Litterfall,litter decomposition and the use of mulch of four indigenous tree species in the Atlantic lowlands of Costa Rica

Litterfall, forest-floor litter biomass and nutrients, short-term litter decomposition and the effects of leaf mulches on initial growth of maize were studied for four indigenous tree species with agroforestry potential:Stryphnodendron microstachyum Poepp. et Endl.(S. excelsum), Vochysia ferruginea Mart,Vochysia guatemalensis Donn. Sm. (V. hondurensis) andHyeronima alchorneoides (O), growing in a young experimental plantation in the Atlantic humid lowlands of Costa Rica. Total annual leaf litterfall was higher inV. ferruginea plots, followed byS. microstachyum, V. guatemalensis andH. alchorneoides; all with values comparable to those reported for other tree species grown in agroforestry combinations in humid tropical regions. Forest-floor litter accumulation was highest underV. ferruginea andV. guatemalensis. Both litterfall and forest-floor litter material had similar patterns in nutrient concentrations: N was higher inS. microstachyum, Ca was higher inV. guatemalensis, K was higher inH. alchorneoides; Mg was higher inV. guatemalensis andH. alchorneoides; H. alchorneoides andV. guatemalensis had the highest P.V. ferruginea litter decomposed more slowly, whileS. microstachyum apparently decomposed faster than the other species. The twoVochysia species showed increases in N and P concentration in decomposing litter after seven weeks in the field,H. alchorneoides showed an increase in litter N and a decrease in litter P, andS. microstachyum showed a net decrease in both N and P over the same time period. The patterns found in the litter bag study were confirmed by results obtained in a tethered-leaves experiment.S. microstachyum andV. ferruginea litters lost more weight when mixed in a 11 proportion than either of them alone. Maize seedlings growing in plots mulched withS. microstachyum andH. alchorneoides leaves showed greatest initial growth, confirming patterns found in decomposition and nutrient release studies. The results show that these species could be used in agroforestry combinations with different advantages according to the specific objectives desired, whether these are soil protection, nutrient recycling, or enhancement of the growth of associated crops.     

Carbon accumulation in aboveground and belowground biomass and soil of different age native forest plantations in the humid tropical lowlands of Costa Rica

Generic or default values to account for biomass and carbon accumulation in tropical forest ecosystems are generally recognized as a major source of errors, making site and species specific data the best way to achieve precise and reliable estimates. The objective of our study was to determine carbon in various components (leaves, branches, stems, structural roots and soil) of single-species plantations of Vochysia guatemalensis and Hieronyma alchorneoides from 0 to 16 years of age. Carbon fraction in the biomass, mean (±standard deviation), for the different pools varied between 38.5 and 49.7% (±3 and 3.8). Accumulated carbon in the biomass increased with the plantation age, with mean annual increments of 7.1 and 5.3 Mg ha⁻¹ year⁻¹ for forest plantations of V. guatemalensis and H. alchorneoides, respectively. At all ages, 66.3% (±10.6) of total biomass was found within the aboveground tree components, while 18.6% (±20.9) was found in structural roots. The soil (0–30 cm) contained 62.2 (±13) and 71.5% (±17.1) of the total carbon (biomass plus soil) under V. guatemalensis and H. alchorneoides, respectively. Mean annual increment for carbon in the soil was 1.7 and 1.3 Mg ha⁻¹ year⁻¹ in V. guatemalensis and H. alchorneoides. Allometric equations were constructed to estimate total biomass and carbon in the biomass which had an R ²aj (adjusted R square) greater than 94.5%. Finally, we compare our results to those that could have resulted from the use of default values, showing how site and species specific data contribute to the overall goal of improving carbon estimates and providing a more reliable account of the mitigation potential of forestry activities on climate change.     

Growth characteristics of some native tree species used in silvopastoral systems in the humid lowlands of Costa Rica

Research for developing silvopastoral systems includes tree species selection and management, and interactions of trees with pastures. In the Caribbean humid lowlands of Costa Rica, total height, diameters at breast height (DBH), survival, basal area and bole volume were compared among three plantations with four species each, in pure stands as well as in the mixture of the four species. In Plantation 1, at 86 months old the pure plantations of Jacaranda copaia and Vochysia guatemalensis, and the mixed plantations (Jacaranda copaia + Vochysia guatemalensis + Calophyllum brasiliense + Stryphnodendron microstachyum) had the largest DBH. Vochysia guatemalensis is one of the most preferred species in the region for reforestation of degraded pastures, while Jacaranda copaia is not preferred because it has low local economic value. In Plantation 2, at 81 months, the greatest DBH was found in Virola koschnyi, Terminalia amazonia and the mixed plantations (Virola koschnyi + Terminalia amazonia + Dipteryx panamensis + Pseudosamanea guachapele). Dipteryx panamensis has good economic value and has a relatively open canopy that produces moderate shade. Virola koschnyi and Terminalia amazonia have denser canopies, are preferred by farmers, and they could be planted in low density in silvopastoral systems. In Plantation 3, at 70 months, the greatest DBH was found in Vochysia ferruginea, Hieronyma alchorneoides and the mixed plantation (Vochysia ferruginea + Hieronyma alchorneoides + Genipa americana + Balizia elegans). Most of these species are currently used in silvopastoral systems in the region. Mixed plantations may combine beneficial effects of the different species' characteristics, with the additional advantage of product diversification. This revised version was published online in June 2006 with corrections to the Cover Date.     

Screening trial of 14 tropical hardwoods with an emphasis on species native to Costa Rica: Fourth year results

Fourth year results are presented from a species screening trial established at the La Selva Biological Station in northeastern Costa Rica. Of the 14 tropical hardwood species evaluated on survivorship, growth, form, and volume the most promising were (survival, diameter and height): Vochysia guatemalensis (84%, 12.2 cm, 10.5 m); Vochysia ferruginea (79%, 9.7 cm, 7.8 m); Hyeronima alchorneoides (83%, 8.1 cm, 7.9 m); Dipteryx panamensis (78%, 5.5 cm, 7.2 m); and Calophyllum brasiliense (80%, 6.3 cm, 6.2 m).

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Resumen Se presentan resultados a los cuatro años de edad de un ensayo de adaptabilidad establecido en la Estación Biológica La Selva ubicado en el noreste de Costa Rica. Las variables evaluadas para todas las especies fueron: sobrevivencia, crecimiento (altura total, diámetro a la altura del pecho), forma, y volumen. Las siguientes especies presentaron los mejores resultados (sobrevivencia, diámetro y altura): Vochysia guatemalensis (84%, 12.2 cm, 10.5 m); Vochysia ferruginea (79%, 9.7 cm, 7.8 m); Hyeronima alchorneoides (83%, 8.1 cm, 7.9 m); Dipteryx panamensis (78%, 5.5 cm, 7.2 m); y Calophyllum brasiliense (80%, 6.3 cm, 6.2 m).

     

Incorporating indigenous knowledge of fodder trees into small-scale silvopastoral systems in Jamaica

A study was conducted to identify and explore indigenous knowledge relating to fodder trees and silvopastoral management systems of small-scale farmers in seasonally dry areas of Jamaica. The objectives of the study were to: 1) explore farmers indigenous knowledge relating to fodder trees and silvopastoral management systems in a subtropical dry climate; 2) examine pastoral land-use systems and technologies in current use; and 3) seek to integrate this information into silvopastoral tree fodder systems. Data were gathered via observation and semistructured interviewing. A snowball sampling strategy was used to purposively select all small-scale farmers who had cattle in the Green Park valley for interview.Of forty farmers interviewed, 37 males (92.5%) and 3 females (7.5%) were primary caretakers of cattle. Twenty-seven of 40 farmers (68%) raised cattle to generate income. During extended droughts farmers are forced to look for alternatives to desiccated, overgrazed pasture grasses, including: 1) local travel to harvest sugar cane tops and to cut Guinea grass (Panicum maximum); 2) purchase of bag feed; and 3) harvest and use of tree fodder. Preferred fodder trees were identified as Bacedar (Guazama ulmifolia), Guango (Albizia saman), Breadnut (Brosimum alicastrum) and Quickstick (Gliricidia sepium).Recommended silvopastoral management systems include fodder hedgerows, three strata forage systems, and living fences. In addition, production of indigenous fodder tree species, introducing improved tree fodder species, and planting improved pasture grasses concurrent with cash trees is suggested.     

Evaluation of plantation and early development of five alternatives to ponderosa pine in silvopastoral systems in northwest Patagonia,Argentina

Previous studies of afforestation in Patagonia indicate that 30–50 % tree cover produces positive effects on the pasture. This coverage level is achieved by applying pruning and strong thinning to reduce the volume of timber production per area unit. From an economic standpoint, in order to not reduce the income level of the system, it is necessary to find tree species that could replace ponderosa pine (Pinus ponderosa) and maximize income per volume unit of wood. We evaluated the feasibility of implementation of five broadleaved tree species (native and exotic) with higher intrinsic wood quality than ponderosa pine. We tested the influence of tree cover generated by a framework of silvopastoral plantation on the increase in survival, regrowth and absolute increase in height, and compared ecophysiological variables (net photosynthetic activity, stomatal conductance, intrinsic water use efficiency) of different species to the status of a traditional plantation without tree cover. Additionally values of air temperature and relative humidity were registered under both cover conditions. Preliminary results support the conclusion that three of these five species could be considered as alternatives to ponderosa pine species for establishing silvopastoral systems with higher timber value. Our Based on the results, the use of tree cover generated under the current plantations of P. ponderosa could increase the success of the establishment of these species.     

Sewage sludge fertilisation of a silvopastoral system with pines in northwestern Spain

The use of municipal sewage sludge as a fertiliser could be a more adequate means of disposal of this residue than its transport to the dump. The objective of this experiment was to evaluate the effects of different doses of sludge and sowing a mixture of cocksfoot (Dactylis glomerata) and clover (Trifolium repens) on pasture production, botanical composition of the pasture, forage quality, and changes in soil chemical properties. Treatments applied were: (1) no fertilisation (L0), (2) fertilisation with 26 Mg ha^–1 of sewage sludge with no sowing (L40), (3) fertilisation with 26 Mg ha^–1 of sewage sludge and sowing with 25 kg of Dactylis glomerata L. cv Artabro (Dg) and 3 kg Trifolium repens cv Huia per hectare and (4) 52 Mg ha^–1 of sewage sludge with no herbage sowing (L80). Plots were established in a silvopastoral system with Pinus radiata D. Don at a density of 1666 trees ha^–1 (3 × 2 m). The improvement of soil fertility due to organic fertiliser application and the use of Dactylis glomerata increased the production and quality of pasture as well as reduced the risk of fire and erosion, as grasses dominated shrubs. Fertilisation with sewage sludge in soils of acid origin increased the concentration of copper in the pasture and, if cocksfoot was not sown, the levels of this element exceeded the maximum tolerable dietary levels for sheep. On the other hand, fertilisation also increased the levels of zinc in pasture, but values were not above toxic limits for sheep, horses and cattle.This revised version was published online in November 2005 with corrections to the Cover Date.     

Do multipurpose companion trees affect high value timber trees in a silvopastoral plantation system?

Establishment of native timber trees on deforested land may contribute to the livelihood of farmers, to improved ecosystem services and to increased greenhouse gas uptake. Here, we present a new silvopastoral planting design to assess species performance and interspecific competition or facilitation effects among native timber and multipurpose trees in Central America. Two timber species, Tabebuia rosea and Cedrela odorata, were established in three low-density planting regimes allowing combined tree and future livestock production: (1) solitary planting, (2) companion planting with Guazuma ulmifolia, and (3) companion planting with the nitrogen-fixing Gliricidia sepium. We quantified survival, growth and reforestation potential of the two timber species subjected to the different planting regimes for the first 2 years after establishment. Nitrogen concentration as well as stable nitrogen and carbon isotope composition (δ¹⁵N, δ¹³C) of leaves of the timber saplings were determined. T. rosea showed higher survival and better growth than C. odorata under varying environmental conditions (soil, concomitant vegetation). Performance of the timber saplings was unaffected by either companion species. Planting regimes had no effect on foliar nitrogen concentration and δ¹⁵N of the two timber species, although δ¹⁵N values indicated nitrogen fixation activity in G. sepium trees. Planting regimes affected foliar δ¹³C values in T. rosea. δ¹³C values were significantly higher in solitarily growing individuals, suggesting lower exposition to water stress conditions in saplings surrounded by companion species. As we found positively correlated growth traits among timber and multipurpose trees, a combined planting may benefit farmers by providing additional goods and services.     

Soil properties and nitrogen availability in silvopastoral plantings of Acacia melanoxylon in North Island, New Zealand

Acacia melanoxylon, a N₂-fixing timber tree occurring naturally in eastern Australia, is now promoted as a component of silvopastoral systems; but the interaction of the tree with pasture and soils has not been adequately studied. This study investigated the effects of Acacia melanoxylon on soil nitrogen (N) levels, N availability, soil pH, bulk density, organic carbon, C:N ratios and soil moisture in three separate silvopastoral sites with contrasting soil types in the North Island of New Zealand. At each site four tree stocking rates were studied (0, 500, 800, and 1700 stems ha^–1). The trees were nine years old at the time of the study. Soil samples from each study site were taken once at three depths (0 to 75 mm, 75 to 150 mm, and 150 to 300 mm), with three replicates per tree stocking rate. Soil analyses showed that although there were differences between soil types, few statistically significant differences occurred due to tree stocking rate. A greenhouse pot trial growing ryegrass (Lolium multiflorum L. cv. Concord) in soil from the A horizon of each soil type from under the trees and the open pasture found that ryegrass yield, N uptake and N supply increased with increasing tree stocking rate. Increased N supply under the trees, coupled with greater soil moisture compared to the open pasture may have accounted for the higher pasture yield under Acacia melanoxylon compared to non dinitrogen fixing tree species. This study suggested that Acacia melanoxylon in a silvopastoral system had the potential to increase soil N availability.This revised version was published online in November 2005 with corrections to the Cover Date.     

Initial agronomic evaluation of Parkia biglobosa in the humid zone of Nigeria

Parkia biglobosa is an important traditional economic tree legume of considerable multipurpose potentials that has not been well researched. It is used for fodder, human food, fuel wood, timber, green manure, medicine, provides shade for forage grasses and livestock and protects soil from heat and it is important in soil nutrient cycling.The objective of this study was to determine the effect of management regimes (cutting frequency and height) on the coppicing ability, fodder production, and nutritive value potentials of cultivated Parkia trees. This initial study has shown that the tree has a potential as fodder for livestock as well as human food and should be further studiedto fully understand its biology, agronomy and feeding value under various agroforestry or silvopastoral systems of humid tropics in West Africa.     

Productivity of two Douglas fir/subclover/sheep agroforests compared to pasture and forest monocultures

Resource sharing between tree and forage plant components in silvopastoral systems includes a complex set of facilitative and competitive interactions. To the extent that facilitation exceeds competition, agroforests are expected to outyield monocultures of their components. Pasture and Douglas fir (Pseudotsuga menziesii) tree production of young agroforests was compared to pasture and forest monocultures under both grid and cluster patterns of tree planting near Corvallis, Oregon, USA, during 1983–1987. The height and diameter growth of forest and agroforest trees was similar, regardless of tree planting pattern. Five-year average annual forage production was 6500, 5800, and 2800 kg ha^–1 on pasture, agroforest, and forest plots, respectively. The total cumulative 1982–1987 above-ground phytomass yield of forage plus trees was similar for pasture and conventional grid forest monocultures. The total productivity of agroforests, however, was over 30% greater than either pasture or forest components grown in monoculture. Approximately 1.6 ha (0.96 ha forest + 0.64 ha pasture) of monocultures would be needed to equal the productivity of 1 ha of agroforest.Oregon Agricultural Experiment Station Technical Paper No. 10, 825.     

Manual recording of minirhizotron data and its application to study the effect of herbicide and nitrogen fertiliser on tree and pasture root growth in a silvopastoral system

The use of a simple counting method using a borescope and minirhizotron system to study the interaction of roots of wild cherry trees (Prunus avium L) and pasture in an upland silvopastoral system is described. Operator variation was greatest when distinguishing different tree root categories (e.g. white, brown, woody) but was not significant in relation to total tree or grass root counts or species differences. Analysis of covariance was used to take these differences into account in a field trial using several operators. The seasonal patterns of tree and pasture root growth were different. Tree roots peaked in May before maximal shoot growth. The peak in pasture roots occurred two weeks earlier. Tree height growth was increased by the use of herbicides to control grass growth but this effect was only apparent during August when the soil was dry and when pasture root counts were reduced.     

Establishment of native tropical timber trees in monoculture and mixed-species plantations: Small-scale effects on tree performance and insect herbivory

Plantations of native timber species established on former pasture are a potential strategy to reduce the logging pressure on remnant natural forests in the tropics. Such plantations may help to mitigate or reverse the negative impacts of land degradation, and they may contribute to the long-term livelihood of livestock farmers. Planting native trees is, however, perceived as a risky activity due to limited knowledge of their performance and due to marked losses of newly established seedlings attributed to insect pests. Our study focuses on the small-scale effects of environmental heterogeneity, tree diversity and insecticide treatment on the performance of three native Central American timber species two years after establishment, and on damage inflicted by insect herbivores. Growth, survival and herbivore leaf damage were quantified for Anacardium excelsum (Anacardiaceae), Cedrela odorata (Meliaceae) and Tabebuia rosea (Bignoniaceae) planted in an experiment in Panama as (1) monocultures, (2) mixed stands, and (3) mixed stands protected by insecticides. Our study revealed that small-scale effects can have a substantial impact on the success of timber trees planted on former pasture. Growth performance and survival of the three species was strongly affected by small-scale environmental heterogeneity, which was expressed as significant differences in growth and survival among different plantation plots at the same study site. Establishment of trees in mixed stands did not have significant effects on tree survival and growth compared to pure stands, although it reduced herbivore pressure in one of the studied tree species. All tree species grew best and suffered lowest leaf damage when protected by insecticides, indicating a general influence of insect herbivory on growth of newly established trees. In contrast to growth performance, survival was not significantly affected by different management practices or herbivory. The large variability among plots in tree survival and growth, and also in the effects of management practices such as planting design and insecticide treatment, emphasizes the importance of small-scale environmental heterogeneity on tree survival and growth.     

Planting slash pine in a dense pasture sod

Recommendations for planting pines usually include providing a competition-free site to assure establishment and good early growth. When combining pines and pasture in agroforestry systems, the possibility of planting pines directly into a pasture sod without site treatment would be economically desirable. In south Georgia, USA, slash pine (Pinus elliottii) was planted into a pasture sod and on well-disked sites; both with and without annual fertilization. Partial weed control was maintained in subsequent years on the cultivated treatments and the pasture sod was mowed periodically.Slash pine establishment and growth rate through 5 years generally was not different among treatments. Survival was 96% intially and 86% after 5 years. The pines grew rapidly with no measurable advantage from cultivation or fertilization.This alternative to providing a competition-free environment needs to be tested under differing climates, soils, tree species, and pasture species. It could be a suitable technique in various situations and of special interest to small, nonindustrial landowners.Cooperative study between the Coastal Plain Experiment Station, University of Georgia, and USDA Forest Service, Tifton, GA 31794, USA.     

Effect of growth conditions on wood density of Spanish Pinus nigra

The natural forests of Laricio Pine (Pinus nigra Subsp. salzmannii) cover in Spain an area of 353,000 ha, producing annually 310,000 m³ of roundwood. This species is considered as the fourth, most important Spanish conifer, being its timber frequently used in construction. The natural area of distribution of Laricio pine in Spain is officially divided into ten different regions of provenance, being only four of them considered as productive. The present study on the influence of growth conditions on extracted wood basic density (EBD) is carried out on 89 P. nigra trees, sampled from the two most productive Spanish regions of provenance (R.7 and R.8) in seven different sites. Cross-sectional disks were cut at 3 m (in 89 trees) and every 3 m height (in 18 trees) from the butt up to 15 m. For assessing the pattern of radial variation of the properties, 20×20×20 mm³ wood samples were taken radially from every cross-sectional disk. In every 20×20×20 mm³ wood sample, age and distance to the pith of the central annual ring was recorded, together with the average growth rate. Extractive content and EBD were measured in every sample. The results lead to the conclusion that there is not a significant relationship between EBD and provenance or growth rate. On the contrary, a significant relationship has been found between EBD and site quality or cambial age. The results also demonstrate that in a tree EBD tends to diminish from the base to the top and from the pith outwards. The main result of the present study is related with the observation that for the same cambial age, annual ring width has no significant influence on EBD, confirming that its utility as a predictor of the quality of timber is highly questionable, at least if the ring curvature is not considered.     

Initial growth of pioneer timber tree species in a Taungya system in the humid lowlands of Costa Rica

The temporary association of annual crops in juvenile tree plantations (Taungya system) can reduce or offset the initial costs of reforestation and hence provide incentives for tree planting on private farms. A replicated systematic spacing design, derived from a Nelder-fan design, was used to study the effects of tree-crop distance on growth and development of the timber species salmwood (Cordia alliodora) or eucalyptus (Eucalyptus deglupta), associated with maize (Zea mays) or cassava (Manihot esculenta), during the first year after transplanting the trees on two sites on alluvial soils in the humid lowlands of Costa Rica. Association with maize did not affect mean tree growth. However, mean eucalyptus and salmwood heights of 7–8 m and 3–4 m, respectively, after one year in pure plantation or associated with maize, were only 4.7 m and 1.4 m, respectively, when associated with cassava. Wider tree – crop spacings were more important for initial salmwood growth than for eucalyptus. It was concluded that the shoot growth characteristics (continuous or episodical growth) of the timber species is a very important consideration when selecting associated agricultural crop species and tree-crop distances.     

Nursery and field establishment techniques to improve seedling growth of three Costa Rican hardwoods

Seedlings of three economically important and ecologicallydifferent native hardwoods, Cordia alliodora (Boraginaceae),Hyeronima alchorneoides (Euphorbiaceae), and Calophyllumbrasiliense (Clusiaceae), were grown in Rootrainers® (abook-type container), paper pots, and plastic bags filled witheither soil, soil with fertilizer, or compost substrates. Aftertransplanting in the field, treatments with and withoutfertilizer and herbicide were applied to all nursery stock types.In the nursery, species responded primarily to substrate type.Cordia grew better in bags of soil with NPK fertilizer andcompost than in unamended soil, probably responding to highernitrogen availability. Despite large treatment differences atplanting, there were no significant differences in plant sizeafter one year in the field between book containers and bags. Theexception were stump plants that were shorter and had highermortality. Hyeronima grew better in compost than in soil with orwithout fertilizer, probably responding to higher phosphorusavailability and lower bulk density of the compost. Plantsproduced in compost were also bigger after one year's fieldgrowth. Plants produced with soil or in paper pots had highermortality. Calophyllum grew less in compost compared to soil andgrew better when micronutrients were added to the compost andsoil. In the field, seedling produced in soil or withmicronutrients had higher survival or growth, respectively. Ingeneral, species grew better with herbicide and fertilizerapplication after transplanting. However, there were nointeractions with nursery treatments. Responses to fieldtreatments were independent and thus additive to the nurserytreatments. Differences in species response can be related tobiomass allocation patterns and ecology of the species.     

Agroforestry tree selection in central Chile: biological nitrogen fixation and early plant growth in six dryland species

Growth rate, resource partitioning, and several biological traits related to biological N₂ fixation for six native or non-native tree species were compared using ¹⁵N isotope dilution techniques. The trees were field grown for six years in a semiarid mediterranean-climate region with five to six months a year of absolute drought. Trees were tested as candidates for new agroforestry systems being developed in central Chile to improve soil fertility and land health, while also increasing productivity and profitability for landowners and animal breeders. Four nitrogen-fixing legume trees (NFTs) were tested: Acacia caven (Mol.) Mol.Prosopis alba Griseb., P. chilensis (Mol.) Steuntz. emend. Burk., and Tagasaste ( Chamaecytisus proliferus L.f. subsp. palmensis (Christ.)Kunkel). Additional, non-nitrogen-fixing trees were the slow-growing native Huingán (Schinus polygamus (Cav.) Caberera and the fast-growing European Ash (Fraxinus excelsior L.). Among the NFTs, highly contrasting patterns in biological nitrogen fixation (BNF) were detected, for N_dfa (proportion of N derived from atmosphere), nodule efficiency (NE = gN fixed g^–1 nodules), and N content in leaves, stems and roots. Tagasaste produced 2.5–25 times more biomass and fixed 4.5 to 30 times more atmospheric nitrogen than the South American Acacia and Prosopis species. N_dfa reached 250 g plant^–1 in Tagastaste, in the sixth year, with NE = maximum 2.68 in the 4th year, and 1.12 in the 6th year. In contrast, Acacia caven had by far the highest NE of the four NFTs – 12.13 in the 4th year and 6.6 in the 6th year. Whereas BNF in Tagasaste peaked in the fourth year, and declined thereafter, BNF in Acacia caven increased steadily over six years. Fraxinus excelsiorand Schinus polygamus had growth rates and biomass accumulation intermediate between that of Tagasaste and the South American NFTs.Results are discussed in relation to agroforestry, restoration of soil fertility, and ecological and economic rehabilitation of damaged ecosystems and landscapes.This revised version was published online in November 2005 with corrections to the Cover Date.     

Selective logging and fire as drivers of alien grass invasion in a Bolivian tropical dry forest

Logging is an integral component of most conceptual models that relate human land-use and climate change to tropical deforestation via positive-feedbacks involving fire. Given that grass invasions can substantially alter fire regimes, we studied grass distributions in a tropical dry forest 1–5 yr after selective logging, and experimentally tested the effect of forest fire on populations of invasive grasses. In unlogged forests and in microhabitats created by selective logging we found a total of four alien and 16 native grass species. Grasses covered 2% of unlogged and 4% of logged forest, with grass cover in logged forest concentrated in areas directly disturbed by logging; log landings and roads had relatively greater grass cover (37% and 17%, respectively) than did skid trails (10%) and felling gaps (8%). Total grass cover and grass species richness increased with canopy openness and were greatest in sites most severely disturbed by logging. The grass flora of these disturbed areas was composed mostly of native ruderal species (e.g., Digitaria insularis, Leptochloa virgata), a native bamboo (Guadua paniculata), and Urochloa (Panicum) maxima, a caespitose C₄ pasture grass introduced from Africa. Urochloa maxima formed monodominant stands (up to 91% cover and 2–3 m tall) and grew on 69% of log landings and 38% of roads. To better understand the potentially synergistic effects of logging and fire on the early stages of grass invasion, we tested the effect of a 12-ha experimental fire on U. maxima populations in a selectively logged forest. Three years after the fire, the area covered by alien grass in burned forest increased fourfold from 400 m² (pre-fire) to 1660 m²; over the same period in a logged but unburned (control) area, U. maxima cover decreased from 398 m² to 276 m². Increased canopy openness due to fire-induced tree mortality corresponded with the greater magnitude of grass invasion following fire. Selective logging of this dry forest on the southern edge of the Amazon Basin promotes alien grass invasion; when coupled with fire, the rate of invasion substantially increased. Recognition of the grass-promoting potential of selective logging is important for understanding the possible fates of tropical forests in fire-prone regions.     

Pasture production and tree growth in a young pine plantation fertilized with inorganic fertilizers and milk sewage in northwestern Spain

The use of organic waste materials such as milk sewage as an organic fertilizer could have the dual advantages of organic-waste disposal and reduced dependence on inorganic fertilizers. The effects of fertilization with (1) conventional mineral fertilization, (2) milk sewage sludge at 40 kg N ha⁻¹ target rate and (3) no fertilization on pasture production and tree growth were examined in an experiment consisting of two pasture mixtures under a one-year-old Pinus radiata plantation with a density of 2500 trees ha⁻¹. The two pasture mixtures were: (1) Dactylis glomerata L. var. saborto (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹); (2) Lolium perenne L. var. Tove (25 kg ha⁻¹) + Trifolium repens L. group Ladino (4 kg ha⁻¹) + Trifolium pratense L. var. Marino (1 kg ha⁻¹). The experiment began in the spring of 1995 using a randomized block design with three replicates in Castro Riberas de Lea (Lugo, Galicia, north-western Spain). Plot size was 12 × 8 m², with a 1 m buffer strip between plots. Two-year data showed that fertilization with either material had a positive effect on pasture production, with no significant difference between the two fertilization treatments. Tree growth in the milk sewage sludge plot was significantly higher than in the control plots. Inorganic fertilization increased pasture production, but affected tree growth negatively. The results show that milk sewage sludge could be used as a fertilizer in silvo-pastoral systems. This revised version was published online in June 2006 with corrections to the Cover Date.