Comparison of field methods for measuring soil respiration: a static alkali absorption method and two dynamic closed chamber methods

The objectives of the present study were to compare the static alkali absorption (AA) and dynamic closed chamber (DC) methods for measuring soil respiration, and to evaluate the effects of methodological differences on estimating annual mean soil respiration rate in a natural forest. For the AA method, we used Kirita’s method [Jpn. J. Ecol. 21 (1971) 119] using an alkali-soaked sponge disc that covers nearly the same area as that covered by a chamber. For the DC method, we used both the LI-6200 system (DC-62 method) and the newer LI-6400 system (DC-64 method) (LI-COR, Lincoln, NE, USA). Comparative measurements were conducted on five occasions during the study period (November 1998–October 1999) at a Quercus serrata forest in Japan. Daily mean soil respiration rates obtained by the AA, DC-62 and DC-64 methods for a 24 h period were in the ranges 205–578, 147–629 and 165–734 mg CO₂ m⁻² h⁻¹, respectively. The daily mean soil respiration rates obtained by the AA method were 79–128% of those obtained by the DC-64 method. When the daily mean soil respiration rate obtained by the DC-64 method was below 300 mg CO₂ m⁻² h⁻¹, the daily mean soil respiration rate obtained by the AA method was an average of 26% higher than that obtained by the DC-64 method. When the daily mean soil respiration rate obtained by the DC-64 method was above 300 mg CO₂ m⁻² h⁻¹, the daily mean soil respiration rate obtained by the AA method was an average of 19% lower than that obtained by the DC-64 method. However, at the present site, there was a little difference between the two methods as for estimating annual mean soil respiration rate, and therefore the AA method improved by Kirita [Jpn. J. Ecol. 21 (1971) 119] is suggested to be a useful method for estimating annual mean soil respiration in the forest. The daily mean soil respiration rates obtained by the DC-62 method were systematically 10–24% lower (an average of 15% lower) than those obtained with the DC-64 method, and the annual mean rate was lower than that estimated by the AA method.     

Fluxes of CO2, CH4 and N2O from drained coniferous forests on organic soils

Fluxes of CO₂, CH₄ and N₂O were measured during two to three years at four sites, located within an area of 9 km² in southern Sweden, using dark static chamber techniques. Three of the sites were drained coniferous forests on moist organic soils that differed in forest productivity and tree species. The fourth site was an undrained tall sedge mire. Although the drained sites were all moist, with average groundwater levels between 17 and 27 cm below the soil surface, the mean annual dark forest floor CO₂ release rate was significantly higher at the drained sites, (0.9–1.9 kg m⁻² y⁻¹) than at the undrained mire site (0.8 to 1.2 kg m⁻² y⁻¹). CH₄ emissions were significantly lower from the drained sites than from the undrained mire (0.0 to 1.6 g m⁻² y⁻¹, compared to 10.6 to 12.2 g m⁻² y⁻¹), while N₂O emissions were significantly lower from the undrained site than from the drained sites (20 to 30 mg m⁻² y⁻¹, compared to 30 to 90 mg m⁻² y⁻¹). There were no clear effects of site productivity or tree species on the soil fluxes of any of the gases. The annual net primary production of the forests was modeled. All drained sites were net sinks, while the undrained mire was a net source of greenhouse gases. The estimated net greenhouse gas exchange of the drained sites was correlated with productivity: the most productive site was the largest net sink and the least productive the smallest net sink for greenhouse gases. The results indicate that, to mitigate the increase of atmospheric greenhouse gases, drained forest sites, which have been unsuccessfully drained or rewetted due to subsidence, should be managed in a way that keeps the groundwater level at a steady state.     

Clear-cutting reduces nitrate leaching in a pine plantation of high natural N status

Following the tree harvest, the biogeochemistry of a catchment is modified by changes in soil temperature and moisture, and nutrient cycling. We monitored soil-solution and stream-water chemistry, and soil properties in a Pinus radiata D. Don plantation in New Zealand before and after clear-cutting and replanting in 1997. The annual rainfall during the study was 1440–1860 mm. The soil was a 1800-year-old pumice soil of high natural N status; the catchment had received large inputs of volcanic N in rain, probably over the 1800 years since the pumice had been deposited. The leaching loss of nitrate-N was 28 kg ha⁻¹ yr⁻¹ in 1996, and then decreased sharply after clear-cutting to 3 kg ha⁻¹ yr⁻¹ in 1998 and <1 kg ha⁻¹ yr⁻¹ in 1999. Weed growth and soil microbial biomass increased during this time, and would have removed much of the N from soil solution in the upper soil layers. Although the catchment was small (8.7 ha), there was a 2-year lag until N decreased in stream-water; the losses of dissolved organic N to stream-water were low. There was no change in soil pH over the 4 years, but spring-water pH appeared to increase, which was consistent with the increase in bicarbonate that accompanied grass/weed growth. The export of cations (mmol_c l⁻¹) in the spring-water was Na>Ca>Mg=K as expected for rhyolitic pumice, and the total concentration was probably controlled by the accompanying anions. The export of anions was NO₃=Cl>SO₄=HCO₃ before harvest and HCO₃=Cl>SO₄=NO₃ after harvest.     

Litterfall, litter decomposition and nutrient dynamics in a subtropical natural oak forest and managed plantation in northeastern India

In northeastern India, subtropical forests are over-exploited for timber, fuel wood and common agricultural practice like shifting cultivation, which are responsible for the degradation of natural forest. In degraded areas, large-scale plantations of different species of Quercus have been raised since 1980 for the production of economic Tasar silk. Conversion of natural forest into plantation affects the process of nutrient cycling due to management practices. Thus, it would be of importance to study the litterfall, litter decomposition process and the factors regulating the rate of litter decay in these ecosystems to improve recommendations for their management and conservation. We recorded litterfall by using litter traps and decomposition of leaf litter by nylon net bag technique to understand the amount of organic matter and nutrient return and their release in soils of forest and plantation in Manipur, northeast India. Total litterfall was 419.9 g m⁻² year⁻¹ in plantation and 547.7 g m⁻² year⁻¹ in forest. Litter decomposition rate was faster at plantation site than the forest in the early stage of litter decomposition whereas the reverse was observed at later stages of decomposition. Stepwise regression analysis showed the significant role of relative humidity and mean temperature on mass loss rates in the forest. Relative humidity, maximum temperature, population of fungi and actinomycetes were the best predictor variables for mass loss rates in plantation. Nutrient retranslocation efficiency and the immobilization of N and P in forest litter were higher than plantation. This suggests that Q. serrata growing in natural ecosystem in oligotrophic condition adapted strong nutrient conservation mechanisms to compete with the other plant species for the meager soil nutrients. The same species in plantation loses these adaptive capabilities because of exogenous supply of nutrients and in the absence of intense competition with other plant species. Thus, the optimization of organic and chemical fertilizer input in plantation is recommended for maintaining the soil fertility level to produce quality leaf for silkworm by conserving essential nutrients in the system.     

Long-term soil potassium availability from a Kanhapludult to an aggrading loblolly pine ecosystem

A long-term (1962 to 1990) forest biogeochemistry study in the southeastern Piedmont of the USA provided estimates of soil K release in response to forest regrowth. We investigated the sources of soil K that buffered the exchangeable K pools during forest growth and we estimated soil K release rates through greenhouse and acid extraction studies for comparison to our field estimate.

In these acid Kanhapludults, derived from granitic-gneiss, the disparity between measured depletions of soil exchangeable K and estimated forest removals indicated a buffering of exchangeable K on the order of 0.31 kmol_c ha⁻¹ per year. Non-exchangeable K extracted by boiling with 1 M HNO₃ exceeded exchangeable K by up to 40-fold. Non-exchangeable K was not depleted during the three decades of stand growth, however, thus was not the long-term source of exchangeable K buffering. Total K in these soils ranged from 0.4 to 3.8% by weight. Mineralogical data indicated a presence of hydroxy-interlayered vermiculite throughout the upper 4 m of soil for <2 μm clay fraction and a presence of micaceous minerals in the 2 to 45 μm silt fraction. XRD analysis of micaceous flakes extracted from 4 to 8 m in the soil indicated a presence of muscovite mica.

Estimated K releases in the greenhouse and extraction studies were generally consistent with long-term results. The accumulation of K during two rotations of pine seedling growth in the greenhouse exceeded the measured depletions in exchangeable and non-exchangeable K over all soil depths tested by 0.007 to 0.026 cmol_c kg⁻¹. Potassium removal by sequential extraction/incubations with 1 mM HCl and 1 mM oxalic acid continued through 24 extractions and K recovered in extract solutions exceeded the sum of depletions in exchangeable and non-exchangeable K pools by 0.001 to 0.028 cmol_c kg⁻¹. These excess removals in plant uptake or solution recovery indicate a release of mineral K. Thirty-day extractions with H⁺-resins in both 1 mM HCl and 1 mM oxalic acid were well fit by the Elovich equation but were not well correlated with plant K uptake in the greenhouse study. The release rate coefficients ranged from 0.012 to 0.025(cmol_c kg⁻¹) h⁻¹.

Extrapolations to annual releases of K in the greenhouse and sequential extraction studies were a similar order of magnitude as long-term releases estimated at the long-term Calhoun plots. Surface soil (0 to 15 cm) releases ranged from 0.15 to 0.65 kmol_c ha⁻¹per year while deeper soils ranged up to 1.54 kmol_c ha⁻¹per year. Results indicate that soils similar to those at Calhoun that contain a similar micaceous and HIV component will be able to supply K at rates adequate to keep pace with demands of forest regrowth even under intensive forest management.     

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.     

Microsite patterns of conifer seedling establishment and growth in a mixed stand in the southern Alps

In many forests of the Alps, permanent forest cover and, therefore, its continuous renewal is the main silvicultural goal. Regenerating these forests must be based on a sound understanding of the ecology of the tree species in question. The regeneration of silver fir (Abies alba Mill.) and Norway spruce (Picea abies (L.) Karst.) in the upper montane zone of the southern central Alps has so far received little attention. The aim of this study was, therefore, to characterize the ecological niche for the establishment and growth of silver fir and Norway spruce seedlings in this zone. The study was conducted as a case study on a plot of 1.5 ha in the upper montane zone of southern Switzerland, at an altitude of 1380 m. The basal area of the mixed Norway spruce–silver fir stand was 43.7 m² ha⁻¹. Seedlings were censused on 375 plots of 1.0 m² area. Seedling density was 1.30 ± 4.25 m⁻² (mean ± standard deviation) for Norway spruce and 0.87 ± 1.50 m⁻² for silver fir. Logistic regression models were used to test the effect of microsite characteristics on seedling occurrence, and general linear models for effects on seedling height growth and biomass increment. Most seedlings received less than 10% light (photosynthetic photon flux density) as compared to values in the open. Silver fir occurrence was positively related to microsites at the edge of canopy trees, but unrelated to ground cover type, light and micro-relief. Norway spruce occurrence was only, and positively, related to the presence of mosses. Height growth and biomass increment of seedlings of both species were only loosely correlated with microsite conditions. Seedlings without canopy cover grew faster than those under canopy cover, probably as a result of light and moisture limitations under the canopy of adult trees. Diffuse radiation was positively correlated with average annual biomass increment of silver fir, but not of Norway spruce seedlings. In general, the results suggest that silver fir seedlings have less specific microsite requirements than Norway spruce seedlings in terms of ground cover. They are also more shade tolerant, and therefore, grow faster than Norway spruce in low-light environments of the upper montane zone of the southern Alps. While the microsite concept can be helpful in designing silvicultural operations, it has limitations when only patch characteristics that are easy to assess are used, and others neglected. Finally, our study suggests that counting the number of green shoots is a promising method to quickly and non-destructively estimate the biomass of a great number of small seedlings.     

Biomass production and allocation, including fine-root turnover, and annual N uptake in lysimeter-grown basket willows

Annual net primary production (NPP) and N uptake were estimated for lysimeter-grown basket willows (Salix viminalis L.) during 3 years after planting. The willows were grown in a stand structure and continuously supplied with water and liquid fertilizer through drip tubes. The lysimeters contained either clay from the site or washed quartz sand. Shoot growth and leaf litter were measured and fine-root dynamics observed in minirhizotrons. Destructive samples were taken annually in late autumn and entire root systems were washed out. Dry mass and N content of all plant parts were determined. Fine-root production was estimated by two methods, based on destructive samplings and observations in minirhizotrons.

The proportion of biomass allocated below ground increased considerably when estimates based on accumulated NPP were compared with those based on standing dry mass. In the first year, 49 and 58% of annual NPP in willows grown in clay and sand, respectively, was belowground. In subsequent years the proportions were 36–38% and 33–40%. Most belowground production was fine roots. Relatively more N was used belowground in the first year than subsequently, but no substrate-induced differences were observed in the allocation pattern. Both annual NPP and N uptake was always higher in plants in clay than in those in sand: in the final 2 years, 21–22 tonnes DM ha⁻¹ year⁻¹ and 190 kg N ha⁻¹ year⁻¹ in clay, and 9–10 tonnes DM ha⁻¹ year⁻¹ and 100 kg N ha⁻¹ year⁻¹ in sand.     

Intensive management modifies soil CO2 efflux in 6-year-old Pinus taeda L. stands

Intensive forestry may reduce net CO₂ emission into atmosphere by storing carbon in living biomass, dead organic matter and soil, and durable wood products. Because quantification of belowground carbon dynamics is important for reliable estimation of the carbon sequestered by intensively managed plantations, we examined soil CO₂ efflux (S_CO2) in a 6-year-old loblolly pine (Pinus taeda L.) plantation in response to weed control (W), weed control plus irrigation (WI), weed control plus irrigation and fertigation (addition of fertilizer to the irrigation water) (WIF), and weed control plus irrigation, fertigation and pest control (WIFP) since plantation establishment. Average S_CO2 ranged from 1.27 to 5.59 μmol m⁻² s⁻¹, and linear models indicated that soil temperature explained up to 56% of the variation in S_CO2. Plot position explained an additional 2–11% of the variation in S_CO2. Soil moisture was only weakly correlated with S_CO2 in the W treatment, and S_CO2 was not significantly correlated to fine root mass. Predicted carbon loss from forest floor respiration ranged between 778 and 966 g C m⁻² year⁻¹ and was 20% lower in the WIF treatment relative to the W treatment. Annual soil carbon loss through soil respiration declined linearly with increasing carbon content in total root biomass (tap + coarse + fine) at age 6.     

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.     

Burning of Amazonian forest in Ariquemes, Rondônia, Brazil: biomass, charcoal formation and burning efficiency

Biomass burning in tropical forests – the normal practice to prepare land for agriculture and ranching – has been a major source of CO₂ emitted to the atmosphere. Mass transformations by burning are still little studied in the tropics. The present study estimated parameters, such as the stock of carbon contained in the biomass, burning efficiency and the formation of charcoal and ashes in a tropical moist forest. Two sets of plots arranged in the form of ‘stars' (720 m² total) were installed in a 3.5 ha area of forest that had been felled for planting pasture at Fazenda Nova Vida, Ariquemes, Rondônia. Each ‘star' had six rays measuring 2 m × 30 m; alternating rays were designated for pre-burn and post-burn measurements. All above-ground biomass present in the plots was weighed directly before the burn in the pre-burn rays and after the burn in the post-burn rays. Pieces of wood with diameter ≥10 cm also had their biomasses estimated from volume estimates, using line-intersect sampling (LIS) in order to increase the area of sampling and to allow volume loss to be estimated as an increment based on individual pieces measured before, and after, the burn at the same point (as opposed to inferring change as a difference between independent estimates of stocks). The initial above-ground biomass (dry weight) before the burn was estimated at 306.5 ± 48.6 (mean ± SE) Mg ha⁻¹, with an additional 4.5 Mg ha⁻¹ for trees left standing. Carbon stock in the initial biomass (including trees left standing) was 141.3 (Mg C) ha⁻¹. After burning, carbon stock was reduced by 36.8% (burning efficiency). The stocks of charcoal and ash formed in the burn were, respectively, 6.4 ± 2.7 and 5.7 ± 1.0 Mg ha⁻¹. The destructive and nondestructive (LIS) methods did not differ significantly (t-test, p > 0.05) in estimating post-burn stocks of wood and charcoal. The results of this study contribute to improving the estimates of parameters needed for global carbon calculations and point to ways in which estimates of these parameters could be further improved.     

Effect of clear-cutting and site preparation on the level and quality of groundwater in some headwater catchments in eastern Finland

The impact of forest management (clear-cutting and site preparation) on stream hydrology has been studied in five small catchments in eastern Finland from 1991 and on groundwater levels and quality from 1994. The period 1992–1996 was a calibration period and in the autumn of 1996, 10% and 30% of the area of two of these catchments were clear-cut according to the forest management plan. Regeneration was carried out by disc-plowing in the autumn of 1998 and planting with Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings in the spring of 1999. The depth and quality of groundwater was monitored with four to nine groundwater wells installed in each catchment. There were 32 wells in all, 16 on upland mineral soils and 16 on peatlands (the perforated part of the pipe was totally put into the underlying mineral soil at eight sites). Sampling was made monthly during spring (March–May) and autumn (November–December) and bimonthly during summer (June–October). The samples were chemically analyzed for pH, electrical conductivity, and concentrations of total P and Fe (before filtration), and total N, NO₃⁻-N, NH₄⁺-N, total P, PO₄³⁻-P, SO₄²⁻-S, Ca²⁺, Mg²⁺, K⁺, Na⁺, Mn, Zn, Fe, Al³⁺, Cl⁻ (after filtration through 0.45 μm membrane filter). Data collected until the end of 2001 are reported. Groundwater was found in the down-slope wells in lower-lying areas, but not in those installed on the slopes with a thin (1–2 m) soil layer. Clear-cutting did not significantly affect groundwater levels in the wells. Nitrate N concentrations increased from 0.03 mg L⁻¹ level after clear-cutting and again after site preparation in the wells on upland soils and peatlands receiving water from the managed parts of the catchment. In one well at the lower edge of the managed area NO₃⁻-N concentrations reached 1–1.4 mg L⁻¹ in 2001 (fifth year after clear-cutting, third after disc-plowing), but mean concentrations remained <0.3 mg L⁻¹. Chloride concentrations also increased (50–100%) after treatments but the concentrations of other solutes showed no significant effect of treatment. It was concluded that changes in groundwater quality and quantity related to the clear-cutting were small and did not represent a danger to water quality or quantity.     

Seedling emergence, growth, and allocation of Oriental bittersweet: effects of seed input, seed bank, and forest floor litter

The establishment of invasive plant populations is controlled by seed input, survival in the soil seed bank, and effects of soil surface disturbance on emergence, growth, and survival. We studied the invasive vine Celastrus orbiculatus Thunb. (Oriental bittersweet) to determine if seedlings in forest understory germinate from the seed bank or from seed rain. We also conducted a greenhouse experiment to investigate the role of leaf litter mass and physical texture on seedling survival, growth, and allocation. In the understory of an invaded mixed hardwood forest, we measured seed input, seedling emergence with seed rain, and seedling emergence without seed rain. Mean seed rain was 168 seeds m⁻²: mean seedling emergence was 107 m⁻², and there was a strong correlation between seed rain and seedling emergence. The ratio of seedlings to seed input (0.61) was close to the seed viability (0.66) leaving very few seeds to enter the seed bank. Seed bank germination under field conditions was low (1 seedling m⁻²). Soil cores were incubated in a greenhouse to determine seed bank viability, and germination from these soil cores did not occur. To determine how litter affects seedling establishment and growth, we measured seedling emergence and biomass allocation in a greenhouse experiment. Seeds were placed below intact and fragmented deciduous leaf litter in amounts ranging from zero to the equivalent of 16 Mg ha⁻¹. Seedling emergence was not affected by fragmented litter, but decreased to <20% as intact litter increased to 16 Mg ha⁻¹. Increasing litter resulted in greater allocation to hypocotyl and less to cotyledon and radicle, and this effect was greater in intact litter. C. orbiculatus seedlings achieve emergence through forest floor litter through plasticity in allocation to hypocotyl growth. The low survival of C. orbiculatus in the seed bank suggests that eradication of seedling advance regeneration and adult plants prior to seed rain may be an effective control strategy. However, the intact forest floor litter of an undisturbed forest will not prevent seedling establishment.     

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.     

Effects of nitrogen on the growth of jack pine competing with Canada blue-joint grass and large-leaved aster

Biomass, leaf area, canopy photosynthesis, photosynthetic nitrogen-use efficiency (PNUE), nitrogen-partitioning ratio (NPR: ratio of nitrogen taken up by jack pine relative to two different competitor species), and nitrogen uptake (NU) of jack pine (Pinus banksiana Lamb.) competing with large-leaved aster (Aster macrophyllus L.) and Canada blue-joint grass (Calamagrostis canadensis (Michx.) Beauv.) were examined at three nitrogen levels in a controlled-environment growth chamber. When grown with large-leaved aster, jack pine biomass, photosynthesis and PNUE (p<0.001) increased as nitrogen level increased. Jack pine biomass, photosynthesis and NPR (p<0.001) decreased as nitrogen level increased when grown with Canada blue-joint grass. At the lowest nitrogen supply level, jack pine photosynthesis decreased as competitor PNUE increased (r²=0.84, p<0.001). Jack pine photosynthesis decreased as NU of large-leaved aster (37.5 mg N l⁻¹: r²=0.75, p<0.001; 100 mg N l⁻¹: r²=0.86, p<0.001) and Canada blue-joint grass (37.5 mg N l⁻¹: r²=0.96, p<0.001; 100 mg N l⁻¹: r²=0.84, p<0.001) increased. NU and PNUE may play an important role in the outcome of interactions between jack pine seedlings and competing forest vegetation in newly planted stands.     

Performance of twelve selected Australian tree species on a saline site in southeast Queensland

The establishment and early growth of 12 species within the genera Eucalyptus, Casuarina, Melaleuca and Tipuana was tested on a saline site in southeast Queensland. Electrical conductivity (EC) in the top 50 cm of soil was measured using an electromagnetic induction method and calibrated against the EC of 1:5 soil:water suspensions. The site was then stratified into five salinity classes: 0.75–1.0, 1.0–1.25, 1.25–1.5, 1.5–1.75 and over 1.75 dSm⁻¹. Relationships were developed for predicting the survival and height production of 18-month-old trees. These regressions explained 15–88% of the variation in survival and 2–66% of the variation in height production. Tree species were grouped by determining the EC level where height production declined by 25% relative to that at 0.75 ds m⁻¹. Casuarina glauca, Melaleuca bracteata, Eucalyptus moluccana, Eucalyptus camaldulensis, Eucalyptus tericornis and Eucalyptus raveretiana were all highly salt tolerant (25% reductions over 1.5 dS m⁻¹). Casuarina cunninghamiana, Eucalyptus grandis, Eucalyptus melliodora and Eucalyptus robusta exhibited moderate salt tolerance (25% reductions between 1.0 and 1.5 dS m⁻¹). The responses to increased salinity of Tipuana tipu and Eucalyptus intermedia (25% reductions at less than 1.0 dS m⁻¹) suggest that these species are not suitable for revegetating similar saline sites.     

Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo: 3. Input–output budgets and consequences for the sustainability of the plantations

Clonal plantations of Eucalyptus have been introduced since 1978 on savanna soils of the coastal plains of Congo. Atmospheric deposition, canopy exchange and transfer through the soil were estimated on the whole rooting depth (6 m) over 3 years, in an experimental design installed in a native savanna and an adjacent 6-year-old Eucalyptus plantation. Complementary measurements after planting the experimental savanna made it possible to establish input–output budgets of nutrients for the whole Eucalyptus rotation and to compare them with the native savanna ecosystem.

In this highly-weathered soil, atmospheric deposits and symbiotic N fixation by a legume species balanced the nutrient budgets in savanna, despite large losses during annual burnings. After afforestation, weeding in the Eucalyptus stands eliminated the leguminous species responsible for a N input by symbiotic fixation of about 20 kg ha⁻¹ year⁻¹. Whereas the budgets of P, K, Ca and Mg were roughly balanced, the current silviculture led to a deficit of about 140 kg N ha⁻¹ in the soil, throughout a 7-year rotation. This deficit was large relative to the pool of total N in the upper soil layer (0–50 cm), which was about 2 t ha⁻¹. Therefore, the sustainability of Congolese plantations will require an increase in N fertilizer inputs over successive rotations to balance the N budget. These results were consistent with field trials of fertilization. Practical consequences of these budgets were identified, in order to: (i) direct field trials of fertilization, (ii) select appropriate methods of soil preparation, weed control and harvest, (iii) highlight the importance of fire prevention in this area, and (iv) support the implementation of field trials aiming at introducing a biological nitrogen fixing understorey in Eucalyptus stands.     

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.     

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.     

Productivity and economics of mixed two-storied spruce and birch stands in Southern Finland simulated with empirical models

The profitability of growing a naturally emerged birch (Betula pendula Roth or Betula pubescens Ehrh.) overstory in a young Norway spruce (Picea abies (L.) Karst.) plantation was examined with empirical stand structure, growth and yield, logging cost, and logging damage models. In the projected alternatives, an overstory of either birch species was thinned to 200–1000 stems per ha at the age of 15 years and retained for another 15 years. Development of the remaining spruce stand was simulated up to rotation age (70–85 years). Alternative treatments included removing the overstory completely at 15 years, and managing a pure spruce stand that was kept free of birch throughout.

Growing a birch overstory of 200–1000 stems per ha up to age 30 years resulted in a 61–93 m³ ha⁻¹ or 9.1–16.8% yield loss for the spruce stand due to growth retardation, and a mortality of 382–498 out of 1900 stems per ha through logging damage. This was compensated for or exceeded by the additional yield of the birch (54–173 m³ ha⁻¹) except for the lowest stocking (200–400 stems per ha) alternatives with B. pubescens. Treatment regimes with a birch overstory were clearly the most profitable alternatives, yielding up to 151% (B. pendula) and 113% (B. pubescens) of the net present value of the pure spruce alternative, at 4% interest rate. Removing the birch overstory already at 15 years was the least profitable alternative with 79 and 83% net present values, respectively. The most profitable treatment with current technology, price, and cost structure appears to be to grow 500–800 birch per ha up to about age 40 years for B. pendula and 45–50 years for B. pubescens.