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.     

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.     

Amazonian deforestation and global warming: carbon stocks in vegetation replacing Brazil''s Amazon forest

Carbon stocks in vegetation replacing forest in Brazilian Amazonia affect net emissions of greenhouse gases from land-use change. A Markov matrix of annual transition probabilities was constructed to estimate landscape composition in 1990 and to project future changes, assuming behavior of farmers and ranchers remains unchanged. The estimated 1990 landscape was 5.4% farmland, 44.8% productive pasture, 2.2% degraded pasture, 2.1% ‘young’ (1970 or later) secondary forest derived from agriculture, 28.1% ‘young’ secondary forest derived from pasture, and 17.4% ‘old’ (pre-1970) secondary forest. The landscape would eventually approach an equilibrium of 4.0% farmland, 43.8% productive pasture, 5.2% degraded pasture, 2.0% secondary forest derived from agriculture, and 44.9% secondary forest derived from pasture. An insignificant amount is regenerated ‘forest’ (defined as secondary forest over 100 years old). Average total biomass (dry matter, including below-ground and dead components) was 43.5 t ha⁻¹ in 1990 in the 410 × 10³ km² deforested by that year for uses other than hydroelectric dams. At equilibrium, average biomass would be 28.5 t ha⁻¹ over all deforested areas (excluding dams). These biomass values are more than double those forming the basis of deforestation emission estimates currently used by the Intergovernmental Panel on Climate Change (IPCC). Although higher replacement landscape biomass decreases net emissions from deforestation, these estimates still imply large net releases.     

Long-term base cation mass balances for Swedish forests and the concept of sustainability

Data from the Swedish Forest Inventory was used to calculate mass balances for base cations Ca, Mg and K for Swedish forests. Using lysimeter and forest survey soil analyses to estimate present base cation leaching from the root zone reveals that weathering plus base cation deposition is not sufficient to support both, the present base cation leaching rate and the present rate of uptake caused by stem growth. Calculations suggest that 96% of the productive forested area may have higher rates of removal than supply for one or more base cation. Under a best-case scenario, assuming less pollution, the present growth rate and 100% efficiency in uptake of available nutrients, the area with more removal than supply would still be at least 30% of the total area. Forest soils are being depleted at a rate where the exchangeable reservoirs have high risk of being severely depleted in the next few decades in central and southern Sweden. During 1983–1985 the depletion rate is calculated to be, on the average, 0.33 keq ha⁻¹ year⁻¹. The weathering rate and present base cation deposition can sustain growth at a level where (80–85)×10⁶m³ stemwood year⁻¹ can be harvested. Any harvested growth beyond this volume must be sustained by artificial means.

For whole-tree harvesting without base cation return, the calculations indicate that it would significantly increase the base saturation depletion rate to an average of 0.62 keq ha⁻¹ year⁻¹, and risk depletion of the soil in less than one-to-two rotation periods almost anywhere in Sweden.

The calculations stress the importance that sustainable forest management must include the management of nutrient fluxes and reservoirs.     

Satellite-based estimation of biomass carbon stocks for northeast China''s forests between 1982 and 1999

Northeast China maintains large areas of primary forest resource and has been experiencing the largest increase in temperature over the past several decades in the country. Therefore, studying its forest biomass carbon (C) stock and the change is important to the sustainable use of forest resources and understanding of the forest C budget in China. In this study, we use forest inventory datasets for three inventory periods of 1984–1988, 1989–1993 and 1994–1998 and NOAA/AVHRR Normalized Difference Vegetation Index (NDVI) data from 1982 to 1999, to estimate forest biomass C stock and its changes in this region over the last two decades. The averaged forest biomass C stock and C density were estimated as 2.10 Pg C (1 Pg = 10¹⁵ g) and 44.65 Mg C ha⁻¹ over the study period. The forest biomass C stock has increased by 7% with an annual rate of 0.0082 Pg C. The largest increase in the C density occurred in two humid mountain areas, Changbai Mountains and northern Xiaoxing’anling Mountains. Climate warming is probably the key driving force for this increase, while anthropogenic activities such as afforestation and deforestation may contribute to variations in the C stocks.     

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.     

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.     

Biogeochemical impacts of clearfelling and reforestation on blanket peatland streams I. phosphorus

Forest drains and streams on blanket peatland in western Ireland were sampled weekly, 1996–2000, using continuous, depth-proportional passive sampling, and analysed for molybdate-reactive phosphorus (MRP) by the acid–antimony-molybdate method. The study area was largely clearfelled and partly reforested with wind-rowing, drainage, planting, and aerially applied rock phosphate equivalent to 70 kg P/ha. Further felled areas were not wind-rowed, drained or fertilised for reforestation. Catchment areas were of the following orders: 1 ha (two forest drains); 10–20 ha (two semi-permanent drains, one permanent stream); 1–3 km² (three permanent streams). Streamwater from three undisturbed closed-canopy-forest catchments had pre-felling median concentrations of MRP (all values are μg MRP l⁻¹) of 9 (catchment approximately 1 km²), 13 (1 ha) and 93 (1 ha). Clearfelling was associated with large increases (maxima 305, 4164 and 3530 μg MRP l⁻¹) in MRP concentrations in each case. Following protracted mechanical operations in four other catchments of ca. 1 km², 20, 10 and 10 ha, with apparently existing elevated MRP concentrations (medians 41, 328, 102, 214 μg MRP l⁻¹) fertilising gave major rises (maxima 218, 2723, 806, 2323 μg MRP l⁻¹). The three smaller catchments showed subsequent exponential-type declines, while the 1 km² catchment had sustained high values (median 74 μg MRP l⁻¹) over the remaining study period. The higher values in this one larger stream were seasonally cyclical, with a late summer maximum. Annual median MRP values above 70 μg l⁻¹ represent a seriously polluted state for these streams, which qualify as waterways under relevant statutes, but it is not clear what implications these results have for downstream river-water quality in larger channels.     

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.     

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.     

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.     

Future development of the Leningrad region forests under nature-oriented forest management

Russian forests are of high importance for the Russian economy, the European wood market, for nature conservation, and for carbon sequestration. However, the ongoing changes in forest management and administration in Russia led to uncertainty about forest ownership, wood harvesting levels, and long-term impacts of alternative management plans. Therefore, better insight in their current and future state is highly desirable. We present a study for the Leningrad region forests in which alternative management regimes for wood production and nature conservation values are balanced in varying ways. The total forest land area in the Leningrad region forest fund is 4.8 million ha. Coniferous species dominate and due to the natural succession occurring, the forests are divers in vertical structurally.

A timber assessment model was used to project the forest until 2040. Five forest management scenarios were run. Special attention was paid to a scenario that simulates recovery of the Russian forest sector in combination with the incorporation of a ‘set-aside for nature conservation’ policy. All scenarios showed that recovery of the forest sector in the Leningrad region is biologically feasible. A sustainable continuous annual production of 10.6 million m³ per year (2.8 m³ ha⁻¹ per year) by 2040 was found. The ‘Recovery with Nature Conservation’ scenario showed that recovery of the forest sector in combination with the establishment of set-aside areas is very well feasible. It was possible to set aside 28% of the forest area for nature conservation while still developing a forest sector to a production level higher than that achieved in the late eighties.

The timber assessment model applied was not specifically designed to incorporate nature-oriented forest management. We, therefore, discuss ways of improving the required methodology to analyse long-term effects of nature-oriented forest management in Europe.     

Characteristic site disturbance due to harvesting and extraction machinery traffic on sensitive forest sites with peat soils

The effects of wood harvesting and extraction machinery traffic on sensitive forest sites with peat soils were characterised with the objective of quantifying the threshold levels beyond which significant site impacts (compaction and rutting) would occur. The treatments involved running the machines in selected extraction racks (i.e., 3 m wide machine routes) while conducting normal wood thinning and extraction operations comprising one and two passes by the harvester and the forwarder with full payload, respectively. Soil disturbance thresholds were established by testing the level of significance of the difference in induced soil damage and compaction before and after machine traffic treatments. For volumetric soil water content lying between 10.0 and 14.9%, threshold cone penetration resistance levels for two 600/55–30.5 tyres were found to range from 594 to 640 kPa for deep-raised peat soil with initial strength lying between 524 and 581 kPa. In general, the proportion of the total rut depth data in each rack that exceeded the threshold level of 21.5 cm was about 5%. The threshold value corresponds to sinkage equivalent to 15% of the overall wheel diameter of the harvester, above which machine mobility would be hampered considerably. In addition, after harvester traffic the mean rut depth per unit rack length was 10.2×10⁻² cm/m, and it ranged from 0.7 to 24.7×10⁻² cm/m.     

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%.     

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.     

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.     

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.     

Fate of N-labelled nitrate in a wet summer under different residue management regimes in young hoop pine plantations

A field study was conducted to investigate the fate of ¹⁵N-labelled nitrate applied at 20 kg N ha⁻¹ in a wet summer to microplots installed in areas under different residue management regimes in second-rotation hoop pine (Araucaria cunninghamii) plantations aged 1–3 years in south-east Queensland, Australia. PVC microplots of 235 mm diameter and 300 mm long were driven into 250 mm soil. There were three replications of each of eight treatments. These were areas just under and between 1-year-old windrows (ca. 2–3 m in width) of harvesting residues spaced 15 m apart, and with and without incorporated foliage residues (20 t DM ha⁻¹); the areas just under and between 2- or 3-year-old windrows spaced 10 m apart. Only 7–29% of the added ¹⁵N was recovered from the top 750 mm of the soil profile with the leaching loss estimated to be 70–86% over the 34-day period. The ¹⁵N loss via denitrification was 3.7–6.3% by directly measuring the ¹⁵N gases emitted. The microplots with the incorporated residues at the 1-year-old site had the highest ¹⁵N loss (6.3%) as compared with the other treatments. The ¹⁵N mass balance method together with the use of bromide (Br) tracer applied at 100 kg Br ha⁻¹ failed to obtain a reliable estimate of the denitrification loss. The microplots at the 1-year-old site had higher ¹⁵N immobilisation rate (7.5–24.7%) compared with those at 2- and 3-year-old sites (2.1–3.6%). Incorporating the residues resulted in an increase in ¹⁵N immobilisation rate (24.5–24.7%) compared with the control without the incorporated residues (8.4–14.3%). These findings suggest that climatic conditions played important roles in controlling the ¹⁵N transformations in the wet summer season and that the residue management regimes could also significantly influence the ¹⁵N transformations. Most of the ¹⁵N loss occurred through leaching, but a considerable amount of the ¹⁵N was lost through denitrification. Bromide proved to be an unsuitable tracer for monitoring the ¹⁵N leaching and movement under the wet summer conditions.     

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.     

Land-use changes alter CO2 flux patterns of a tall-grass Andropogon field and a savanna-woodland continuum in the Orinoco lowlands

Land use changes in the savannas of the Orinoco lowlands have resulted in a mosaic of vegetation. To elucidate how these changes have affected carbon exchanges with the atmosphere, we measured CO2 fluxes by eddy covariance and soil CO2 efflux systems along a disturbance gradient beginning with a cultivated tall-grass Andropogon field (S1) and extending over three savanna sites with increasing woody cover growing above native herbaceous vegetation. The savanna sites included a herbaceous savanna (S2), a tree savanna (S3) and a woodland savanna (S4). During the wet season, maximum diurnal net ecosystem exchange (NEE) over the S1-S4 sites was 6.6-9.3, 6.6-7.9, 10.6-11.3 and 9.3-10.6 micromol m(-2) s(-1), respectively. The rate of CO2 uptake over S1 was lower than that for C4 grasses elsewhere because of pasture degradation. Soil respiration and temperature were exponentially related when soil water content (theta) was above 0.083 m(3) m(-3); however, soil respiration declined markedly as theta decreased from 0.083-0.090 to 0.033-0.056 m(3) m(-3). There were bursts of CO2 emission when dry soils were rewetted by rainfall. During the wet season, all sites constituted carbon sinks with maximum net daily ecosystem production (NEP) of 2.1, 1.7, 2.1 and 2.1 g C m(-2) day(-1), respectively. During the dry season, the savanna sites (S2-S4) became carbon sources with maximum emission fluxes of -0.5, -1.4 and -1.6 g C m(-2) day(-1), respectively, whereas the tall-grass field (S1) remained a carbon sink with a maximum NEP of 0.3 g C m(-2) day(-1) at the end of the season. For all measurement periods, annual NEP of sites S1-S4 was 366, 6, 116 and 139 g C m(-2), respectively. Comparisons of carbon source/sink dynamics across a wide range of savannas indicate that savanna carbon budgets can change in sign and magnitude. On an annual basis, gross primary production over the S1-S4 stands was 797, 803, 136 and 1230 g C m(-2), respectively. Net primary productivity (NPP) of the S1-S4 stands, calculated from eddy covariance measurements as the daily sum of NEE and day and night heterotrophic respiration was 498, 169, 181 and 402 g C m-2 year-1, respectively. These values were slightly higher than NPP based on harvest measurements (432, 162, 176 and 386 g C m(-2) year(-1), respectively), presumably because fine roots were incompletely harvested. Soil water content limited carbon uptake at all sites, and water-use efficiency (WUE) was related to rainfall dynamics. During the dry season, all sites except the cultivated tall-grass Andropogon field (S1) had a negative WUE. Although our results are specific to the Orinoco vegetational mosaic, the effects of land-use practices on the controls and physiological functions of the studied ecosystems may be generalized to other savannas.