils. High, early growth responses to phosphatic fertiliser occurred when applied to individual trees at the time of planting compared with slower initial growth when fertiliser was broadcast applied. There were long-term responses to broadcast treatments of phosphorus or nitrogen/phosphorus at time of planting, but it took several growing seasons for the trees to access the applied phosphorus. Later-age applications of phosphorus also gave significant responses with and without nitrogen and the level of response was related to the prior nutritional status of the stand. There were additional responses to high levels of nitrogen when the initial phosphorus deficiency was overcome. Application of phosphorus or nitrogen/phosphorus after thinning resulted in significant increases in growth. Combining trial results and assuming treatments were additive, it was concluded that productivity could be increased from 8 m³ ha^–1 y^–1 to about 16 m³ ha^–1 y^–1 over 24 years. The trials show there could be a range of nutritional management strategies where fertilisers are applied at various times in the rotation process and the appropriate one would be selected in terms of management objective, the cost and outcomes.     

TAPER IN PINUS PINASTER

In dealing with taper over bark in young stands of Pinus pinaster of 15 years of age on the Coastal Sands at Ludlow in Western Australia, after introductory remarks on locality and thinning grade, the effect on taper of variation in age, breast—high girth, and canopy class is discussed. The influence of thinning on taper when all these factors are held constant is then considered.

It was found that: —

Taper decreases with age for a given breast-high girth.

In trees of the same age taper increases with greater breast-high girth.

In trees of the same age and the same breast-high girth the apparent variation in taper with canopy class was not shown to be significant on the data.

In trees of the same age, breast-high girth and canopy class, taper did not increase with wider spacing.     

The effect of tree spacing on the production of flowers in Eucalyptus nitens

The effect of spacing on the production of flowers and capsules in Eucalyptus nitens was studied in two spacing trials located in northern Tasmania. Tree density in these trials ranged from 468 to 4216 stems ha^?1. Reproductive structures were collected in litter traps and these data were used to calculate reproductive output on a per tree and per hectare basis. Between 8735 (5-y-old site, 1333 stems ha”¹) to 234098 (13-y-old site, 1082 stems ha”¹) flowers were produced per hectare over a single flowering season at these two study sites at the planting densities expected of a pulpwood plantation. This represented 8 and 211 flowers per tree respectively. As tree density decreased, the production of flowers and capsules increased on both a per-tree and per-hectare basis. It is estimated that the number of flowers per hectare is likely to be anywhere between between 1.4 and 10-fold greater under the spacing expected in sawlog regimes (250 trees ha^?1). This difference in reproductive output between plantations of E. nitens that use different spacing regimes is one of the many factors that need to be considered in assessing the risk of wilding establishment or hybridisation with adjacent native eucalypts.     

Differences between wildfire and clearfelling on the structure of montane ash forests of Victoria and their implications for fauna dependent on tree hollows

Some differences between the effects of clearfelling and wildfire on the vegetation structure of montane ash forests in the Central Highlands of Victoria are reported. The significance of these differences is considered in terms of their possible impacts on wildlife, particularly those species which are dependent on hollows in trees.

Distinct differences between the vegetation of montane ash forests which have been clearfelled and those burnt by wildfire include: –(1) the range in ages of trees in a stand, and, (2) the spatial pattern, abundance and longevity of hollow-bearing trees. In stands burnt by wildfire, the longevity of hollow-bearing trees may be more than 50 years and several forms of such trees can be present. These trees are removed from clearfelled forest. In unlogged stands and stands regenerating from selective logging the spacing of hollow-bearing trees was typically regular in distribution. In contrast, the spatial distribution of hollow-bearing trees was clustered in stands which had regenerated after clearfelling.

These differences may have a significant detrimental impact on wildlife and lead to a reduction in the abundance of some vertebrates in a given area of forest. Species such as arboreal marsupials, owls and cockatoos, which are reliant on a hollow-bearing trees for shelter and breeding, will probably be those most affected.     

Mortality of Eucalyptus marginata (jarrah) and E. calophylla (marri) trees following stem injection: effects of herbicide,dose, season,and spacing of injections

Mortalities of Eucalyptus marginata Donn ex Smith (jarrah) and E. calophylla R.Br, (marri) trees injected with Roundup^TM (360 g glyphosate l^?1 ) or Tordon Timber Control^TM (50 g picloram l^?1 and 100 g triclopyr l^?1 ) (P+T) were determined at doses of 5 to 80 grams of active ingredient (g.a.i.) m^?2 of stem basal area for glyphosate, and 2 to 40 g.a.i. m^?2 for P+T. The effect of spacing of injections around the tree bole was examined. Seasonal variation in mortality was examined in relation to atmospheric vapour pressure deficit (v.p.d.) and soil water deficits.

Glyphosate was more effective in killing E. marginata trees, whereas P+T tended to be more effective in killing E. calophylla trees. The optimum doses of glyphosate and P+T for E. marginata trees were 20 g.a.i. m^?2 and 15 g.a.i. m^?2 respectively, and for E. calophylla 80 g.a.i. m^?2 and 15 g.a.i. m^?2 respectively. Spacing of injections around the circumference of the tree did not have a significant effect on the mortality of E. marginata trees injected with glyphosate. However, mortality of E. calophylla trees injected with P+T declined from 89% at 10–15 cm spacing to 70% at 25–50 cm spacing.

There were strong seasonal effects with both herbicides and in both species, with mortality higher (70 - 90 % mortality) in seasons other than summer (30 - 50 % mortality). For E. marginata trees injected with glyphosate, mortality declined linearly with increasing v.p.d., from 95% at a v.p.d. of 0.5 kPa, to 25% at 4.5 kPa. For E. calophylla trees injected with P+T, mortality declined linearly with increasing v.p.d., from 85% mortality at a v.p.d. of 0.5 kPa, to 50% at 3.0 kPa. Stem injection of both species should only be carried out when the vapour pressure deficit on the day prior to stem injection is less than 1.5 kPa.     

Biomass and nutrient distribution in relation Pine in relation to previous land use I. Biomass

Above-ground biomass data are compared for 21-year-old radiata pine stands on three sites which had different types of land use prior to plantation establishment. The three sites, previously under native forest, grazed pasture and a pasture site subsequently used for crop production (cultivated) formed a continuous section of plantation in the 1968 age class. Biomass equations were developed for each tree component in relation to stem diameter for each site. There were no significant differences between sites in the coefficients for stemwood and bark, but different coefficients were required to relate stem diameter to the biomass of branches in the dead crown and foliage in the lower live crown. Stands on previously cultivated pasture (pasture followed by crops) accumulated 362,745 kg ha^?1 in the above-ground biomass; 40% and 13% more than the ex-native forest (259,183 kg ha^?1) and ex-pasture (320,055 kg ha^?1) sites, respectively. Trees on the previously cultivated pasture allocated a larger proportion of the total biomass to branches (17.4%) compared with the ex-native forest and ex-pasture trees (11.1%). This is attributed to reduced stocking and enriched soil resulting from different previous land use practices. The high frequency of multi-leadered trees in the ex-cultivated pasture increased the non-merchantable components to 26.7% of the total biomass.     

Eucalyptus growth in relation to combined nitrogen and phosphorus fertiliser and soil chemistry in Tasmania

Two experiments were established to determine the early nutrient requirements for plantations of Eucalyptus globulus (Nabowla site) and E. nitens (Westfield site) in Tasmania, Australia. Nitrogen (N) and phosphorus (P) were broadcast together, 2 and 26 months after planting, at cumulative rates of up to 1200 kg N ha^?1 600 kg P ha^?1. Soil chemistry was characterised at each site in three profiles, and in all plots at the Westfield site. Nabowla has a relatively infertile, poorly structured soil, low in N and P, and receives about 800 mm of rainfall per annum. Growth was very poor without fertiliser, but trees responded rapidly and strongly to added NP. Stem volume increased directly with the rate of applied fertiliser, with no evidence that the response had peaked at the highest rate. Even at the highest rate of NP fertiliser, however, productivity was relatively low (mean annual increment about 15 m ha^?1 y^?1). Establishment of plantations on such sites is unlikely to be economically viable, with or without fertiliser, but fertile ex-pasture sites that are otherwise similar should be considered. Westfield has a relatively fertile, well-structured soil, and receives 1400–1500 mm of rainfall per annum. Response to NP fertiliser was delayed at this site, with first responses measured 33 months after planting. The response in stem volume was sigmoidal in relation to fertiliser rate, with a plateau at an application rate of 400 kg N ha^?1 and 200 kg P ha^?1. This rate of fertiliser is expected to increase mean annual increment from 20 (without fertiliser) to 25 m ha^?1 y^?1. Splitting applications of fertiliser at the highest rate did not alter the growth response.

Although high rates of N and P fertiliser may be required to maximise growth of eucalypt plantations in Tasmania on ex-forest sites, rates required during the first few years might be lower than those reported here if the timing and placement of fertiliser is optimised. Soil chemical analyses were indicative of N and P requirements, and about a third of the variation in growth across the Westfield site was accounted for by natural variability in initial concentrations of total P and exchangeable K in surface soil.     

Wood properties of Eucalyptus globulus at three sites in Western Australia: effects of fertiliser and plantation stocking

Wood properties of 10-year-old trees in Eucalyptus globulus plantations at three sites in Western Australia were examined. Silvicultural treatments applied at age two years were stocking (unthinned, 1250 stems ha^–1; thinned to 600 stems ha^–1 or 300 stems ha^–1) and nitrogen fertiliser application (0 or 250 kg ha^–1 elemental nitrogen) in factorial combination. The three sites differed markedly in their annual rainfall (620–1100 mm), open-pan evaporation and soil water-holding capacity. Wood cores were collected at breast height from a total of 263 trees (~15 trees for each site-by-treatment combination), and radial samples prepared for analysis by SilviScan to produce radial profiles of air-dry density and microfibril angle (MFA) and modulus of elasticity (MOE), and by radial near infrared (NIR) surface scanning to produce radial profiles of NIR-predicted Kraft pulp yield (KPY) and cellulose content (CC). Sampling interval was 0.025 mm for density and 5 mm for the other properties. For wood property mean values (i.e. wood property averages of each pith-to-cambium sample), sites differed significantly only in air-dry density. The Boyup Brook site, which had low annual rainfall, the lowest climate wetness index and soil water-storage capacity and the slowest diameter growth, had the highest mean wood density (648 kg m^–3), while Scott River, with the highest rainfall, had mean density that was 10% lower. The Wellstead site (low rainfall but highest soil water-storage capacity) was intermediate for density. The only other significant differences for mean wood properties were caused by fertiliser addition, which reduced NIR-predicted KPY from 54.6% (without fertiliser) to 54.1% and predicted CC from 43.7% to 43.1%. Clear radial trends were evident for all wood properties. Density, MOE, KPY and CC all increased from pith to the cambium, while MFA declined. From the innermost (pith) 10% to the outermost (cambial) 10% of the radius, density increased on average by 21%, MOE by 103%, KPY by 9% and cellulose by 11%, while MFA declined by 47%. NIR calibrations developed using the SilviScan and NIR spectral data explained 71% of variance in MOE for an independent set of radial wood samples of E. globulus from Victoria, but less than 50% of variance for density and MFA. Implications for paper pulp and veneer manufacture are briefly considered.