Agroforestry systems: sources of sinks of greenhouse gases?

The prominent role of forestry and agroforestry systems in the flux and long-term storage of carbon (C) in the terrestrial biosphere has increased global interest in these land-use options to stabilize greenhouse gas (GHG) emissions. Preliminary assessments suggest that some agroforestry systems (e.g., agrosilvicultural) can be CO₂ sinks and temporarily store C, while other systems (e.g., ruminant-based silvopastoral systems) are probably sources of GHG (e.g., CH₄).Agroforestry systems can be significant sources of GHG emissions, especially at low latitudes. Practices such as tillage, burning, manuring, chemical fertilization, and frequent disturbance can lead to emission of CO₂, CH₄, and N₂O from soils and vegetation to the atmosphere. Establishment and management of agroforestry systems incompatible with prevailing edaphic and climatic conditions can accelerate soil GHG emissions. Non-sustainable agroforestry systems are quickly degraded, and woody and herbaceous crops can become significant GHG sources. Silvopastoral systems can result in soil compaction and erosion with significant loss of labile C and N compounds to the atmosphere. Ruminant-based silvopastoral systems and rice paddy agrisilvicultural systems are well documented sources of CH₄ which significantly contribute to the global CH₄ budget.Early assessments of national and global terrestrial CO₂ sinks reveal two primary beneficial attributes of agroforestry systems: 1) direct near-term C storage (decades to centuries) in trees and soils, and, 2) potential to offset immediate GHG emissions associated with deforestation and subsequent shifting agriculture. Within the tropical latitudes, it is estimated that one ha of sustainable agroforestry can provide goods and services which potentially offset 5–20 ha of deforestation. At a global scale, agroforestry systems could potentially be established on 585–1275×10⁶ ha of technically suitable land, and these systems could store 12–228 (median 95) Mg C ha^–1 under current climate and edaphic conditions.The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Nitrogen dynamics across silvicultural canopy gaps in young forests of western Oregon

Silvicultural canopy gaps are emerging as an alternative management tool to accelerate development of complex forest structure in young, even-aged forests of the Pacific Northwest. The effect of gap creation on available nitrogen (N) is of concern to managers because N is often a limiting nutrient in Pacific Northwest forests. We investigated patterns of N availability in the forest floor and upper mineral soil (0–10 cm) across 6–8-year-old silvicultural canopy gaps in three 50–70-year-old Douglas-fir forests spanning a wide range of soil N capital in the Coast Range and Cascade Mountains of western Oregon. We used extractable ammonium (NH₄⁺) and nitrate (NO₃⁻) pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ ion exchange resin (IER) concentrations to quantify N availability along north-south transects run through the centers of 0.4 and 0.1 ha gaps. In addition, we measured several factors known to influence N availability, including litterfall, moisture, temperature, and decomposition rates. In general, gap-forest differences in N availability were more pronounced in the mineral soil than in the forest floor. Mineral soil extractable NH₄⁺ and NO₃⁻ pools, net N mineralization and nitrification rates, and NH₄⁺ and NO₃⁻ IER concentrations were all significantly elevated in gaps relative to adjacent forest, and in several cases exhibited significantly greater spatial variability in gaps than forest. Nitrogen availability along the edges of gaps more often resembled levels in the adjacent forest than in gap centers. For the majority of response variables, there were no significant differences between northern and southern transect positions, nor between 0.4 and 0.1 ha gaps. Forest floor and mineral soil gravimetric percent moisture and temperature showed few differences along transects, while litterfall carbon (C) inputs and litterfall C:N ratios in gaps were significantly lower than in the adjacent forest. Reciprocal transfer incubations of mineral soil samples between gap and forest positions revealed that soil originating from gaps had greater net nitrification rates than forest samples, regardless of incubation environment. Overall, our results suggest that increased N availability in 6–8-year-old silvicultural gaps in young western Oregon forests may be due more to the quality and quantity of litterfall inputs resulting from early-seral species colonizing gaps than by changes in temperature and moisture conditions caused by gap creation.     

Effect of shifting cultivation on soil physical and chemical properties in Bandarban hill district, Bangladesh

This study reports the effects of shifting cultivation at slashing stage on soil physicochemical properties at Bandarban Sadar Upazila in Chittagong Hill Tracts of Bangladesh. At this initial stage of ...     

How well does Kirtland's warbler management emulate the effects of natural disturbance on stand structure in Michigan jack pine forests?

Jack pine (Pinus banksiana) forests in parts of northern Lower Michigan have been managed with 30 years of extensive clearcut harvesting followed by planted stand establishment in order to maintain habitat for the endangered Kirtland's warbler (Dendroica kirtlandii). We used two, parallel chronosequences to study how this management has affected the structural development of jack pine stands relative to the historically dominant disturbance regime of stand-replacing wildfire. Each chronosequence consisted of three young stands (age range 3–6 years), three intermediate-aged stands (age range of 12–17 years) and three mature stands (age range of 39–69 years). Average stem density in young plantations (2300 stems/ha) was lower than the average for young, fire-origin stands (11,000) and varied over a much narrower range among stands (1403–2667 for plantations and 1552–24,192 in fire stands). In addition, within-stand patchiness of stem density was also much higher in the wildfire sites for young and intermediate ages. Plantation sites possessed very little dead wood at young ages (averaging 3 snags/ha and 12 m³/ha CWD) compared to young fire-origin stands (averaging 252 snags/ha and 49 m³/ha CWD). In contrast, mature plantations had similar levels of dead wood (averaging 269 snags/ha and 22 m³/ha CWD) as mature fire-origin stands (averaging 557 snags/ha and 12 m³/ha CWD). Differences between the plantation and fire-origin chronosequences were driven mainly by young- and intermediate-aged stands, whereas mature stands were typically quite similar in all structural features. Our results show clearly that forest management aimed at preserving and enhancing the population of a single endangered species results in greatly simplified habitat structure at the stand level, and suggest that this simplification is perpetuated across the landscape as well. Of particular concern are the effects of extensive harvesting and planting on the availability of snags and CWD.     

Influence of coarse wood and pine saplings on nitrogen mineralization and microbial communities in young post-fire Pinus contorta

Nitrogen (N) limits productivity in many coniferous forests of the western US, but the influence of post-fire structure on N cycling rates in early successional stands is not well understood. We asked if the heterogeneity created by downed wood and regenerating pine saplings affected N mineralization and microbial community composition in 15-yr old lodgepole pine (Pinus contorta var. latifolia) stands established after the 1988 fires in Yellowstone National Park (Wyoming, USA). In three 0.25-ha plots, we measured annual in situ net N mineralization in mineral soil using resin cores (n = 100 per plot) under pine saplings, downed wood (legacy logs that survived the fire, and fire-killed trees that had fallen and were contacting or elevated above the ground), and in bare mineral soil. Annual in situ net N mineralization and net nitrification rates were both greater in bare mineral soil (8.4 ± 0.6 and 3.6 ± 0.3 mg N kg_soil⁻¹ yr⁻¹, respectively) than under pine saplings, contact logs, or elevated logs (ca. 3.9 ± 0.5 and 0.8 ± 0.1 mg N kg_soil⁻¹ yr⁻¹, respectively). Net nitrification was positively related to net N mineralization under all treatments except for elevated logs. In laboratory incubations using ¹⁵N pool dilution, NH₄⁺ consumption exceeded gross production by a factor of two in all treatments, but consumption and gross production were similar among treatments. Contrary to our initial hypothesis, microbial community composition also did not vary among treatments. Thus, two- to three-fold differences in in situ net N mineralization rates occurred despite the similarity in microbial communities and laboratory measures of gross production and consumption of NH₄⁺ among treatments. These results suggest the importance of microclimate on in situ annual soil N transformations, and differences among sites suggest that broader scale landscape conditions may also be important.     

The effect of cutting height and frequency on the forage,wood and seed production of sixSesbania sesban accessions

The forage, wood and seed production of sixSesbania sesban accessions was assessed under irrigated conditions for two cutting frequencies and heights. Control trees were left uncut to measure their seed production potential. The trial was conducted over an 18-month period. The fastest growing accession produced almost 10 t/ha total dry matter (DM) after six months of growth, 40% of it being leaves. Total DM yield was higher at the six-month cutting interval compared to the three months' interval with yields between 25–42 t/ha/year. Some accessions could not sustain their high level of production but showed drastic drops in yield after repeated cutting. In general leaf DM production increased with increased cutting height. When cut at 150 cm the DM leaf yield at the three-month cutting interval ranged form 9.7–18.2 t/ha. More plants survived at the three-month cutting frequency. Seed yields varied considerably between accessions (0.02–1.56 t/ha at the six months' interval). After 18 months of undisturbed growth the trees yielded 36.5–83.7 t/ha total DM comprising 21% leaves. Fresh wood biomass ranged between 56.4–138.0 t/ha and seed yields 2.7–6.6 t/ha.     

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.     

Shifting cultivation on the tidal floodplains of Amazônia: impacts on soil nutrient status

Studies of the impact of traditional agroforestry practices on soil nutrient status in the Amazon Basin have largely been limited to upland sites, many of which are oligotrophic. However, rural population density in Amazonia is highest on the floodplains of the major rivers and shifting cultivation has been practiced on the floodplains for millennia. We established a slash-and-burn cultivation plot and an untreated forest plot in tidal floodplain successional forest in the Islands of Abaetetuba, Pará, Brazil, and sampled soils (0–10 cm) during four phases of the agroforest cycle (n = 9 in each plot for each sampling period): (1) prior to clearing, (2) following burning, (3) harvest one (rice at four months), and (4) harvest two (sugar cane at 15 months). During the course of this pilot experiment, background temporal variability in SOC, total N and available mineral nutrient pools (P, K, Ca, Mg) exceeded treatment effects, suggesting that soil nutrient pools in this environment are well-buffered against the effects of shifting cultivation. These results contrast markedly with those reported for Amazonian upland sites where nutrient pulses and declines associated with the agroforest cycle have been demonstrated and may restrict the potential for low-input, continuous cultivation. Physical limitations imposed by flooding and fluvial erosion, rather than nutrient constraints, restrict the potential of agricultural intensification in the tidal floodplains. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root development in a Sesbania sesban fallow-maize system in Eastern Zambia

Roots of trees (Sesbania sesban) and crops (Zea mays) were quantified during two tree/crop cycles in a sequential tree — crop system at Chipata, Eastern Zambia. The experiment included one- and two-year fallows as well as fertilized and unfertilized controls. The roots of S. sesban represent a standing biomass in the soil of 3 Mg ha^t-1 in the top 1.5 m after two years, with 45–60% and 70–75% being in the top 25 and 50 cm respectively. S. sesban fallow improved early rooting and growth of the following maize crop. Increased soil infiltration was also observed in the two-year fallow treatment, as well as decreased bulk density and resistance to penetration in the soil. No differences between maize root parameters could be detected at tasselling, nor differences between nutrient status of the different treatments. Study results indicate that under the drought-prone conditions of Eastern Zambia, where improved soil physical conditions are important for early deep rooting of crops and access to water and nutrients, tree roots could play an important role in the fallow effect. Further studies are required to assess the relative importance of the improvement of soil chemical and physical properties.Submitted as ICRAF Journal Article # 95/28.     

Loss of carbon sequestration potential after several decades of shifting cultivation in the Southern Yucatán

Cumulative losses from shifting cultivation in the tropics can affect the local to regional to global balance of carbon and nutrient cycles. We determined whether shifting cultivation in the Southern Yucatán causes feedbacks that limit future forest productivity and carbon sequestration potential. Specifically, we tested how the recovery of carbon stocks changes with each additional cultivation-fallow cycle. Live aboveground biomass, coarse woody debris, fine woody debris, forest floor litter and soil were sampled in 53 sites (39 secondary forests 2–25 years old, with one to four cultivation-fallow cycles, and 14 mature forests) along a precipitation gradient in Campeche and Quintana Roo, Mexico. From the first to the third or fourth cultivation-fallow cycle, mean carbon stocks in live aboveground biomass debris declined 64%. From the first to the third cycle, coarse woody debris declined by 85%. Despite declining inputs to soil with each cultivation-fallow cycle, soil carbon stocks did not further decline after the initial conversion from mature to secondary forest. The combined aboveground and soil carbon stock declined almost 36% after conversion from mature forest, however two additional cultivation cycles did not promote further significant decline, largely because of the stability of the soil carbon pool. Although age was the dominant factor in predicting total carbon stocks of secondary forests under shifting cultivation, the number of cultivation-fallow cycles should not be neglected. Understanding change beyond the first cycle of deforestation will enhance forest management at a local scale by improving predictions of secondary forest productivity and related agricultural productivity. A multi-cycle approach to deforestation is critical for regional and national evaluation of forest-based carbon sequestration. Finally, models of the global carbon cycle can be better constrained with more accurate quantification of carbon fluxes from land-use change.     

A diagnostic survey of shifting cultivation in northern Laos: targeting research to improve sustainability and productivity

About 25% of Laos' four million people practise shifting cultivation (mainly of rice) on a third of the country's cropped area. Official policy is to eliminate shifting cultivation by the year 2000. Diagnostic surveys of shifting cultivation were conducted in Luang Prabang and Oudomsay Provinces in northern Laos to understand the practice from a farmer's perspective, to observe fields, and to identify and give priority to problems and research to address problems. Weeds, low and possibly declining soil fertility, intensification of the cropping cycle, rats (plus birds, wild pigs), and insects lowered rice yields or reduced system sustainability. The forest ecosystem has been degraded by logging, burning, and rice monocropping; and potentials for environmental rehabilitation through natural succession are minimal. Farmers cannot adopt high labor and cash cost innovations; and improved fallow is needed as an intermediate step prior to crop diversification, adoption of agroforestry technologies, and sedentary agriculture.     

Chemical properties of forest soils along a fly-ash deposition gradient in eastern Germany

Chemical characteristics of forest soils subjected to long-term deposition of alkaline and acid air pollutants were analysed in spruce (Picea abies (L.) Karst.) stands in eastern Germany. Three forest sites along an emission gradient of 3, 6, and 15 km downwind of a coal-fired power plant were selected, representing high, intermediate, and low fly-ash input rates. Past emissions caused an accumulation of mineral fly-ash constituents in the organic layer, resulting in an atypically high mass of organic horizons of forest soils, especially in the F and H horizons. Total mass of organic layers at the site with heavy deposition loads was as high as 128 t ha^–1, compared to 58 t ha^–1 at the low input site. Fly-ash deposition significantly increased the pH values in the L, F and H horizons and mineral topsoil (0–10 cm). Significantly higher concentrations of NH₄Cl-extractable cations (i.e. effective cation exchange capacities) and base saturations of >66% were found in the humic horizons at sites where the pH was increased due to the direct and indirect (i.e. higher proportions of deciduous trees) effects of fly-ash emissions. Stocks of basic cations were dominated by Ca²⁺ and decreased significantly along the fly-ash deposition gradient from 33.6 to 5.3 kmol_c ha^–1. Proportions of water-soluble basic cations out of the total potentially exchangeable (i.e. NH₄Cl-extractable) basic cations generally increased in the forest soil with decreasing deposition loads following the cation exchange capacity and base saturation along the fly-ash gradient. Higher proportions of monovalent cations, such as K⁺ and Na⁺, were observed in the water extracts from fly-ash-affected forest soils, while the NH₄Cl-extracts were dominated by bivalent cations, such as Ca²⁺ and Mg²⁺. These results suggest a greater leaching tendency for monovalent cations in these soils. Stocks of organic C and total N in the humus layer decreased from sites with high fly-ash deposition levels to sites with low levels, from 57.4 to 46.4 t C ha^–1 and from 2.43 to 1.99 t N ha^–1. The C/N ratios of the organic horizons varied from 22 to 25, revealing no distinct pattern along the fly-ash gradient. Measurements of hot-water-extractable and water-soluble organic C suggested a reduced availability or a faster decomposition of soil organic matter in soils with historically high fly-ash loads.     

Comparison of soil nitrogen dynamics under beech,Norway spruce and Scots pine in central Germany

To investigate the effect of tree species on soil N dynamics in temperate forest ecosystems, total N (Nt), microbial N (Nmic), net N mineralization, net nitrification, and other soil chemical properties were comparatively examined in beech (64–68 years old) and Norway spruce (53–55 years old) on sites 1 and 2, and beech and Scots pine (45 years old) on site 3. The initial soil conditions of the two corresponding stands at each site were similar; soil types were dystric Planosol (site 1), stagnic Gleysols (site 2), and Podzols (site 3). In organic layers (LOf₁, Of₂, Oh), Nmic and Nmic/Nt, averaged over three sampling times (Aug., Nov., Apr.), were higher under the beech stands than under the corresponding coniferous ones. However, the Nmic in the organic layers under beech had a greater temporal variation. Incubation (10 weeks, 22 °C, samples from November) results showed that the net N mineralization rates in organic layers were relatively high with values of 8.1 to 24.8 mg N kg^–1 d^–1. Between the two corresponding stands, the differences in net N mineralization rates in most of the organic layers were very small. In contrast, initial net nitrification rates (0.2–17.1 mg N kg^–1 day^–1) were considerably lower in most of the organic layers under the conifer than under the beech. In the mineral soil (0–10 cm), Nmic values ranged from 4.1–72.7 mg kg^–1, following a clear sequence: August>November>April. Nmic values under the beech stands were significantly higher than those under the corresponding coniferous stands for samples from August and April, but not from November. The net N mineralization rates were very low in all the mineral soils studied (0.05–0.33 mg N kg^–1 day^–1), and no significant difference appeared between the two contrasting tree species.     

Natural resources of miombo woodland and recent changes in agricultural and land-use practices

Miombo woodland covers a large area of central Africa, south of the equator. The renewable natural resources of miombo, i.e., timber, poles, wood fuel, beeswax and honey, mushrooms, edible caterpillars, wild fruits and livestock browse are discussed and roughly evaluated. A brief account is given of the salient ecological features of miombo, including the role of fire and its timing in determining the tree population dynamics, and the parts played by tsetse fly and wildlife populations in the history of human occupation in the region.Under the traditional system of agriculture practised in northern Zambia, a form of shifting cultivation known as chitimene, the woodland is the agricultural fallow. Owing to population growth, changes in social attitudes and economic considerations, the chitimene system can no longer be practised. An important aspect of the changes taking place is the replacement of finger millet, a grain relatively rich in protein, by maize, giving a poorer diet and resulting in the need for measures to restore soil fertility periodically. One such measure discussed is the introduction of an edible leguminous fallow crop to maintain productivity.with the inevitable reduction in the area of the miombo woodlands which accompanies the changes in the agricultural system, their natural resources are becoming scarcer. Their importance in the economy is not appreciated by the people until there is a local shortage.The value of exotic pine and eucalyptus plantations, which grow well in the high rainfall areas (1000 mm⁺), and their economic future are considered.     

Post-fire soil nitrogen content and vegetation composition in Sub-Boreal spruce forests of British Columbia's central interior,Canada

Forest fires are known to influence nutrient cycling, particularly soil nitrogen (N), as well as plant succession in northern forest ecosystems. However, few studies have addressed the dynamics of soil N and its relationship to vegetation composition after fire in these forests. To investigate soil N content and vegetation establishment after wildfire, 13 sites of varying age class were selected in the Sub-Boreal spruce zone of the central interior of British Columbia, Canada. Sites varied in time since the last forest fire and were grouped into three seral age classes: (a) early-seral (<14 years), (b) mid-seral (50–80 years) and (c) late-seral (>140 years). At each site, we estimated the percent cover occupied by trees, shrubs, herbs and mosses. In addition, the soil samples collected from the forest floor and mineral horizons were analyzed for the concentrations of total N, mineralizable N, available NO₃⁻-N and available NH₄⁺-N. Results indicated that soil N in both the forest floor and mineral horizons varied between the three seral age classes following wildfire. Significant differences in mineralizable N, available NO₃⁻-N and available NH₄⁺-N levels with respect to time indicated that available soil N content changes after forest fire. Percent tree and shrub cover was significantly correlated to the amount of available NH₄⁺-N and mineralizable N contents in the forest floor. In the mineral horizons, percent tree cover was significantly correlated to the available NH₄⁺-N, while herb cover was significantly correlated with available NO₃⁻-N. Moss cover was significantly correlated with total N, available NO₃⁻-N and mineralizable N in the forest floor and available NO₃⁻-N in the mineral horizons. We identified several unique species of shrubs and herbs for each seral age class and suggest that plant species are most likely influencing the soil N levels by their contributions to the chemical composition and physical characteristics of the organic matter.     

Biomass burning in Brazil's Amazonian “arc of deforestation”: Burning efficiency and charcoal formation in a fire after mechanized clearing at Feliz Natal, Mato Grosso

Estimates of greenhouse-gas emissions from deforestation are highly uncertain because of high variability in key parameters and because of the limited number of studies providing field measurements of these parameters. One such parameter is burning efficiency, which determines how much of the original forest's aboveground carbon stock will be released in the burn, as well as how much will later be released by decay and how much will remain as charcoal. In this paper we examined the fate of biomass from a semideciduous tropical forest in the “arc of deforestation,” where clearing activity is concentrated along the southern edge of the Amazon forest. We estimated carbon content, charcoal formation and burning efficiency by direct measurements (cutting and weighing) and by line-intersect sampling (LIS) done along the axis of each plot before and after burning of felled vegetation. The total aboveground dry biomass found here (219.3 Mg ha⁻¹) is lower than the values found in studies that have been done in other parts of the Amazon region. Values for burning efficiency (65%) and charcoal formation (6.0%, or 5.98 Mg C ha⁻¹) were much higher than those found in past studies in tropical areas. The percentage of trunk biomass lost in burning (49%) was substantially higher than has been found in previous studies. This difference may be explained by the concentration of more stems in the smaller diameter classes and the low humidity of the fuel (the dry season was unusually long in 2007, the year of the burn). This study provides the first measurements of forest burning parameters for a group of forest types that is now undergoing rapid deforestation. The burning parameters estimated here indicate substantially higher burning efficiency than has been found in other Amazonian forest types. Quantification of burning efficiency is critical to estimates of trace-gas emissions from deforestation.     

Response of Coniferous Forest Ecosystems on Mineral Soils to Nutrient Additions: A Review of Swedish Experiences

Nitrogen (N) is the only nutrient that promotes forest growth when given individually. An extra stem growth of 15 m 3 ha -1 is obtained during a 10 yr period following an application of 150 kg N ha -1 . Larger growth increases have often been the result of more intensive N fertilization. Lime or wood ash give a minor growth stimulation on sites with a carbon (C) to N ratio below 30 in the humus layer, while the opposite effect prevails on N-poor sites. Nutrients given as soluble fertilizers are readily taken up by trees. Boron deficiency may be induced in northern Sweden after N fertilization or liming. The ground vegetation may be altered by single-shot N fertilization, but long-term effects occur only for intensive regimes. Lime or wood ash may modify the flora if soil pH is significantly altered: the change will be in response to N availability. Fruit-body production of mycorrhizal fungi is disfavoured by chronic N input, but also by lime or ash. However, the mycorrhizal structures on root tips are less affected. Faunistic studies are not common and those present are mostly devoted to soil fauna. A practical N dose of 150 kg N ha -1 has no clear effect, but higher doses may reduce the abundance in some groups. Hardened wood ash does not significantly affect the soil fauna. Lime favours snails and earthworms, while other groups are often disfavoured. The response of aquatic fauna to terrestrial treatments has hardly been studied. N fertilization generally results in insignificant effects on fish and benthic fauna. Lime and wood ash reduce the acidity of the topsoil, but practical doses (2-3 t ha -1 ) are too low to raise the alkalinity of runoff unless outflow areas are treated. N fertilizer use in forestry and N-free fertilizers lack effects on acidification. N fertilization may, however, be strongly acidifying if nitrification is induced and followed by nitrate leaching. N fertilization often results in increased long-term C retention in trees and soil, but does not promote significant N 2 O losses. N may temporarily reduce CH 4 oxidation in soil, but there are indications of a long-term opposite effect. Lime and poorly hardened wood ash may cause losses of C from N-rich soils. Only a few per cent of added N are leached to surface water following practical N fertilization, while N-free fertilizers do not induce N leaching. Soil incubations and soil-water studies suggest an increased risk for nitrate formation and leaching where lime or wood have been added to N-rich soils, but increased leaching to surface water has not been observed. Wood ash causes a temporal increase in bioavailability of cadmium (Cd). Other fertilizers may indirectly increase the availability of heavy metals. Wood ash may contain radioactive caesium 137Cs, but addition of such ash does not increase radioactivity in plants and soil.     

Site index curves for Norway spruce (Picea abies (L.) karst.) planted on abandoned farm land

Growth data were collected from 157 Norway spruce (Picea abies (L.) Karst.) stands planted on farm land in Sweden from 55 to 66° N. The mean age of the stands was 41 years (range, 25–91), the mean stand density 1 640 stems ha^–1 (range, 400–3 722), and the mean diameter at breast height (outside bark) 25 cm (range, 12–48). The height growth was measured in 56 stands during the initial five years after plantation and followed systematically until the stands were 30 years old. Early height growth for spruces growing on clay soils was lower than for trees growing on sand, till and peat soils. The height increment for 5-year-old spruces predicted the height increment for mature spruces (30–50 years old).Site index curves were constructed for total age. Curves for H₄₀ (dominant height at 40 years total age) were made for both northern (50 stands, 61–66° N) and southern (107 stands, 55–61° N) Sweden. Site index curves for H₅₀ at total age are presented for Sweden as a whole (Lat. 55–66° N) and southern Sweden only. Curves based on breast height age were also constructed for H₄₀ and H₅₀. Curves fitted for H₄₀ at breast height were well in accordance with the curves presented in the past for spruce on forest lands by Nordic studies. The curves from the present study have slower growth for young spruces than curves for forest land. For 40–90-year-old spruce stands, curves from the present study indicate taller heights than from forest land curves.     

A theoretical study for North Vietnam of alternative agroforestry systems to pure cassava

Cassava is recognized in the region as the second most important crop after paddy rice. In Vinh Phu province of Vietnam, it is usually grown on highly erodible slopes of the small hills surrounding paddy fields. Cassava crop land is generally in annual use until the yield is less than 3–4 t fresh tubers/ha. This constant cropping system rapidly depletes the soil as fertilizers are infrequently used and crop residues are usually removed from the fields. Erosion is a major problem as the soil is exposed through hand cultivation and regular weeding during a cropping season which coincides with the wet season. Sustainable and productive cropping systems are needed.In order to take a first step towards sustainability, this paper presents several possible agroforestry systems in which cassava could be intercropped with a number of nitrogen fixing trees and shrubs. Several theoretical combinations have been examined assuming a critical lower production limit of 3–4 t fresh tubers/ha. Those with sustained high potential yields are recommended for future field experimentation.     

Short-term impacts of harvesting and burning disturbances on physical and chemical characteristics of forest soils in western Newfoundland,Canada

The short-term impacts of prescribed burning were determined for the physical and chemical properties of a forest soil on a clear-cut in western Newfoundland, Canada. The experimental site was predominantly covered by black spruce trees (Picea mariana Mill.) that were harvested in 1996. In August 1998, prescribed burning removed most of the logging residues left on the site after harvesting, all above-ground surface vegetation, and parts of the upper humus layer. In October 1998, field samples were taken from four replicated burned plots and four replicated unburned plots. In each study plot, soil samples were taken from the organic layer (F+H) and from the top 10 cm of the mineral soil. In the burned treatments, mass of the humus layer (F+H) was reduced by 24% and, because of the accretion of basic ash materials, acidity of the humus layer was also reduced by up to 1 unit. In the organic layer and the mineral soil, total contents of Mg, Ca, and P, extractable Mg and Ca, available P, sum of NH₄Cl-extractable cations, and C/N ratios were increased by burning, while total C and N as well as total and extractable K remained unaffected by burning.