Effects of experimental acidification,nitrogen addition and liming on ground vegetation in a mature stand of Norway spruce (Picea abies (L.) Karst.) in SE Sweden

During the period 1976–1991, a combined experiment of acidification, liming and nitrogen addition in a mature spruce stand was conducted at Farabol in south-east Sweden. The aim of this study was to investigate the effects of these treatments on the ground vegetation 0, 1, 5 and 15 years after experimental establishment. The treatment regimes were nitrogen (200 kg N ha⁻¹, repeated three times at 4–5-year intervals, totally 600 kg N ha⁻¹), sulphur powder (50 and 100 kg S ha⁻¹ a⁻¹, totally 600 and 1200 kg ha⁻¹), sulphur plus nitrogen (600+600 kg ha⁻¹) and limestone (500 kg ha⁻¹ a⁻¹, i.e. totally 6000 kg ha⁻¹). The results showed that nitrogen addition and liming promoted the abundance of the grass Deschampsia flexuosa, while acidification had a negative effect on D. flexuosa and herbs in the field layer. There was a negative reaction giving immediate damage to the bryophytes in connection with additions of nitrogen, sulphur powder and lime. The magnitude of damage and the capacity to recover varied among species as well as among treatments. The recovery from immediate damage after liming was much faster than after the treatments with sulphur powder and/or nitrogen. A negative interaction between sulphur powder and nitrogen was found for herbs and mosses where the combined effects were stronger than the effects of a single treatment alone. Acidification also had a negative effect on the total number of species. The results of this study showed that acidification and nitrogen deposition could negatively influence forest vegetation by changing the nutrient availability in the soils. Liming led to an improved growth of the forest ground vegetation and the flora changed towards a more nitrophilic species composition.     

Stand development and production dynamics of loblolly pine under a range of cultural treatments in north-central Florida USA

The objectives of this study were to examine the effects of stand development and soil nutrient supply on processes affecting the productivity of loblolly pine (Pinus taeda L.) over a period approximately equal to a pulpwood rotation (18 years). The experiment consisted of a 2×2 factorial combination of complete and sustained weed control and annual fertilization treatments (C: control treatment, F: fertilization, W: weed control, FW: combined fertilization and weed control), located on a Spodosol in north-central Florida, USA. The reduction of soil nutrient limitations through fertilization or control of competing vegetation resulted in dramatic increases in almost every measure of productivity investigated, including height (19.7 m in the FW treatment versus 12.5 m in the C treatment at age 18 years), basal area (FW=44.2 m² ha⁻¹, F=39.6 m² ha⁻¹, W=36.6 m² ha⁻¹, C=19.9 m² ha⁻¹ at age 16 years), stemwood biomass accumulation (114 Mg ha⁻¹ in FW versus 42.8 Mg ha⁻¹ in C at age 18 years), foliar nitrogen concentration (1.53% in plots receiving fertilization versus 1.06% in unfertilized plots at age 17 years) and leaf area index (age 16-year peak projected of approximately 3.3 at age 9–10 years in F and FW plots, 2.5 in the W treatment and 1.5 in the C plots). Cultural treatments also decreased the growth ring earlywood/latewood ratio, and accelerated the juvenile wood to mature wood transition. While soil nutrient supply was a major determinant of productivity, production changes that occurred within treatments over the course of stand development were equally dramatic. For example, between age 8 and 15 years, stemwood PAI in the FW treatment declined by 275%; similarly large reductions occurred in the F and W treatments over the same time period. The reductions in PAI in the treated plots were linearly related to stand BA, suggesting the decline in productivity was associated with the onset of inter-tree competition. Responses of stemwood PAI to re-fertilization treatments at age 15 years suggests that the declines in growth and growth efficiency with time were partially attributable to nutrient limitations.     

Litter production and organic matter accumulation in exclosures of the Tigray highlands,Ethiopia

To determine annual litter production of regenerating forest areas in the Tigray highlands of northern Ethiopia monthly litter production was monitored over a two-year period in areas with varying degree of vegetation cover restoration. Total annual litter production varied from 30 to 425 g m⁻² and increased significantly where areas were closed for a longer time. Litter production was depending on vegetation cover through an exponential relation and was influenced also by soil fertility. Leaf litter typically constituted between 70 and 85% of total litter production, while contributions of woody and reproductive litter varied according to species composition. Strong seasonality in litterfall was explained by pronounced seasonal variation in rainfall. Standing crop of litter built up once an area was closed for grazing, increasing from around 20 g m⁻² in degraded grazing lands to nearly 600 g m⁻² in an old exclosure. Litter accumulation was mainly determined by litter input, but was also influenced by litter quality, species composition and microclimate development in the restoring forest areas. A detailed study of nine dominant shrub and tree species revealed three distinct litter production patterns, corresponding to drought-deciduous species, evergreen species and (semi-)evergreen Acacia species respectively.     

Forest floor vegetation plays an important role in photosynthetic production of boreal forests

We estimated gross photosynthetic production (GPP) of the forest floor vegetation in a 40-year-old Scots pine stand in southern Finland with three different methods: measurements of CO₂ exchange of single leaves of field and ground layer species, measurement campaigns of forest floor net CO₂ efflux at different irradiances with a manually operated soil chamber, and continuous measurements of forest floor net CO₂ efflux with an automatic transparent chamber system. We upscaled the measured light response curves from the manual soil chambers using the biomass distribution of the forest floor species, a modelled seasonal pattern of photosynthetic capacity and a model of light extinction down the canopy. Leaf gas exchange measurements as well as measurements of net CO₂ efflux with the manual chamber indicated saturation of photosynthesis at relatively low (50–400 μmol m⁻² s⁻¹) light levels. Leaf and patch level measurements gave similar rates of photosynthetic CO₂ fixation per unit leaf biomass suggesting that reduction in photosynthetic production due to within-patch shading was small. Upscaling of photosynthetic production to the stand level and continuous measurements with the automatic soil chambers indicated that momentary photosynthetic production by the forest floor vegetation in the summer was typically about 2 μmol m⁻² (ground) s⁻¹. Cumulative upscaled GPP over the period of no snow (from 20 April to 20 November) in year 2003 was 131 g C m⁻². Continuous measurements with the automatic soil chamber system were in line with the upscaling, the cumulative GPP being 83 g C m⁻² and the seasonal pattern of photosynthetic rate similar to that of the upscaled photosynthesis.     

Fine-root distribution and morphology in an acidic Norway spruce (Picea abies (L.) Karst.) stand in SW Sweden in relation to granulated wood ash application

Wood ash is recommended as a compensatory fertiliser to counteract the effects of acidic deposition on forest ecosystems. Spatial distribution of biomass, necromass and morphology parameters of the fine roots (diameter classes <1, 1–2, <2 mm) of Norway spruce (Picea abies (L.) Karst.) were analysed in response to fertilisation with granulated wood ash (GWA) in a long-term field experiment in SW Sweden. GWA was applied as a single dose of 3200 kg ha⁻¹ and the fine roots were sampled 9 years later by soil coring. Soil cores were divided into 1-cm strata within the top 0–2.5 cm humus limits, the lower humus below 2.5 cm (with varying thickness) and the mineral soil to 50 cm depth (from ground surface). Total fine-root biomass in the control (C) and GWA treatment, 256 ± 20 and 258 ± 25 g m⁻², respectively, and length 2072 ± 182 and 1800 ± 198 m m⁻², respectively, did not differ statistically from each other. Total fine-root necromass in the 1–2 mm fraction was significantly higher in C than in the GWA treatment, 130 ± 12 and 80 ± 10 g m⁻², respectively. Fine-root biomass in the <1 mm fraction was significantly greater in the lower humus in the GWA treatment, but this did not affect the total biomass in the <1 mm fraction in the whole soil profile. The biomass-to-necromass ratio (1–2 mm) was significantly higher in the GWA treatment in the 0–30 cm soil layer than in the corresponding layer of the control. Specific root length (SRL) was lower in the GWA treatment than in the control in the 0–5 cm soil layer. The lower necromass and SRL were more clearly related to the GWA treatment, whereas the difference in the vertical distribution of biomass may have been related to the thicker humus layer in the GWA plots.     

The analyses of physiological and morphological attributes of 10 tree species for early determination of their suitability to afforest degraded landscapes in the Aral Sea Basin of Uzbekistan

The establishment of woody fallow systems is an option for the improvement of degraded agricultural landscape within the ecologically deteriorated Aral Sea area in northwest Uzbekistan. Growth and development of ten tree species, differing in tolerance to drought and salinity, were studied over 24 months. To determine species suitability for afforestation, conventional diameter and height measurements were compared to Relative Growth Rate (RGR) and its underlying components, Net Assimilation Rate (NAR), Specific Leaf Area (SLA) and Leaf Weight Ratio (LWR), as well as the Crop Growth Rate (CGR) as a function of NAR and Leaf Area Index (LAI). RGRs varied between 0.46 and 2.16 mg g⁻¹ day⁻¹ and showed highly significant differences among species, but also between years, which reduces the parameter's suitability for species selection. The same is true for NAR and SLA. CGR values ranged from 0.01 to 0.71 g m⁻² day⁻¹, increased with age of the trees, and showed significant species differences. CGR correlated better with NAR (r = 0.89) and SLA (r = 0.86) than RGR. Overall CGR correlated highly with the RGRs in height and diameter (in 80% and 71% of all cases, respectively). Thus, CGR appears to be a suitable indicator complementing RGR, NAR and SLA estimates to decide on species suitability for afforestation. The intensive measurements for RGR should be done later, rather than earlier in the tree life cycle. At an early age, CGR and diameter measurements are more meaningful. A combination of estimates, not a single factor, best assesses the long-term sustainable growth under natural conditions and allows early selection of species suitable for afforestation of degraded patches in the agricultural landscape. Ranking of all parameters concurrently showed the high potential of Elaeagnus angustifolia and Populus euphratica, which matched previous rankings based on total biomass production and financial added value.     

Nutrient uptake and growth of young trees in a P-deficient soil: Tree species and phosphorus effects

Phosphorus deficiency is widespread in the subhumid highlands of eastern Africa but there are few data on the effect of P deficiency on the growth of agroforestry tree species. We studied the effect of P application on growth, nutrient uptake and dry matter partitioning in young trees of Calliandra calothyrsus, Cedrela serrulata, Eucalyptus grandis, Grevillea robusta, Markhamia lutea, Senna spectabilis, and Sesbania sesban on a P-deficient soil (Kandiudalfic Eutrudox, bicarbonate-EDTA extractable P = 1 mg kg⁻¹) in western Kenya. The trees were grown at two P levels (control and 500 kg added P ha⁻¹) at 1 m² spacing in a randomized complete block design with three replications. Leaf K concentrations were in the low range for all species (5–9 mg g⁻¹) and K deficiency may have limited responses to P. Averaged over species, P addition increased aboveground shoot dry matter by a factor of 2.6 at 62 and 124 days, but the response decreased to 1.3 at 325 days. The increases at 62 days were large in sesbania (5.4) and eucalyptus (3.2) but small in calliandra (1.4) and markhamia (1.1). Relative response to P was more strongly correlated with shoot growth rate per unit root length among species than with shoot growth rate alone. Calliandra, which had high early growth rate but low response to added P, had an exceptionally high root length (6.0 km m⁻²) compared with the other species (0.3–2.1 km m⁻²). P addition increased N and P content but decreased final shoot K content in sesbania and calliandra, and had little effect on K content in the other species. The high-yielding species (eucalyptus, sesbania and calliandra) accumulated more than 30 g N and 2 g P m⁻² in shoots in 325 days of growth. The proportion of total shoot N in wood (branch + stem) was in a higher range (67–75%) in the shrubby species (sesbania, calliandra, senna) than in the upperstorey tree species (38–43%). Slow early shoot growth relative to total root length, and high specific root length (root length per unit root mass) are proposed as criteria for the selection of species and provenances that are well adapted to P deficient soils.     

A low-cost method of mycorrhizal inoculation improves growth of Acacia tortilis seedlings in the nursery

A pot experiment in Kenya examined the effects of mycorrhizal and rhizobial inoculation on the growth and symbiont infection of Acacia tortilis seedlings in unsterile soil, in comparison with sterile and unsterile soil controls. Three mixed, arbuscular mycorrhizal inocula, originating from A. tortilis, Terminalia brownii, T. spinosa and Prosopis juliflora (produced under nursery conditions), were used to inoculate the seedlings, which also received a standard, mixed, rhizobial inoculum. Mycorrhizal infection and plant growth were significantly affected by experimental treatment (p<0.001). Some mycorrhizal infection (presumably with indigenous soil-borne inoculum) occurred in the unsterile soil control, but not in the sterile soil control, indicating that transfer of inoculum between pots in the experiment did not occur. Mycorrhizal formation was significantly more extensive in inoculated plants, which also had better shoot and root growth. 24 weeks after inoculation, plants which had received inoculum of A. tortilis origin were 105 mm tall and their roots were 56% mycorrhizal, whereas the controls most representative of normal nursery treatment were only 77 mm tall and 29% mycorrhizal. Nodulation was erratic and poor in all treatments. The results suggest that mycorrhizal inoculation can improve both mycorrhizal infection and growth of tree seedlings in unsterile nursery soil. The methods of culture and inoculation described here could easily be adopted by nurseries at little cost.     

Biomass and nutrient accumulation in pure and mixed plantations of indigenous tree species grown on poor soils in the humid tropics of Costa Rica

Aboveground biomass and nutrients and soil chemical characteristics were examined in young plantations of four indigenous tree species: Hieronyma alchorneoides, Vochysia ferruginea, Pithecellobium elegans, and Genipa americana, growing in mixed and pure stands at La Selva Biological Station, Costa Rica. Total tree biomass production rates ranged from about 5.2 Mg ha⁻¹ year⁻¹ for G. americana to 10.3 Mg ha⁻¹ year⁻¹ for H. alchorneoides pure stands, and for the species mixture it was about 8.9 Mg ha⁻¹ year⁻¹. Branches and foliage formed 25–35% of total tree biomass but they represented about 50% of total tree nutrients. H. alchorneoides, the four species mixture, and P. elegans had the greatest accumulations of total aboveground nutrients per hectare. The importance of the plantation floor as a nutrient compartment varied temporally. When forest floor litter biomass was at its peak, plantation floor litter N, Ca, and Mg were roughly equal to, or greater than stem nutrients for all species except for P. elegans. For P. elegans, the plantation floor consistently represented a very low proportion of total aboveground nutrients. G. americana and V. ferruginea trees showed 55–60% less biomass accumulation in mixed than in pure stands while H. alchorneoides and P. elegans trees grew 40–50% more rapidly in mixture. P. elegans foliage had 60% lower Ca but higher P concentrations in mixed than in pure stands, and G. americana had higher foliar Mg in mixed than in pure stands. V. ferruginea stands had the highest concentrations of soil Ca, Mg, and organic matter, particularly in the top layers. Relative to pure plantations, soil nutrient concentrations in mixed plantations were intermediate for N, P, and K, but lower for Ca and Mg. The results of this study can be used in the selection of tree species and harvest designs to favor productivity and nutrient conservation.     

Stand development after different thinnings in two uneven-aged Picea abies forests in Sweden

Two field experiments, located in Central and Northern Sweden, were used to study the influence of standing volume on volume increment and ingrowth in uneven-aged Norway spruce (Picea abies (L.) Karst.) stands subjected to different thinnings. Each experiment had a 3 × 2 factorial block design with two replications. Treatments were thinning grade, removing about 45, 65, and 85% of pre-thinning basal area, and thinning type, removing the larger or the smaller trees, respectively. Each site also had two untreated control plots. Plot size was 0.25 ha. Volume increment was 0.5–6.8 m³ ha⁻¹ year⁻¹ for the plots, and significantly positively (p < 0.01) correlated with standing volume. Within treatment pairs, plots thinned from Above had consistently higher volume increment than plots thinned from Below. Ingrowth ranged from 3 to 33 stems ha⁻¹ year⁻¹, with an average of 14 and 21 stems ha⁻¹ year⁻¹ at the northern and southern site, respectively. At the southern site ingrowth was significantly negatively (p < 0.01) correlated with standing volume, but not at the northern site. Mean annual mortality after thinning was 2 and 7 stems ha⁻¹ year⁻¹at the northern and southern site, respectively.     

Applying PROMOD spatially across Tasmania with sensitivity analysis to screen for prospective Eucalyptus globulus plantation sites

When screening areas as prospective plantation sites two criteria are of interest: the expected yield at that site and the uncertainty associated with this yield expectation. This paper describes a methodology to enable spatial estimates of uncertainty to be attached to predictions of tree growth in relation to variations in soil depth, nutrient status, and drainage. An environmental sensitivity analysis was applied using the growth model PROMOD to produce site productivity and suitability maps for Eucalyptus globulus (Labill.) on a 1000 m regular grid of Tasmania. Nine productivity surfaces were computed, one for each of three assumed soil-water holding capacities (for soil depths of 0.5, 1.0, and 1.5 m — the common classes in Tasmania), combined with three permutations of soil-nutrient rating (the expected rating and one class above and one below this value), and incorporating potential waterlogging estimates. The mean and coefficient of variation was calculated for each 1000 m grid cell in the nine subsequent productivity surfaces. A final plantation suitability grid was then mapped to show areas of the State with predicted high (>25 m³ ha⁻¹per year), moderate (15–25 m³ ha⁻¹ per year), and low productivity (<15 m³ ha⁻¹ per year), each with corresponding high variability (coefficient of variation (CV>30%)) and low variability classes (CV<30%). As no areas had both, high productivity and high variability, only five plantation suitability classes were mapped.Those areas indicated as high productivity/low variation sites have a high probability of being successful E. globulus plantations. Low productivity sites with either high or low variation are very unlikely to be favourable for this species. Moderate productivity sites with low variation are likely to be viable potential plantation sites with relatively low risk, particularly with nutrient amelioration. Moderate productivity sites with high variation might warrant further field investigation to determine if there are major environmental factors or combinations of these, such as soil depth (water holding capacity), nutrient status, or drainage that are actually going to be limiting reasonable growth at these sites.     

Dynamics of above- and below-ground biomass and nutrient accumulation in an age sequence of Nothofagus antarctica forest of Southern Patagonia

Nothofagus antarctica (Forster f.) Oersted is a deciduous tree species, which naturally grows on poorly drained or drier eastern sites in the Andes Mountain near Patagonian steppe. Above- and below-ground biomass and nutrients pools were measured in pure even-aged stands at different ages (5–220 years) and crown classes. Functions were fitted for total biomass and nutrients accumulation, and root/shoot ratio of individual trees against age. Total biomass accumulated for mature dominant trees was eight times greater than mature suppressed trees. Biomass root/shoot ratio decreased with age from 1.8 to a steady-state of 0.5. All nutrients concentration (except Ca) decreased with age and varied according to the degree of crown suppression classes. Nutrient concentrations varied between biomass pool components following the order leaves > bark > small branches > fine roots > medium roots > rooten wood > coarse roots > sapwood > heartwood. Total nutrient accumulation followed the order dominant > codominant > intermediate > suppressed trees and its accumulation rate varied over time, e.g. P accumulation rate of dominant trees increased from 0.17 g tree⁻¹ year⁻¹ during regeneration to 1.39 g tree⁻¹ year⁻¹ in mature trees. Nutrients uptake reached a peak during the period of maximum biomass production, and root/shoot ratio of nutrients decreased from its maximum value at 5 years of age (0.6, 4.0, 0.9, 1.5, 1.0 and 2.6 for N, P, K, Ca, S and Mg, respectively) to a steady-state asymptote beyond 50 years of age. Thus, accumulation of nutrients in roots was greater during the regeneration phase of stand development, and nutrient accumulation increased in above-ground over time. Also, nutrient use efficiency increased in mature trees (111–220 years) and decreased in suppressed crown classes. The equations developed for individual trees have been used to estimate stand biomass and nutrient accumulation from forest inventories data. Total stand biomass varied from 62.5 to 133.4 t ha⁻¹ and total nutrients accumulation ranged from 3 kg Mg ha⁻¹ to 1235 kg Ca ha⁻¹. Proposed equations can be used for practical purposes such as to estimate pasture nutrients requirement in a silvopastoral system based on nutrients supply from leaf litter returns, or to determine amelioration practices like debarking stems before harvesting.     

Simulation of carbon and water budgets of a Douglas-fir forest

The forest growth/hydrology model FORGRO–SWIF, consisting of a forest growth and a soil water model, was applied to quantify the inter-annual variability of the carbon and water budgets of a Douglas-fir forest (Pseudotsuga menziessii (Mirb.) Franco) in The Netherlands. With these budgets, the water use efficiency, the amount of water needed to fix a certain amount of carbon, and its variability was estimated. After testing the model performance in simulating daily carbon and transpiration fluxes, and soil water contents of this forest ecosystem, the model was applied to a 10-year period of meteorological data. Two forest parameterisations were used: the non-thinned situation of 1995, and the thinned situation in 1996. Relations between forest water use and forest growth were quantified with the model. The model performed satisfactory, an R² value for daily carbon fluxes of 0.58 and for daily transpiration fluxes 0.81. The forest showed to be a clear carbon sink, in the climax situation between 1000 and 1210 g C m⁻² per year. In the thinned situation the carbon uptake was more than halved to values between 430 and 620 g C m⁻² per year. The calculated yearly WUE’s for the forest were between 2.5 and 4.3 g C m⁻² mm⁻¹ and for the total ecosystem between 1.1 and 2.0 g C m⁻² mm⁻¹. The thinned forest had clearly lower WUE’s than the non-thinned forest. The importance of including interception evaporation as forest water use is discussed, and the results showed the importance of integration of forest growth and forest water use for calculating yearly carbon and water budgets.     

Mineral nitrogen dynamics following soil compaction and cultivation during hoop pine plantation establishment

The effects of soil compaction and cultivation on soil mineral N dynamics were investigated through an 18-month, in situ N mineralisation experiment during the inter-rotation and early establishment period of a second rotation (2R) hoop pine (Araucaria cunninghamii Aiton ex A. Cunn) plantation in southeast Queensland, Australia. Treatments were 0, 1 and 16 passes of a fully laden forwarder (gross weight, 40.2 Mg) and cultivation by disc plough (zero cultivation and cultivation). Nitrate N was the dominant form of mineral N throughout the 18-month sampling period in both non-cultivated and cultivated soils, varying between 10 and 40 kg ha⁻¹ whilst ammonium N remained <10 kg ha⁻¹. Compaction had no significant effect on N mineralisation or nitrification. However, the remediation of the effects of compaction on soil through the use of the disc plough had significant impacts on N mineralisation, nitrification and N leaching. On a seasonal basis, the mean net N mineralisation increased from around 30 to 53 kg ha⁻¹, nitrification from 28 to 43 kg ha⁻¹ and nitrate N leaching from around 10 to 73 kg ha⁻¹ following cultivation.     

Ideal subsoiling moisture content of Latosols used in forest plantations

The interaction between moisture and soil significantly affects the performance of soil tillage equipment. This interaction has not been sufficiently studied for the subsoiling of forest areas in Brazil. For this reason, this study was conducted with the objectives of evaluating the degree of soil disturbance as a function of increasing clay and moisture contents, the relationship between soil resistance to penetration plus bulk density and moisture content, and establishing an ideal soil moisture interval for subsoiling, as a function of soil disturbance and bulk density. The research plots were established in a sandy clay loamy dystrophic Red Latosol (LVd-1), a kaolinite-rich clayey dystrophic Red Latosol (LVd-2), and a clayey dystrophic Red Latosol (LVd-3). The higher clay and organic matter contents in the LVd-3 imparted lower soil resistance to penetration, in view of the greater water adsorption of this soil. The three Latosols presented an inverse and quadratic relation between soil disturbance and moisture content increase. The increase in clay and kaolinite contents in these soils caused lower soil maximum densities and higher amounts of water required to reach their maximum densities. The LVd-1 showed better subsoiling conditions between the moisture contents of 0.07 and 0.13 cm³ cm⁻³, the LVd-3 between moisture contents of 0.14 and 0.27 cm³ cm⁻³, while the kaolinitic LVd-2 presented the lowest water range for subsoiling when compared to the other soils, between the moisture contents of 0.12 and 0.19 cm³ cm⁻³. The subsoiling water interval was based on two parameters (standard Proctor test and soil disturbance area) that may present much variation. These limitations suggest that new studies should be conducted to determine whether this interval should be adopted as an index for consideration when deciding upon the best condition for soil tillage.     

Cumulated deposition of strong acid and sulphur compounds to a spruce forest

Deposition of acid to forest is mainly a result of dry-deposition of SO₂ and wet-deposition of acid dissolved in rain water. Ten years of measurements in the forest and 40 years of regional air pollution measurements, combined with historical sulphur emission inventories, make it possible to calculate the cumulated atmospheric deposition of strong acid. The concentration trend of atmospheric sulphur compounds measured during the last two decades until 1996 fits well with the model calculated emission and concentration pattern reported. Wet-deposition of sulphate was measured before 1955 in Denmark, these data also fit well with model calculated estimates. Acid-deposition during the last 30 years was estimated to be 4.0 eqv. m⁻² (40 keqv. ha⁻¹) and cumulated deposition for the last 90 years was 7.8 eqv. m⁻² (78 keqv. ha⁻¹). The acid inputs during these periods were equivalent to 100% and 200%, respectively, of the present top soil pool of exchangeable magnesium, potassium and calcium. The development of acid soils has led to aluminium being the major base cation in soil water.     

Litter of multipurpose trees in Kerala,India: variations in the amount,quality, decay rates and release of nutrients

In a field study involving 8–9 year-old woodlots of nine fast growing tree species in Kerala, India, the amount and nutrient content of litterfall were monitored. Decay rate constants and litter half-lives were estimated by fitting a single exponential model to the litter decomposition data. Annual litter production ranged from 3.43 Mg ha⁻¹ (Pterocarpus) to 12.69 Mg ha⁻¹ (Acacia). Litterfall of Acacia, Ailanthus, Pterocarpus and Casuarina followed a uni-modal distribution pattern. Nutrient content of litter samples showed considerable variations, owing to species and sampling time. Based on mean litter N content, the nine species were broadly divided into high, medium and low detrital N species. Litterfall accounted for substantial nutrient recycling within the system. Results of the litter bag study revealed that residual litter mass declined exponentially with time for Ailanthus, Pterocarpus, Casuarina and Leucaena. Paraserianthes showed a linear trend, while Emblica and the two Artocarpus species exhibited a bi-phasic pattern of mass loss. Both initial lignin content and lignin–N ratio had a negative, although modest influence on decay rate coefficients, while initial N content exerted a positive influence. Nutrient release from the decomposing litter followed either a tri-phasic pattern characterised by an initial accumulation, followed by a rapid release and a final slower release phase, or a bi-phasic pattern that is devoid of the initial accumulation phase.     

Carbon sources and sinks in high-elevation spruce–fir forests of the Southeastern US

This paper examines carbon (C) pools, fluxes, and net ecosystem balance for a high-elevation red spruce–Fraser fir forest [Picea rubens Sarg./Abies fraseri (Pursh.) Poir.] in the Great Smoky Mountains National Park (GSMNP), based on measurements in fifty-four 20 m × 20 m permanent plots located between 1525 and 1970 m elevation. Forest floor and mineral soil C was determined from destructive sampling of the O horizon and incremental soil cores (to a depth of 50 cm) in each plot. Overstory C pools and net C sequestration in live trees was estimated from periodic inventories between 1993 and 2003. The CO₂ release from standing and downed wood was based on biomass and C concentration estimates and published decomposition constants by decay class and species. Soil respiration was measured in situ between 2002 and 2004 in a subset of eight plots along an elevation gradient. Litterfall was collected from a total of 16 plots over a 2–5-year period.The forest contained on average 403 Mg C ha⁻¹, almost half of which stored belowground. Live trees, predominantly spruce, represented a large but highly variable C pool (mean: 126 Mg C ha⁻¹, CV = 39%); while dead wood (61 Mg C ha⁻¹), mostly fir, accounted for as much as 15% of total ecosystem C. The 10-year mean C sequestration in living trees was 2700 kg C ha⁻¹ year⁻¹, but increased from 2180 kg C ha⁻¹ year⁻¹ in 1993–1998 to 3110 kg C ha⁻¹ year⁻¹ in 1998–2003, especially at higher elevations. Dead wood also increased during that period, releasing on average 1600 kg C ha⁻¹ year⁻¹. Estimated net soil C efflux ranged between 1000 and 1450 kg C ha⁻¹ year⁻¹, depending on the calculation of total belowground C allocation. Based on current flux estimates, this old-growth system was close to C neutral.     

Regional influences of soil available water-holding capacity and climate,and leaf area index on simulated loblolly pine productivity

We simulated loblolly pine (Pinus taeda L.) net canopy assimilation, using BIOMASS version 13.0, for the southeastern United States (1° latitude by 1° longitude grid cells) using a 44-year historical climate record, estimates of available water-holding capacity from a natural resource conservation soils database, and two contrasting leaf area indices (LAI) (low; peak LAI of 1.5 m² m⁻² projected, and high; 3.5 m² m⁻²). Median (50th percentile) available water-holding capacity varied from 100 to 250 mm across the forest type for a normalized 1.25 m soil profile. Climate also varied considerably (growing season precipitation ranged from 200 to 1600 mm while mean growing season temperature ranged from 13° to 26°C). Net canopy assimilation ranged from 9.3 to 19.2 Mg C ha⁻¹ a⁻¹ for high LAI and the 95th percentile of available water-holding capacity simulations.We examined the influence of soil available water-holding capacity, and annual variation in temperature and precipitation, on net canopy assimilation for three cells of similar latitude. An asymptotic, hyperbolic relationship was found between the 44-year average net canopy assimilation and soil available water-holding capacity. Shallow soils had, naturally, low water-holding capacity (<100 mm) and, subsequently, low productivity. However, median available water-holding capacity (125–150 mm) was sufficient to maintain near maximum production potential in these cells.Simulations were also conduced to examine the direct affects of soil available water on photosynthesis (P_N) and stomatal conductance (g_S) on net canopy assimilation. In the absence of water limitations on P_N and g_S, net canopy assimilation increased by only 10% or less over most of the loblolly pine region (when compared to simulations for median available water-holding capacity with water influences in place). However, the production differences between high and low LAI, at the median soil available water-holding capacity, ranged from 30% to 60% across the loblolly pine range. Vapor pressure deficit was found to dramatically reduce productivity for stands of similar LAI, incident radiation, rainfall, and available water-holding capacity. Thus, these simulations suggest that, regionally, loblolly pine productivity may be more limited by low LAI than by soil available water-holding capacity (for soils of median available water-holding capacity or greater). In addition, high atmospheric forcing for water vapor will reduce net assimilation for regions of otherwise favorable available water and LAI.     

Long-term growth and ecophysiological responses of a southeastern Oklahoma loblolly pine plantation to early rotation thinning

A thinning levels study was initiated in a 9-year-old loblolly pine (Pinus taeda L.) plantation containing 26.6 m² ha⁻¹ basal area during the spring of 1984 in southeastern Oklahoma. Thinning treatments consisted of (1) three control plots (BA100), (2) three plots thinned to approximately 50% of the original basal area (BA50) and (3) three plots that were thinned to 25% of the original basal area (BA25). In 1987 the BA50 and BA25 plots were both rethinned to a basal area of 12 m² ha⁻¹. No other thinnings were done through age 24.The control plots have attained a basal area of 45.3 m² ha⁻¹ and basal area is now starting to decline. The BA25 and BA50 plots have basal areas between 34 and 35 m² ha⁻¹. Mortality has averaged about 90 trees ha⁻¹ per year from age 10 to age 24 on the control plot, declining from 2078 trees ha⁻¹ at age 10 to 827 trees ha⁻¹ at age 24. Mortality losses in the BA25 and BA50 plots have been only 3.2–7.7 trees ha⁻¹ per year over the entire study period. Cumulative stem biomass lost to mortality was 10.5, 16.0 and 61 Mg ha⁻¹, respectively, for the BA25, BA50 and BA100 treatments. Cumulative standing live biomass at age 24 in the BA100 treatment is 132 Mg ha⁻¹. Cumulative standing live biomass in the BA25 and BA50 treatments at age 24 is 86 and 79%, respectively, of that observed in the BA100 treatment. These results suggest wide ranges of residual stand densities left after an early thinning will produce a high percentage of the potential total maximum standing stem biomass. Diameter distributions at age 24 show only 33% of the trees in the BA100 treatments have the dimensions to be sawtimber (≥30 cm) but 92 and 95% of the trees in the BA25 and BA50, respectively, are sawtimber dimension or larger. Mean annual stem biomass production (MAI) of the BA100 treatment is 7.5 Mg ha⁻¹ per year at age 24. MAI of the thinned treatments is about 5.1 Mg ha⁻¹ per year and is converging to that of the BA100 treatment. The basis for this convergence is not that the live trees in the BA100 treatment are producing live biomass less rapidly than the thinned plots, but that mortality losses in the BA100 plot are much higher. Current annual stemwood production in all treatments is often limited by the severe summer droughts that occur in this region. The wide variations in weather experienced at this site also result in variations in earlywood:latewood ratio and ring specific gravity.