Soil-solution chemistry in black locust, pine/mixed-hardwoods and oak/hickory forest stands in the southern Appalachians, U.S.A.

Soil-solution chemistry was measured over a 15-month period in three forest stands of contrasting nitrogen mineralization and nitrification rates in the southern Appalachians of North Carolina, U.S.A., using porous-cup lysimeters. In a black-locust-dominated stand, soil solution NO₃---N was 3.73 and 5.04 mg l⁻¹ at 30- and 60-cm depth respectively, and dissolved organic N ( ) was 0.718 and 0.582 mg l⁻¹ respectively. Values at 30 and 60 cm for a pine/mixed-hardwood stand were 0.032 and 0.058 mg l⁻¹ NO₃---N, and 0.201 and 0.168 mg l⁻¹ (values are means over the whole duration of the study). At both depths, soil solution conductivity, pH, Ca, Mg, K and PO₄---P were higher in black locust than in pine/mixed-hardwoods, and there were no differences in soil solution Na. In an oak/hickory stand, soil solution NO₃---N at 30-cm depth was 0.008 mg l⁻¹, and was 0.357 mg l⁻¹. At 30-cm depth, soil-solution conductivity, Ca, Mg and PO₄---P were higher in black locust than in oak-hickory, with no differences in pH, K and Na; , pH and K were higher in oak/hickory than in pine/mixed-hardwoods. In the oak/hickory and pine/mixed-hardwoods forest stands, with relatively lower soil N turnover rates, was a major portion of soil solution N.     

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.     

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.     

Nitrate dynamics after clear felling monitored by in vivo nitrate reductase activity (NRA) and natural N abundance of Deschampsia flexuosa (L.) Trin.

The N dynamics following clear felling, focusing on NO₃⁻ turnover, were studied at four forested sites in southern Sweden. Two different methods were used to study N availability: (i) an in vivo nitrate reductase activity (NRA) bioassay and (ii) measurements of natural abundance of stable N isotopes in leaves of the grass species Deschampsia flexuosa, and in organic soil horizons. At each of the four sites, six plots were established and each year, for 5 consecutive years (1989–1993), one plot per site was felled. Thus, in 1993 there were five plots with different ages since clear felling and one control (closed forest) plot at each site. NRA was analyzed three times annually during the years 1989–1993. Samples for grass and soil analysis of δ¹⁵N, total N and soil pH were taken in 1993 only. NRA rapidly increased after the felling and remained high throughout the studied period. This suggests that there was an increased pool of plant-available soil NO₃⁻ more than 5 years after clear felling. Despite differences in site productivity and N deposition between the four sites, no significant differences in NRA were found between the sites. There were also rapid changes in δ¹⁵N in leaves of D. flexuosa, coinciding with the increases in NRA, during the first 3 years after felling. In contrast to NRA, shoot δ¹⁵N decreased 3–4 years after the felling at three out of four sites. Variations in the δ¹⁵N figures between sites may have been largely due to between-site differences in field-layer retention of N. At two of the sites, where NO₃⁻ leaching was also measured, a correlation was found between the NO₃⁻ concentration in the water and the difference in δ¹⁵N between D. flexuosa leaves from felled and closed forest plots. The data presented here suggest that NO₃⁻ leakage after clear felling is a rapid process, which is influenced by the development of field-layer biomass after the felling. Furthermore, losses of NO₃⁻ through leaching rapidly change the natural abundance of the plant available N pools in the soil.     

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.     

Thinning, burning, and thin-burn fuel treatment effects on soil properties in a Sierra Nevada mixed-conifer forest

More than a century of fire exclusion and past timber management practices in many Sierra Nevada mixed-conifer forests have led to increased stand densities and fuel accumulation, with a corresponding risk of large, high severity wildfires. To reduce hazardous fuel accumulations and restore the health and natural processes of forest ecosystems, fuel management programs often employ thinning and prescribed fire treatments, both alone and in combination. We evaluated forest floor and mineral soil chemical and physical characteristics following these treatments in a managed Sierra Nevada mixed-conifer forest using a fully replicated study design with four separate treatments: THIN, BURN, THIN + BURN, and an untreated CONTROL. Compared to the CONTROL, the BURN and THIN + BURN treatments consumed a large amount of the forest floor, reducing the mass and depth by more than 80%. These treatments reduced the forest floor C and N pools by more than 85%, resulting in reductions of 25 Mg C ha⁻¹ and more than 700 kg N ha⁻¹ from the forest floor. Despite these large losses from the organic horizons, no significant differences in mineral soil total C and N pools were detected among treatments. Compared with the CONTROL and THIN treatments, the BURN and THIN + BURN significantly increased the mineral soil NO₃-N concentration, pool of inorganic N, pH, and exposed bare soil. The THIN + BURN treatment significantly increased the concentrations of NH₄-N and exchangeable Ca relative to the CONTROL. No significant differences in the net rates of nitrification, N mineralization, or bulk density were detected among the four treatments. The BURN treatment reduced mineral soil C concentration and CEC, while the THIN + BURN treatment had the greatest increase in inorganic N. Fire effects on soil pH and inorganic N were moderated in skid trails due to reduced fuel continuity and consumption. In light of the current management emphasis on hazardous fuels reduction, we recommend that researchers investigating fire effects in harvested stands include skid trail influences in their study design.     

Effects of intensive harvesting on nutrient capitals of three forest types in New England

Effects of whole-tree clearcutting are being studied in three major forest types in the northeastern United States: a spruce-fir forest in central Maine, a northern hardwood forest in New Hampshire, and a central hardwood forest in Connecticut. At each site we sampled total and extractable nutrient capitals, inputs and outputs of nutrient ions in precipitation and streamflow, nutrient removals in harvested products, and nutrient accumulation in regrowth. Depending upon location, combined losses of nutrients in harvested products and increased leaching to streams were in the ranges of 374–558 kg ha⁻¹ for Ca, 135–253 kg ha⁻¹ for K, 50–65 kg ha⁻¹ for Mg, 248–379 kg ha⁻¹ for N, and 19–54 kg ha⁻¹ for P. Opportunities for replacing these losses over the next rotation are best for N. Data on inputs in precipitation versus outputs in streamflow indicate that, once effects of harvest subside, most N in precipitation will stay within the forest. By contrast, Ca shows a net output of 8–15 kg ha⁻¹ year⁻¹ from uncut watersheds, and the added leaching losses due to harvest may have a serious impact on Ca capital. This is especially the case for the Connecticut site, where total site capital for Ca is only about 4000 kg ha⁻¹.     

Use of a gradient of N-deposition to calculate effect-related soil and vegetation measures in deciduous forests

Deposition of N and S has increased since the 1950s in most European countries and N accumulates in ecosystems that are not N saturated. This study shows long-term effects of a (modelled) N deposition of 7–17 kg N ha⁻¹ per year on biological and chemical processes in soil, vegetation composition, and functional types of field-layer plant species in deciduous forests. Soil pH largely determined the response of the soil processes, emphasising the importance to compare soils of similar acidity regarding the effects of N deposition. The most pronounced effects were demonstrated for the most acid study plots. When we compared regions with a deposition of 7 and 17 kg N ha⁻¹ per year we found a 40–80% higher soil N mineralisation rate, 2–90% higher nitrification rate and 10–25% lower C:N ratio in the region with the highest deposition. Similar but smaller differences were indicated when regions with a deposition of 7 and 10 kg N ha⁻¹ per year were compared. Number of species was lower in the regions with the highest deposition. Literature data for plants on N concentration, nitrate reductase activity (NRA), growth rates, morphology and height were calculated on a site basis. They varied to different extent between the regions. The N concentration was 7–24% higher in the regions with the highest N deposition. We argue that the effect-related critical load based on our results should be set to a N deposition of 7–10 kg N ha⁻¹ per year. Critical loads for a subdivision of deciduous forests would give lower critical loads for the most acid soils compared to less acid soil.     

Carbon uptake by secondary forests in Brazilian Amazonia

Estimating the contribution of deforestation to greenhouse gas emissions requires calculations of the uptake of carbon by the vegetation that replaces the forest, as well as the emissions from burning and decay of forest biomass and from altered emissions and uptakes by the soil. The role of regeneration in offsetting emissions from deforestation in the Brazilian Legal Amazon has sometimes been exaggerated. Unlike many other tropical areas, cattle pasture (rather than shifting cultivation) usually replaces forest in Brazilian Amazonia. Degraded cattle pastures regenerate secondary forests more slowly than do fallows in shifting cultivation systems, leading to lower uptake of carbon. The calculations presented here indicate that in 1990 the 410 × 10³ km² deforested landscape was taking up 29 × 10⁶ t of carbon (C) annually (0.7 t C ha⁻¹ year⁻¹). This does not include the emissions from clearing of secondary forests, which in 1990 released an estimated 27 × 10⁶ t C, almost completely offsetting the uptake from the landscape. Were the present land-use change processes to continue, carbon uptake would rise to 365 × 10⁶ t annually (0.9 t C ha⁻¹ year⁻¹) in 2090 in the 3.9 × 10⁶ km⁶ area that would have been deforested by that year. The 1990 rate of emissions from deforestation in the region greatly exceeded the uptake from regrowth of replacement vegetation.     

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.     

Required sample size for estimating soil respiration rates in large areas of two tropical forests and of two types of plantation in Malaysia

We estimated the required sample sizes for estimating large-scale soil respiration (for areas from 1 to 2 ha) in four ecosystems (primary and secondary forests, and oil palm and rubber plantations) in Malaysia. The soil respiration rates were 769 ± 329 mg CO₂ m⁻² h⁻¹ in the primary forest (2 ha, 50 sample points), 708 ± 300 mg CO₂ m⁻² h⁻¹ in the secondary forest (2 ha, 50 points), 815 ± 363 mg CO₂ m⁻² h⁻¹ in the oil palm plantation (1 ha, 25 points), and 450 ± 178 mg CO₂ m⁻² h⁻¹ in the rubber plantation (1 ha, 25 points). According to our sample size analysis, the number of measurement points required to determine the mean soil respiration rate at each site with an error in the mean of no more than 10% ranged from 67 to 85 at the 95% probability level. These results suggest that evaluating the spatial heterogeneity of soil respiration rates in the tropics may require more measurement points than in temperate forests.     

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.     

Soil denitrification rates in a subalpine watershed of the eastern Sierra Nevada

Denitrification rates in soils of six subalpine plant communities in an eastern Sierra Nevada watershed were determined by the acetylene inhibition method. Soil atmosphere samples were collected monthly from June 1986 through May 1987 in a riparian, wet meadow, dry meadow, north-facing forest, south-facing forest and barren site and analyzed for N₂O content using gas chromatography. Soil temperature, moisture, organic matter, C,N,C:N ratio, NO₃N and pH were examined to assess their effects on denitrification rates. Mean denitrification rates for the year varied from 103.3 μg m⁻²h⁻¹ in the north-facing forest to 120.2 μg m⁻²h⁻¹ at the barren site, but did not differ significantly among any of the six plant communities. However, comparisons among months within individual communities revealed that the denitrification rates in each community varied significantly over the year, and in three of the six sites significant correlations between denitrification rates and other soil parameters were detected. Soil acidity was positively correlated with denitrification rate in the riparian and wet meadow communities, and in the dry meadow, soil moisture was positively correlated while soil temperature and organic matter were negatively correlated with denitrification rate. Comparisons among sites within individual months revealed significant differences in denitrification rates in June, September, October and January, but no single site consistently exhibited the highest or lowest rate in all 4 months, and only in October, when denitrification rates were positively correlated with soil temperature and moisture, was variation in denitrification rates among sites explained by other soil parameters. For the six plant communities overall, soil denitrification rates were highly variable from June to October, increased sharply from October to December, and then declined from December to May.     

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.     

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.     

Topographical differences in soil N transformation using15N dilution method along a slope in a conifer plantation forest in Japan

Soil N transformation was investigated using¹⁵N dilution method along a slope on a conifer plantation forest. Although there was no significant difference in the net N mineralization rates by laboratory incubation, net nitrification rates increased downslope. Gross N transformation by¹⁵N dilution method showed a distinct difference not only on the rates, but also on the main process between the lower and the upper of the slope. Half of minelarized N was immobilized and the other half was left in NH ₄ ⁺ pool at the upper part of the slope, while all of mineralized N was used for immobilization or nitrification and NH ₄ ⁺ pool decreased at the lower of the slope. Soil N transformations were classified into two groups: one was shown below 773 m and the other was shown above 782 m. The incubation with nitrification inhibitor showed that nitrification was mainly conducted by autotrophs irrespective of the position of the slope. Microbial biomass and microbial C/N were similar among the sites. However, the gross mineralization rate was higher below 773 m than above 782 m under similar respiration rates. This suggests that the substrate quality may be one of the controlling factors for soil N transformation. Extractable organic C/N was similar to microbial C/N at the lower of the slope. It indicated that the substrate was more decomposable below 773 m. It is considered that soil N transformation is affected by topographical gradient of moisture and nutrient which makes plant growth and decomposition rate different.     

Estimation of litter fall and seed production of Acacia mangium in a forest plantation in South Sumatra, Indonesia

Annual litter fall of Acacia mangium in the period of September 1995 to August 1996 was estimated at 5939 kg ha⁻¹ year⁻¹ and from September 1995 to August 1996 at 6048 kg ha⁻¹ year⁻¹, with the highest seasonal production in the dry season. The litter fall was dominated mainly by leaves, 4446 kg (75%) and 4137 kg (68%), respectively. Seed production in the litter fall was estimated at 42.4 kg ha⁻¹ year⁻¹ (4.1 million seeds ha⁻¹) and 39 kg ha⁻¹ year⁻¹ (3.8 million seeds ha⁻¹), with the highest in the dry season from June to October. The accumulated litter fall in the forest floor together with shrubs and grass provide a high fuel load, increasing fire risk.     

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%.     

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.     

采伐方式对森林土壤氮初级转化速率和净氮转化速率的影响

目的 为探明不同采伐方式下森林土壤氮素的释放和保存能力,揭示采伐对森林土壤氮素循环的影响。 方法 本研究通过室内培养试验,采用15N同位素成对标记技术和FLUAZ数值优化模型研究了择伐和皆伐方式下寒温带阔叶混交林土壤氮初级转化速率和净氮转化速率特征。 结果 保留带处理土壤氮初级矿化速率、净氮矿化速率、氮初级固定速率、初级硝化速率和净硝化速率分别为4.16、1.86、2.32、0.368和0.343 mg∙kg−1∙d−1。与保留带处理相比,择伐和皆伐处理土壤氮初级矿化速率分别显著降低了32.2%和61.8%,净氮矿化速率分别显著降低了43.1%和61.5%,氮初级固定速率分别显著降低了23.3%和63.4%。择伐对土壤初级硝化速率和净硝化速率没有显著影响,但皆伐处理土壤初级硝化速率和净硝化速率分别显著降低了23.6%和33.3%。相关分析结果表明,土壤有机碳和水溶性有机碳含量的变化是影响氮初级矿化速率和初级固定速率的主要因素,pH是影响硝化速率的主要因素。 结论 皆伐后土壤铵态氮固定速率的下降程度大于初级硝化速率,导致gn/ia和NO3−/NH4+值显著提高,增加了硝态氮淋溶风险。而择伐处理的gn/ia和NO3−/NH4+值与保留带处理没有显著差异,是一种相对可取的森林采伐方式。