Modelling the yield of Pinus radiata on a site limited by water and nitrogen

A process-based model is described and applied to a range of Pinus radiata D. Don stands, aged 9–12 years, growing on stabilised sand dunes in a stocking × fertiliser experiment in Woodhill State Forest, New Zealand. The model requires inputs of daily weather data (maximum and minimum air temperatures and rainfall), physical characteristics of the site (longitude, latitude, rootzone depth and relationship between root-zone soil matric potential and volumetric water-content) and crop (stocking, crown dimensions and leaf-area index) and crop physiological parameters (e.g., maximum stomatal conductance). The model was used to simulate components of the forest water-balance and annual net photosynthesis for a defined crop canopy architecture. Simulated daily root-zone water storage in both open and closed canopy stands generally agreed with monthly measurements made over a complete year. Simulated net annual photosynthesis ranged from 23 to 33 t C ha⁻¹ year⁻¹ and comparison with measured stem-volume increments of 12–38 m³ ha⁻¹ year⁻¹ over the same time periods resulted in a strong positive correlation. Ratios of stem-volume increment to net photosynthesis suggested that fertilised and unfertilised stands had a 26 and 14%, respetively, allocation of C to stem growth. Simulations using weather data for a dry year with 941 mm year⁻¹ rainfall indicated that annual net photosynthesis and transpiration of fully stocked stands were reduced by 41 and 45%, respectively, compared to those in a wet year with 153 mm year⁻¹ rainfall. Operational applications of the model to forest management in quantifying environmental requirements for stand growth and examining silvicultural alternatives are discussed.     

Nitrogen mineralization in irrigated plantations of shisham and mulberry

Ammonification and nitrification rates and nitrogen uptake were measured using the buried-bag technique in irrigated mixed plantations of shisham (Dalbergia sissoo Roxb. ex DC.) and mulberry (Morus alba L.). Nitrogen transformations were rapid in these stands, particularly following thinning to reduce stand density. In young stands, net N mineralization was 26.72 mg N kg⁻¹ soil month⁻¹ (approximately 480 kg ha⁻¹ month⁻¹), but, as the end of the 22-year rotation approached, nitrification slowed to 13.41 mg N kg⁻¹ soil month⁻¹ (approximately 241 kg ha⁻¹ month⁻¹). N₂-fixing shisham appeared to respond after thinning only to the increased space and temporarily reduced competition for light and moisture, but mulberry appeared to benefit greatly from the nitrogen released through mineralization following thinning.     

Biomass distribution and productivity ofPinus edulis—Juniperus monosperma woodlands of north-central Arizona

Above-ground biomass distribution, leaf area, above-ground net primary productivity and foliage characteristics were determined for 90- and 350-year-oldPinus edulis-Juniperus monosperma ecosystems on the Colorado Plateau of northern Arizona. These ecosystems have low biomass, leaf area and primary productivity compared with forests in wetter environments. Biomass of the 350-year-old pinyon-juniper stand examined in this study was 54.1 mg ha⁻¹; that of the 90-year-old stand was 23.7 mg ha⁻¹. Above-ground net primary production averaged 2.12 mg ha⁻¹ year⁻¹ for the young and 2.88 mg ha⁻¹ year⁻¹ for the mature stand; tree production was about 80% of these values for both stands. Projected ecosystem leaf area (LAI) of the stands was 1.72 m² m⁻² and 1.85 m² m⁻², respectively. Production efficiency (dry matter production per unit leaf area) was 0.129 kg m⁻² year⁻¹ for the young, and 0.160 kg m⁻² year⁻¹ for the mature stand. Production efficiency of the study sites was below the 0.188 kg m⁻² year⁻¹ reported for xeric, pure juniper stands in the northern Great Basin. Biomass of pinyon-juniper ecosystems of northern Arizona is generally below the 60–121 mg ha⁻¹ reported for pinyon-juniper stands of the western Great Basin in Nevada. A climatic gradient with summer precipitation decreasing between southeast Arizona and northwest Nevada occurs in the pinyon-juniper region. Great Basin pinyon-juniper ecosystems lie at the dry-summer end of this gradient while pinyon-juniper ecosystems of the Colorado Plateau lie at about the middle of this gradient. In spite of wetter summers, pinyon-juniper ecosystems of northern Arizona are less productive than those of the Great Basin.     

Dynamics and potentials of carbon sequestration in managed stands and wood products in Finland under changing climatic conditions

Carbon (C) sequestration was studied in managed boreal forest stands and in wood products under current and changing climate in Finland. The C flows were simulated with a gap-type forest model interfaced with a wood product model. Sites in the simulations represented medium fertile southern and northern Finland sites, and stands were pure Scots pine and Norway spruce stands or mixtures of silver and pubescent birch.

Changing climate increased C sequestration clearly in northern Finland, but in southern Finland sequestration even decreased. Temperature is currently the major factor limiting tree growth in northern Finland. In southern Finland, the total average C balance over the 150 year period increased slightly in Scots pine stands and wood products, from 0.78 Mg C ha⁻¹ per year to 0.84 Mg C ha⁻¹ per year, while in birch stands and wood products the increase was larger, from 0.64 Mg C ha⁻¹ per year to 0.92 Mg C ha⁻¹ per year. In Norway spruce stands and wood products, the total average balance decreased substantially, from 0.96 Mg C ha⁻¹ per year to 0.32 Mg C ha⁻¹ per year. In northern Finland, the total average C balance of the 150 year period increased under changing climate, regardless of tree species: in Scots pine stands and wood products from 1.10 Mg C ha⁻¹ per year to 1.42 Mg C ha⁻¹ per year, in Norway spruce stands and wood products from 0.69 Mg C ha⁻¹ per year to 0.99 Mg C ha⁻¹ per year, and in birch stands and wood products from 0.43 Mg C ha⁻¹ per year to 0.60 Mg C ha⁻¹ per year.

C sequestration in unmanaged stands was larger than in managed systems, regardless of climate. However, wood products should be included in C sequestration assessments since 12–55% of the total 45–214 Mg C ha⁻¹ after 150 years' simulation was in products, depending on tree species, climate and location. The largest C flow from managed system back into the atmosphere was from litter, 36–47% of the total flow, from vegetation 22–32%, from soil organic matter 25–30%. Emissions from the production process and burning of discarded products were 1–6% of the total flow, and emissions from landfills less than 1%.     

Forest structure and C stocks in natural Fagus sylvatica forest in southern Europe: The effects of past management

Managed forests often differ substantially from undisturbed forests in terms of tree structure and diversity. By altering the forest structure, management may affect the C stored in biomass and soil. A survey of 58 natural stands located in the south-westernmost limit of European beech forests was carried out to assess how the C pools are affected by the changes in tree structural diversity resulting from past management. The mean tree density, basal area and the number of large trees found in unmanaged forests were similar to those corresponding to virgin beech forests in Central Europe, whereas large live trees were totally absent from partially cut stands. Analysis of the Evenness index and the Gini coefficient indicated high structural diversity in the three stand types. The results of the Kolmogorov–Smirnov test used to compare the diameter distributions of each group revealed significant differences between stand types in terms of distributions of total tree species and of Fagus sylvatica.

The mean C stocks in the whole ecosystem – trees, litter layer and mineral soil – ranged from 220 to 770 Mg ha⁻¹ (average 380 Mg ha⁻¹). Tree biomass (above and belowground), which averaged 293 Mg C ha⁻¹, constituted the main C pool of the system (50–97%). The statistical test (Kolmogorov–Smirnov) revealed differences in the distribution of C pools in tree biomass between unmanaged and partially cut stands. As a consequence of the presence of large trees, in some unmanaged stands the C stock in tree biomass was as high as 500–600 Mg C ha⁻¹. In the partially cut stands, most of the C was mainly accumulated in trees smaller than 20 cm dbh, whereas in unmanaged stands the 30% of tree C pool was found in trees larger than 50 cm dbh. Furthermore, many unmanaged stands showed a larger C pool in the litter layer. The C content of mineral soils ranged from 40 to 260 Mg C ha⁻¹ and it was especially high in umbrisols. In conclusion, the implementation of protective measures in these fragile ecosystems may help to maintain the highly heterogeneous tree structure and enhance the role of both soils and trees as long-term C sinks.     

Nitrogen-fertilization impacts on carbon sequestration and flux in managed coastal Douglas-fir stands of the Pacific Northwest

We examined whether N-fertilization and soil origin of Douglas-fir [Psuedotsuga menziesii (Mirb.) Franco] stands in western Washington state could affect C sequestration in both the tree biomass and in soils, as well as the flux of dissolved organic carbon (DOC) through the soil profile. This study utilized four forest sites that were initially established between 1972 and 1980 as part of Regional Forest Nutrition Research Project (RFNRP). Two of the soils were derived from coarse-textured glacial outwash and two from finer-textured volcanic-source material, primarily tephra, both common soil types for forestry in the region. Between 1972 and 1996 fertilized sites received either three or four additions of 224 kg N ha⁻¹ as urea (672–896 kg N ha⁻¹ total). Due to enhanced tree growth, the N-fertilized sites (161 Mg C ha⁻¹) had an average of 20% more C in the tree biomass compared to unfertilized sites (135 Mg C ha⁻¹). Overall, N-fertilized soils (260 Mg C ha⁻¹) had 48% more soil C compared to unfertilized soils (175 Mg C ha⁻¹). The finer-textured volcanic-origin soils (348 Mg C ha⁻¹) had 299% more C than glacial outwash soils (87.2 Mg C ha⁻¹), independent of N-fertilization. Soil-solution DOC collected by lysimeters also appeared to be higher in N-fertilized, upper soil horizons compared to unfertilized controls but it was unclear what fraction of the difference was lost from decomposition or contributed to deep-profile soil C by leaching and adsorption. When soil, understory vegetation and live-tree C compartments are pooled and compared by treatment, N-fertilized plots had an average of 110 Mg C ha⁻¹ more than unfertilized controls. These results indicate these sites generally responded to N-fertilization with increased C sequestration, but differences in stand and soil response to N-fertilization might be partially explained by soil origin and texture.     

Nitrogen deposition in Casuarina equisetifolia (Forst.) plantation stands in the dry tropics of Sonbhadra, India

Periodic variations in the concentration, deposition and canopy impact of different forms of N on annual N deposition through rainfall, throughfall and stemflow in 5 and 8 year old stands of Casuarina equisetifolia were studied. Throughfall and stemflow ranged from 70 to 76% and 5–6% of annual precipitation respectively. The total N deposition by rainfall was 11.1 kg ha⁻¹ year⁻¹, and by throughfall was 13.6 kg ha⁻¹ year⁻¹ and 16.5 kg ha⁻¹ year⁻¹ in 5-year-old and 8-year old plantations, respectively. The quantities of N deposited through stemflow in the two plantations were nearly identical, accounting for 1.6 kg ha⁻¹ year⁻¹. Observations of the monthly deposition of NH₄,N, NO₃-N, Kjeldahl-N and organic-N revealed that maximum deposition occurred in July and the minimum in September. Organic-N deposition was 17% less (5-year) than the rainwater content. Net deposition of N, as an effect of canopy, was 7–8.7 kg ha⁻¹ year⁻¹, which was added directly to the available nutrient pool of soil.     

Structure, allometry, and biomass of plantation Metasequoia glyptostroboides in Japan

We quantified structural features and the aboveground biomass of the deciduous conifer, Metasequoia glyptostroboides (Hu and Cheng) in six plantations in central Japan. In order to derive biomass estimates we dissected 14 M. glyptostroboides trees into three structural components (stem wood, branch wood and foliage) to develop allometric equations relating the mass of these components and of the whole tree to diameter at breast height (DBH). We found robust relationships at the branch and whole tree level that allow accurate prediction of component and whole tree biomass. Dominant tree height was similar within five older (>40 years) plantations (27–33 m) and shorter in a 20-year-old plantation (18 m). Average stem diameter varied from 12.8 cm in the youngest stand to greater than 35 cm in the oldest stand.

Metasequoia have relatively compact crowns distributed over the top 30% of the tree although the youngest stand had the deepest crown relative to tree height (up to 38%). At the individual tree level in older stands, 87% of the aboveground biomass was allocated to the stem, 9% to branch wood and 4% to foliage. We found little difference in the relative distribution of above ground biomass among the stands with the exception of lower foliage biomass in larger diameter trees. Total aboveground biomass of the older stands varied twofold, ranging from a maximum of 450 Mg ha⁻¹ in a 42-year-old stand to a minimum of 196 Mg ha⁻¹ in a 48-year-old stand. Total above ground biomass of the 20-year-old stand was 176 Mg ha⁻¹.     

Biomass and carbon sequestration of ponderosa pine plantations and native cypress forests in northwest Patagonia

Fast growth tree plantations and secondary forests are considered highly efficient carbon sinks. In northwest Patagonia, more than 2 million ha of rangelands are suitable for forestry, and tree plantation or native forest restoration could largely contribute to climate change mitigation. The commonest baseline is the heavily grazed gramineous steppe of Festuca pallescens (St. Yves) Parodi. To assess the carbon sequestration potential of ponderosa pine (Pinus ponderosa (Dougl.) Laws) plantations and native cypress (Austrocedrus chilensis (Don) Flor. et Boutl.), individual above and below ground biomass models were developed, and scaled to stand level in forests between 600 and 1500 annual rainfall. To calculate the carbon sequestration baseline, the pasture biomass was simulated. Also, soil carbon at two depths was assessed in paired pine-cypress-pasture sample plots, the same as the litter carbon content of both forest types. Individual stem, foliage, branch and root log linear equations adjusted for pine and cypress trees presented similar slopes (P>0.05), although some differed in the elevations. Biomass carbon was 52.3 Mg ha⁻¹ (S.D.=30.6) for pine stands and 73.2 Mg ha⁻¹ (S.D.=95.4) for cypress forests, given stand volumes of 148.1 and 168.4 m³ ha⁻¹, respectively. Soil carbon (litter included) was 86.3 Mg ha⁻¹ (S.D.=46.5) for pine stands and 116.5 Mg ha⁻¹ (S.D.=38.5) for cypress. Root/shoot ratio was 19.5 and 11.4%, respectively. The low r/s value for cypress may account for differences in nutrient cycling and water uptake potential. At stand level, differences in foliage, taproot and soil carbon compartments were highly significative (P<0.01) between both forest types. In pine stands, both biomass and soil carbon were highly explained by the rainfall gradient (r²=0.94). Nevertheless, such a relationship was not found for cypress, possibly due to stand and soil disturbances in sample plots. The carbon baseline estimated in pasture biomass, including litter, was 2.6 Mg ha⁻¹ (S.D.=0.8). Since no differences in soil carbon were found between pasture and both forest types, additionality should be accounted only by biomass. However, the replacement of pasture by pine plantations may decrease the soil carbon storage, at least during the first years. On the other hand, the soil may be a more relevant compartment of sequestered carbon in cypress forests, and if pine plantation replaces cypress forests, soil carbon losses could cause a negative balance.     

Pre-commercial thinning effects on growth, yield and mortality in even-aged paper birch stands in British Columbia

The aim of this study was to quantify 5-year growth, yield and mortality responses of 9- to 13-year-old naturally regenerated, even-aged paper birch (Betula papyrifera Marsh.) stands to pre-commercial thinning in interior British Columbia. The study included four residual densities (9902–21,807 stems ha⁻¹ (unthinned control), 3000, 1000 and 400 stems ha⁻¹) and four sites with 3-fold within-site replication in a randomised block design. The largest, straightest, undamaged trees were selected to leave during thinning. Thinning reduced stand basal area from 5.90 m² ha⁻¹ in the control to 2.50, 1.53 and 0.85 m² ha⁻¹ in the three thinning treatments, representing 42, 26 and 15% of control basal area, respectively. After 5 years, total stand volume per plot remained lower in the three thinning treatments than the control (50.20, 30.07, 18.99 and 11.86 m³ in the control, 3000, 1000 and 400 stems ha⁻¹ treatments), whereas mean stand diameter, diameter increment, height, and height increment were increased by thinning, and top height (tallest 100 trees ha⁻¹) was unaffected. When a select group of crop trees (largest 250 trees ha⁻¹) in the thinning treatments was compared with the equivalent group in the control, there was a significant increase in mean diameter, diameter increment, basal area, basal area increment, and volume increment. Mean height, height increment, top height, and total volume were unaffected by thinning. Crop tree diameter increment was the greatest following thinning to 400 stems ha⁻¹ for all diameter classes. Thinning to 1000 stems ha⁻¹ resulted in lower diameter increment than thinning to 400 stems ha⁻¹ but tended to have higher volume increment. Dominant trees responded similarly to subdominant trees at 400 stems ha⁻¹, but showed the greatest response at 3000 stems ha⁻¹. Results suggest that pre-commercial thinning of 9–13-year-old stands to 1000 stems ha⁻¹ would improve growth of individual trees without seriously under-utilising site resources.     

Some nutrient dynamics associated with litterfall and litter decomposition in hoop pine plantations of southeast Queensland, Australia

Litterfall was collected over a 12-month period with littertraps in hoop pine (Araucaria cunninghamii) plantations aged 10, 14 and 62 years in southeast Queensland, Australia. The bulk of litterfall occurred during spring, mainly as hoop pine foliage with the annual litterfall ranging between 6.0 and 10.9 t ha⁻¹, respectively, for the younger stands (10 and 14 years) and the mature 62-year old stand. The amount of nitrogen (N) and phosphorous (P) recycled annually through litterfall was lower in the younger stands (28–37 kg N ha⁻¹ and 4.4–5.3 kg P ha⁻¹) compared with that of the mature stand (85 N ha⁻¹ and 6.2 kg P ha⁻¹). The N and P retranslocated during senescence varied across the three stands studied with a trend for N and P retranslocation to increase as availability of soil mineral-N decreased.

Decomposition of the hoop pine foliage component of litter was also studied in the same stands using a litterbag technique and mass-balance analysis. The estimated half-life of hoop pine foliage mass ranged between 1.5 and 1.8 years. Litter-mass loss was strongly correlated with litter substrate quality indicators of N, C, P, C/N ratio, lignin, lignin/N ratio and polyphenols. During the course of the study, there was no difference in litter-mass loss between the stands of different ages. During the 15-month period, the order of element release from the hoop pine litter was K>Na>C>Mg>P, with N, Ca and Mn generally demonstrating varying degrees of net accumulation. During the course of the study, the lignin/C ratio of the hoop pine litter increased from 0.61 to 0.96. This suggested that the litter-C was predominantly in a recalcitrant form and, therefore, the associated N was unlikely to be rapidly released in the hoop pine litter layer.     

Interaction between moisture, nutrients and growth of white spruce in interior Alaska

Two thinning and fertilization studies, the first in 1969 and the second in 1971, were established to evaluate the question of nutrient limitation to tree growth and the consequences of stand manipulation of soil moisture supply. Fertilizer was applied yearly for the first 5 years in both studies; growth response has been measured through 1987. Results indicate that thinning is necessary to obtain a growth response to fertilizer applied at the rate of 111 kg nitrogen ha⁻¹. The response to fertilization after fertilization ended lasted for 4 years in plots thinned to 800 stems ha⁻¹, while a significant response continued for only 2 years in plots thinned to 1600 stems ha⁻¹.

A soil water-balance model was calibrated for the control and treatment plots of these two studies. Soil water-deficits were estimated and correlated with yearly average basal-area growth per tree. Results indicated that there is a correlation between seasonal soil-moisture deficit and growth during the years when soil moisture was measured for the unthinned control plots (r² = −0.787, P = 0.002) but not for the thinned and fertilized plots (r² = −0.652, P = 0.057).     

Effects of harvesting intensity on carbon stocks in eastern Canadian red spruce (Picea rubens) forests: An exploratory analysis using the CBM-CFS3 simulation model

Carbon stocks and stock changes in a chronosequence of 24 red spruce (Picea rubens Sarg.) dominated stands in Nova Scotia, Canada, were compared against predictions from the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3). Regression analysis of the observed versus simulated total ecosystem C stocks indicates the model's predictions accounted for 81.1% of the variation in the observed biomass data and for 63.2% of the variation in total ecosystem C data; however, the simultaneous F-test for bias was significant. Discrepancy between the observed and simulated total ecosystem C data was primarily caused by differences in dead organic matter C pool estimates, with the model consistently predicting higher soil C throughout stand development. Changes to model parameters were not warranted however, as the field data measured only a portion of the mineral soil profile represented in the model.

Clear-cut and partial-cut harvesting scenarios for red spruce stands were simulated to examine the impacts of clear-cut and partial-cut harvesting on C stocks. Total ecosystem C increased in the partial-cut stand throughout the 240-year simulation from 308.9 to 327.3 Mg C ha⁻¹, while it decreased in the clear-cut stand to 305.8 Mg C ha⁻¹. Enhanced C sequestration in the partial-cut stand was a consequence of the residual standing biomass providing a continuous source of litterfall and reducing decomposition rates of the forest floor. Choice of harvest system clearly affects forest ecosystem C stocks, but also affects the amount of C removed from forests to meet society's needs. Over the period of the simulation, partial cutting provided 115.6 Mg C ha⁻¹ of merchantable timber, while clear-cutting provided 132.4 Mg C ha⁻¹. Strategies aimed at using forest management to mitigate atmospheric C increases need to assess both the direct impacts on the forest ecosystem and the indirect impacts through product and energy substitution associated with the use and storage of harvested biomass.     

An empirical cohort model for management of Terra Firme forests in the Brazilian Amazon

A model to project forest growth in the Terra Firme forests of the eastern Amazon is described. It is based on 12–17 years measurements from experimental plots at Jarí and Tapajós. Forest stands are represented by cohorts of species group, diameter, and defect. There are 54 species groups, with a robust diameter increment function fitted to each, tables of mortality by crown and defect status, and recruit lists by disturbance level and locality. Stand level functions partition trees by crown status, and modify growth for stand density. Recruitment is a function of basal-area losses. Evaluation compares model performance with two experiments involving heavy felling in Tapajos State Forest. At one site, total bole volume growth of all species over 45 cm DBH was 2.56 m³ ha⁻¹ year⁻¹ over 17 years, whereas the model projected 3.13 m³ ha⁻¹ year⁻¹. At the other site, actual growth over 12 years was 0.39 m³ ha⁻¹ year⁻¹, with the model giving an identical result. Both felled and control plots are compared in the study and accurately simulated. Some weaknesses in the model are discussed.     

Deposition of atmospheric constituents and its impact on nutrient budgets of oak forests (Quercus petraea and Quercus robur) in Lower Austria

Geochemical processes in central European oak ecosystems (Quercus petraea and Quercus robur) suffering stand decline were studied in two oak stands of the Weinviertel, Lower Austria, about 30 km north of Vienna. Stores of chemical elements were determined by soil and biomass inventories. Deposition input was monitored over a 2 year period by bulk sampling of throughfall. Soil solution chemistry was studied by tension lysimetry over a 1 year period. Mineral nutrition of oak was judged by foliar analysis. Bulk deposition rates were 10–12 kg N ha⁻¹ year⁻¹, and 15–20 kg S ha⁻¹ year⁻¹. Total annual nitrogen gain is high. Both systems lose calcium and magnesium. Foliar nutrient levels indicate sufficient nutrition with main mineral nutrients, except for magnesium, which is in moderately low supply. Based on these findings, the hypothesis that pollutant deposition has been the cause of a sudden and severe appearance of decline symptoms in the second half of the 1980s must be dismissed. The data on deposition rates and ecosystem nutrient status, however, indicate that the soil of both systems is acidifying, nitrogen stores are increasing, and magnesium pools are depleted. If deposition of pollutants continues at current rates, a slow but steady degradation of many oak ecosystems in the Austrian Weinviertel is inevitable.     

Nutrient accumulation and role of atmospheric deposition in coniferous stands

Three stand types on drained wetlands, all 31 years old, were studied. The stands were: (1) Scots pine, unfertilized; (2) Scots pine, fertilized; and (3) Norway spruce, fertilized. Amounts of nutrients (N, K, Ca, Mg, P, S, B, Fe, Mn, Zn, Cu) in above-ground biomass for all three stand types could be simulated precisely by a curvilinear regression model, with stand volume on bark as regressor. Net H⁺ production of the fertilized pine was estimated to be 661 mol H⁺ ha⁻¹ year⁻¹ from establishment to 31 years of age. The corresponding value for spruce was 1232 mol H⁺ ha⁻¹ year⁻¹. Atmospheric inputs to the pine and spruce sites were 695 and 516 mol H⁺ ha⁻¹ year⁻¹, respectively. Atmospheric input of N was 2.3 and 1.3 times the accumulation in the biomass of unfertilized and fertilized pine, whereas the value for spruce was 0.7. The corresponding ratios for S were 43, 19, and 11.     

Growth, biomass production and nutrient accumulation by seven tree species irrigated with municipal effluent at Wodonga, Australia

In the Murray-Darling basin, irrigation of tree crops is being evaluated as an alternative method for the disposal of municipal effluent. A study was carried out at Wodonga in which seven tree species were irrigated with effluent for a period of 4 years. Irrigation was calculated weekly on the basis of pan evaporation and rainfall during the preceding week. Annual irrigation varied between 1190 mm and 1750 mm with a total input over the 4-year-period of 4940 mm.

Height and diameter growth varied significantly between species. At age 4, mean dominant height of Eucalyptus grandis, E. saligna and Populus deltoides × P. nigra ranged from 14.3 to 15.0 m compared with 6.6 to 9.8 m for Casuarina cunninghamiana, E. camaldulensis, P. deltoides and Pinus radiata. Wood production of the faster-growing species (E. grandis and E. saligna) was approximately 130 m³ ha⁻¹, or around 32 m³ ha⁻¹ year⁻¹ over a 4-year period. This was nearly three-fold the production of the other native species and twice that of Pi. radiata. Volume growth of P. deltoides × P. nigra (85 m³ ha⁻¹) was significantly greater than that of P. deltoides (42 m³ ha⁻¹).

Accumulation of nutrients in the above-ground biomass varied significantly between species and ranged from 24 to 41 g m⁻² for N, 2.6 to 5.9 g m⁻² for P, 0.5 to 9.2 g m⁻² for Na, 12 to 27 g m⁻² for K, 7 to 52 g m⁻² for Ca and 3.1 to 7.9 g m⁻² for Mg. Nutrient accumulation was generally greater in species with a comparatively large crown biomass relative to stem size such as C. cunninghamiana and E. camadulensis. Average nutrient accumulation by trees as a percentage of input from effluent was estimated at 19% for N, 9% for P, 1% for Na, 14% for K, 52% for Ca and 32% for Mg.

Results of this study indicate the importance of selecting species on the basis of not only growth but also nutrient accumulation to optimise renovation of wastewater by tree plantations.     

Stand growth scenarios for Tectona grandis plantations in Costa Rica

Management scenarios with rotation lengths of 20 and 30 years were developed for different site qualities (high, medium and low) under two different management options (high individual tree growth versus high stand growth) for teak (Tectona grandis L.f.) in Costa Rica. The scenarios are based on data collected in different regions in Costa Rica, representing different site conditions, offering a variety of possible management options for high-quality teak yield.

Three competition indices were used for modeling the competition and for the definition of intensities and the plantation age at thinning. The maximum site occupation (MSO) and the Reineke density index (RDI) provide conservative stand density management limits, resulting in the need to execute several thinning frequently. The competition factor (CF) matches the field observations and seems to be more appropriate for the growth characteristics of the species.

Final stand densities varied between 120 and 447 trees ha⁻¹, with mean diameter at breast height (dbh) of 24.9–47.8 cm, and mean total heights between 23.0 and 32.4 m, depending on rotation length and site quality. The mean annual increment of total volume (MAI_Vol) at the end of the rotation varied from 11.3 to 24.9 m³ ha⁻¹ year⁻¹, accumulating a total volume over rotation of 268–524 m³ ha⁻¹.

The most suitable scenario for teak plantations for high-quality sites is the 30-year-rotation scenario with five thinnings of intensities between 20 and 50% (of the standing trees) at the ages of 4, 8, 12, 18 and 24 years. After the sectioning of the merchantable stem in 4-m length logs, the merchantable volume varied between 145 and 386 m³ ha⁻¹, with an estimated heartwood volume of 45–195 m³ ha⁻¹, both depending on rotation length and site quality.     

Effects of water availability and fertilization on nitrogen cycling in a stand of Pinus radiata

Nitrogen cycling was studied for four years (1983–1987) in an N-deficient 10-year-old stand of Pinus radiata growing on a yellow podzolic soil which had a low water-holding capacity. Trees were subjected to combinations of irrigation of N-fertilization resulting in a wide range of N uptake and tree growth. Net mineralization, plant uptake and leaching of soil N was monitored using a sequential coring and in-situ incubation technique. Nitrogen concentrations were measuredd monthly in live needles and litterfall. Average rates of weight loss and release of N from decomposing litter were estimated over a 3-year period using a budgeting approach.

Trees responded only to N (not to P, and there was no N×P interaction), but there was a large positive interaction between N supply and water availability. Response to fertilizer averaged + 24% over a 4-year period, but was zero during a growing-season which contained a 4-month drought. Irrigation alone increased growth by 60%, but in combination with high N availability growth increased 2–3 fold. Annual uptake of N ranged from <10 (irrigated plots in years 2 and 3 after enhanced mineralization during the initial year) to 166 kg ha⁻¹ (during a wet growing season following heavy N fertilization). Although soil mineral-N concentrations were elevated for only about 1 year after fertilization, fertilization enhanced rates of N mineralization throughout the soil N mineralization may have resulted from re-mineralization of the large quantity (147 kg soil N mineralization may have resulted from re-mineralization of the large quantity (147 kg ha⁻¹) of fertilizer N immobilized by the soil during the initial 8 months after fertilization, or the N released from decomposition of fine roots having higher N content. Nitrification was negligible in unfertilized soils, but increased markedly 50–100 days after fertilization and resulted in the leaching of about 60 kg N ha⁻¹ during autumn and winter of the first year after fertilization. Fertilized soils have continued to nitrify readily. Irrigation increased rates of weight loss and N release from decomposing litter.

The rate of N uptake by trees markedly affected the concentrations of N in newly emerging and older needles, and the concentration of N in needlefall. The weighted mean concentration of N in annual needlefall ranged from 0.42% in the irrigated-only plot (most N-stressed) to 0.94% in the heavily fertilized plot during the first year after treatment. These weighted concentrations are a useful index of N uptake from the soil and of growth rate where water supply is not limiting. Except for the initial year after heavy N fertilization, annual uptake of N was equivalent to annual soil N mineralization, and N uptake was positively linearly correlated with annual basal-area increment of trees.     

Estimation of shoot biomass productivity in Estonian Salix plantations

Shoot biomass production was estimated in two Estonian short rotation forest (SRF) plantations during the first rotation cycle (1994–1997). The plantations were established with six clones of Salix viminalis and one clone of Salix dasyclados in 1993. The plantation, located on well-composed organic soil, was characterised by higher productivity (6.2 t DM ha⁻¹ per year) compared with the plantation on poor mineral soil (5.2 t DM ha⁻¹ per year). Fertilisation of the latter plantation increased its productivity to 11.0 t DM ha⁻¹ per year, which is the value close to a predicted maximum for Swedish climatic conditions. In fertilised plots, clone 81090 of S. dasyclados was characterised by the highest productivity among all clones, but also by high stool mortality. Clones 78021 and 78183 of S. viminalis had the most stable and relatively high productivity and can therefore be recommended as promising planting material for SRF in Estonia.

When estimating production, the use of proper allometric relations between shoot dry weight and diameter is of crucial importance. Additional measurements on 1-year-old shoots in 1998 showed that besides shoot age also clone and fertilisation are significant factors influencing allometric relations. The dry weight of fertilised shoots was about 10% lower than that of non-fertilised shoots of the same height and diameter. Older shoots were heavier than younger shoots with a similar diameter.