Atlantic Lowlands Of Costa Rica Research Articles

Distribution and photosynthetic assimilation of rosulate aroid epiphytes in a Costa Rican lowland rainforest

Relatively few studies are available to characterize the distribution, abundance and ecophysiology of aroid epiphytes, distinct from aroid hemiepiphytes which are more common and abundant. The rosulate growth form is restricted to a relatively small number of aroid species. Field and experimental growth studies and ecophysiological measurements on rosulate aroids were carried out at La Selva Biological Station in the premontane rainforest of the Atlantic lowlands of Costa Rica. Rosulate aroids occur in habitats from forest understory to high canopy, and from old growth wet forest to secondary forest in Costa Rica. The distribution of rosulate and acaulescent aroid epiphytes were determined in multiple stands of old growth and secondary tropical forests at La Selva. Anthurium upalense and Philodendron wendlandii exhibited a preference for upper canopies on trees over 50 cm dbh where irradiance levels are high for much of the day. Anthurium consobrinum and A. bakeri occurred lower in the canopy in more shaded positions. To evaluate how these habitat preferences are related to plant traits, controlled shadehouse growth experiments were conducted to characterize species response to different ambient light environments. We applied conditions of high light (50% full sun), medium light (10% full sun), and low light (2% full sun) to simulate a gradient of canopy and understory light environments. Anthurium consobrinum and A. bakeri exhibited photosynthetic traits characteristic of shade-adapted understory epiphytes. In contrast, A. upalense and Ph. wendlandii, typically high canopy species, had maximum photosynthetic rates of 10.5 and 8.7 µmol m−2 s−1, respectively, higher than those previously reported for other vascular epiphytes. The variability in distribution of epiphytic aroids in Costa Rican premontane rainforest is consistent with their diversity of ecological and ecophysiological traits demonstrating adaptation to diverse canopy habitats in this aroid growth form.

Review of the genus Cruziohyla (Anura: Phyllomedusidae), with description of a new species.

The presented work summarises new and existing phenotypic and phylogenetic information for the genus Cruziohyla. Data based on morphology and skin peptide profiling supports the identification of a separate new species. Specimens of Cruziohyla calcarifer (Boulenger, 1902) occurring in Ecuador, Colombia, two localities in Panama, and one in the south east Atlantic lowlands of Costa Rica, distinctly differ from those occurring along the Atlantic versant of Central America from Panama northwards through Costa Rica, Nicaragua, to Honduras. A new species-Cruziohyla sylviae sp. n.-(the type locality: Alto Colorado in Costa Rica)-is diagnosed and described using an integrated approach from morphological and molecular data. Phylogenetic analysis of DNA sequences of the 16S rRNA gene confirms the new species having equal minimum 6.2% genetic divergence from both true C. calcarifer and Cruziohyla craspedopus.

Canopy height and ground elevation in a mixed‐land‐use lowland Neotropical rain forest landscape

We obtained spatially extensive canopy height measurements using airborne remote sensing to characterize the structure and dynamics of a tropical rain forest landscape. Light detection and ranging (LiDAR) is a remote‐sensing technology that acquires measurements of canopy height and ground elevation. By recording the return time of laser pulses emitted by aircraft‐mounted sensors, LiDAR systems quantify the structure and geometry of individual trees and canopy height and enable estimation of the vertical and horizontal distribution of biomass using millions of accurate height measurements. This data set contains 127 849 839 records from 128 square kilometers of tropical wet forest in the Atlantic lowlands of Costa Rica (mean sampling density is 1.99 observations/m2). The study area includes all 16 square kilometers of mixed‐land‐use forest at the La Selva Biological Station and the lower flanks of Braulio Carrillo National Park. It contains a mosaic of historical and contemporary land use that is representative of contemporary tropical forest landscapes. Field studies demonstrated that LiDAR measurements were precise and accurate throughout the topographic and structural conditions at the site. Each record includes: easting and northing mapped coordinates (UTM Zone 16 North), height above ground (m), interpolated ground elevation (m), and six remote‐sensing descriptors (point classification, the return number, the number of returns for the given pulse, intensity, scan angle, and the time of emission of the laser pulse). The data can be applied to a wide range of questions in basic and applied science, and are a valuable resource from a well‐studied tropical rain forest for teaching and education. They also provide a quantitative baseline against which future conditions can be assessed.The complete data sets corresponding to abstracts published in the Data Papers section of the journal are published electronically in Ecological Archives at 〈http://esapubs.org/archive〉. (The accession number for each Data Paper is given directly beneath the title.)

Phenology and Stem Diameter Increment Seasonality in a Costa Rican Wet Tropical Forest

ABSTRACTThe relationship between phenology and tree stem diameter increment is largely unexplored in tropical species, especially in wet tropical forests. To explore links between these phenomena, we measured stem diameter increment and phenology of ten canopy tree species from a range of functional types in the Atlantic lowlands of Costa Rica to test for seasonal and interannual patterns. We measured stem diameter increment using band dendrometers and visually assessed leaf and reproductive phenology monthly from 1997 to 2000. We categorized the species into groups based on patterns of leaf exchange and reproduction. Species were either deciduous with synchronous or asynchronous leaf drop, or evergreen with continuous or seasonal leaf flushing. Flowering occurred supra‐annually, annually, or continuously. Of the ten species studied, four species,Cecropia insignis, Dipteryx panamensis, Lecythis ampla,andSimarouba amara, had consistent seasonal stem diameter increment patterns in both years.Dipteryx panamensisandL. amplawere deciduous with synchronized leaf drop. Cecropia insigniswas evergreen and produced new leaves continuously.Simarouba amara, also evergreen, exchanged leaves over a brief period once a year. We tested whether stem diameter increment was correlated to phenology using logistic regression. Leaflessness significantly explained patterns in stem diameter increment but reproductive phenology did not. Deciduous trees were 2.6–9.3 times more likely to grow less than average the month following leaffall than in months when trees had full crowns.

Thermal Ecology Of The Neotropical Army Ant Eciton Burchellii

I explored the thermal ecology of Eciton burchellii, a New World army ant, in primary forest and forest fragments in the Atlantic lowlands of Costa Rica in 2002 and 2003. My primary objective was to determine whether high surface temperatures in pastures surrounding forest fragments posed a thermal barrier to ant colonies within those fragments; secondarily, I assessed whether thermal gradients within continuous moist forest were sufficient to elicit avoidance reactions from foraging colonies. E. burchellii colonies in forest fragments avoided entering open pasture in full sun (51.3 degrees C) on 100% of all edge interactions; however, edges were readily crossed where artificial shaded areas had previously been installed. Ant raids in primary forest avoided artificially established temperatures >43 degrees C but tolerated 45.5 degrees C in the presence of prey baits. Captive ants held at 43 degrees C survived 18.5 min; at temperatures of 51.3 degrees C survival time was only 2.8 min. Ants running on established foraging trails increased running velocity by 18% when substrate temperature was raised from 28.4 degrees to 38.0 degrees C, and they abandoned trails at temperatures >43 degrees C. The standard deviation (s) of temperatures on active raid trails in continuous forest was 2.13 degrees C, while nearby systematic sampling revealed a greater background standard deviation of 4.13 degrees C. E. burchellii colonies in this region appear to be living surprisingly near their upper limits of thermal tolerance. The heat of open pastures alone is sufficient to prevent their exiting forest fragments, or entering similarly hot areas within continuous forest. Shaded vegetative corridors are sufficient to permit mobility between isolated fragments, and their preservation should be encouraged. Despite views that tropical lowland moist forests have an essentially homogenous microclimate, army ants appear to avoid local hot spots on the forest floor, steering daily foraging trails to follow cooler routes than would be expected by chance. As deforestation remakes tropical landscapes worldwide, it is important to consider the thermal consequences of these actions and their effect on keystone organisms such as army ants. Changes in global climate patterns are likely to affect even evergreen tropical systems whose organisms may be sensitive to finer microclimatic variation than previously suspected.

Benefits and costs of intercropping reforestation in the Atlantic lowlands of Costa Rica.

Agroforestry options as a means of promoting reforestation were testedby establishing with 10 farmers simple comparisons between pure plantations andtimber trees with annual or perennial intercrops. Two year-old intercroppedtrees on-farm had 22–48% greater height and 24–38% greater diameterthan trees in pure plantations. The exception was Cordiaalliodora that did not respond significantly to intercropping withcassava (Manihot esculenta). Only intercrops with the mostvaluable crops on agricultural soils produced a return to labour above thedailywage rate. On non-agricultural soils, farmers only intercrop a small area oftheir plantations with subsistence crops to meet home consumption needs. In thecontext of Costa Rica intercropping appeared to have a limited potential tofinance the reforestation costs of farmers.

Performance of forest plantations in small and medium-sized farms in the Atlantic lowlands of Costa Rica

Exotic tree species predominate in reforestation in tropical regions worldwide. However, some native species are suitable for plantations, providing a wider variety of products. Adequate government programs of incentives, coupled with good technical advice to farmers, are needed to stimulate reforestation, especially among small and medium-sized farmers with limited financial resources. This project evaluates growth of native and exotic tree species on plantations in small and medium-sized farms in the Atlantic humid lowlands of Costa Rica, Central America. A total of 210 pure plantations, ranging in age from 6 to 11 years, were evaluated on 123 farms that had used government incentives for reforestation, and had received technical advice from local non-government organizations. For each species, seven plantations were chosen at random for study. In each plantation, plots of 15 trees each were chosen systematically for evaluation of diameter at breast height (dbh), total height, number of trees per hectare, tree form and spacing. Terminalia amazonia (J.F. Gmel) Exell, Hieronyma alchorneoides Allemao, and Vochysia guatemalensis Donn. Sm. were the most frequent species found in plantations in the region of study. Gmelina arborea Roxb. (exotic) and V. guatemalensis (native) had the highest mean annual diameter increment with 2.90 and 2.59 cm, respectively. Calophyllum brasiliense Cambess. (native) had the lowest mean annual diameter increment (1.48 cm). G. arborea and V. guatemalensis had the highest mean annual volume increment, followed by Tectona grandis (L.f) Lam. (exotic), and T. amazonia and Cordia alliodora (R&P) Cham. (both native). Although G. arborea had the greatest mean annual diameter increment, it had the lowest plantation density and problems with form. V. guatemalensis and T. amazonia, two native species, were the most promising species for reforestation, due to good growth in volume, good form, and adaptability to a variety of sites. The poor form of exotic species was related to the low intensity management methods of small farmers. Exotic species had the highest performance variability between sites, while native species showed relatively high growth homogeneity. Therefore, native species seem more promising than exotics for general use across varying ecological conditions.

Initial Shoot and Root Growth Patterns of Cordia alliodora in Agroforestry Systems with Perennial Crops in the Atlantic Lowlands of Costa Rica

ABSTRACT Through a participatory research approach with small farmers in the tropical humid Atlantic lowlands of Costa Rica, the importance of selecting appropriate propagation methods for Cordia alliodora (common name: laurel) in perennial crop fields was studied. Trees were established on alluvial loam soils (Typic Tropofluvents) as container plants, stumps or direct seeding, either without crops (Experiments A1, B1) or in association with plantain (Experiment B2: 918 plants ha−1), Musa x paradisiaca (L.) AAB, or a mixture of plantain (553 plants ha−1) and cacao, Theobroma cacao L. (1111 plants ha−1) (Experiments A2: cacao established at the same time as laurel, Experiment A3: cacao established 16 months before laurel). Six months after planting, B1-trees were three times higher than B2-trees, except for trees established by direct seeding which remained smaller in both experiments and were twice as high in B1. At age twelve months, perennial crops in A2 and A3 did not affect tree height compared to A1. However, plant biomass of container plants and stumps decreased significantly when they were planted with the crops. The reduced tree growth in association with the crops mainly was related to the reduction of light availability. Stumps had higher root/shoot ratios than container plants due to the fact that the stump material consisted mainly of tap root from which shoot and roots had to be regenerated and probably also due to the removal of suppressed shoots from stumps six months after establishment. It was concluded that both container and stump plants can be used successfully to establish laurel in plantain or plantain-cacao fields.

Performance of Klinki (Araucaria hunsteinii K. Schuman) in the Humid Tropics of Costa Rica

Araucaria hunsteinii K. Schuman (Syn. A. klinki Lauterbach), locally known as Klinki, is a tropical conifer from Papua New Guinea, which was introduced to Costa Rica in 1966 by the Tropical Agriculture Research and Training Center (CATTE). Due to its capacity for carbon sequestration and long-term storage, and wood production, EARTH University, located in the Atlantic lowlands of Costa Rica, established two research plots with the species in October 1993 to monitor its growth in the humid tropical lowland conditions. Results show that the species performs well under such soil and climate conditions. It seems probable that Klinki may be considered as a promising species for commercial timber production and for long-term carbon sequestration in the tropics.

Can forestry be a competitive land use option?: Model simulations from humid tropical Costa Rica

Model simulations are carried out to study options for managed natural forest and melina ( Gmelina arborea) and teak ( Tectona grandis) plantations on land suited for agriculture in the humid tropical Atlantic lowlands of Costa Rica. Tools used are a linear programming model that maximizes regional economic surplus, and the so-called technical coefficient generators that quantify input and outputs of forestry and competing crop and pasture-based beef cattle production systems. Model scenarios are carried out for varying prices of wood, beef and labor, under conditions of unlimited and restricted wood markets. Results indicate that teak and melina plantations are attractive land use options while managed natural forest is not. With unlimited wood demand, teak occupies 70% of the study area and total yearly teak production exceeds current Costa Rican wood consumption, indicating the need to include market mechanisms in the study. The area under teak is relatively insensitive to price changes between –40 and +60% of the base value of $95 m −3 (1996 stumpage price). The area under melina changes considerably with price changes of ±20% of the base value of US$16 m −3. Natural forest revenues must rise by 440% of the current $32 m −3 to make it economically more attractive than other land use options, especially pasture for beef production and melina plantations. Increasing labor costs are likely to favor tree plantations. Increasing beef prices considerably reduce the competitiveness of melina, though not that of teak.

Is the number of tree species in small tropical forest plots nonrandom?

Tree species richness is remarkably high in many tropical forests, even at very fine spatial scales. However, the study of fine-scale richness is complicated by the rarefaction effect: that is, a trivial correlation between the number of individuals and the number of species. We developed null models to test whether fine-scale species richness differs from random expectation, and applied these models to a dataset of 1170 100 m2circular plots in the old-growth portion of La Selva Biological Station in the Atlantic Lowlands of Costa Rica. Although species richness in these plots was close to its theoretical maximum, we found that it was frequently lower than null expectation. This was a result of slightly clumped distributions within species. We found no relationships between species richness at the 100 m2scale and soil type or topography, after accounting for the effects of density

Model simulation of changes in N2O and NO emissions with conversion of tropical rain forests to pastures in the Costa Rican Atlantic Zone

Nitrous oxide (N2O) and nitric oxide (NO) are among the trace gases of concern because of their importance in global climate and atmospheric chemistry. Modeling techniques are needed for simulating the spatial and temporal dynamics of N2O and NO emissions from soils into the atmosphere. In this study, we modified the ecosystem model CENTURY to simulate changes in N2O and NO soil emissions through the process of converting tropical moist forests to pastures in the Atlantic Lowlands of Costa Rica. Measurements of water‐filled pore space (WFPS) and fluxes of N2O and NO from a chronosequence of pastures were used for calibration and testing of the model. It was found that the N2O+NO — WFPS and N2O:NO — WFPS relationships as developed from primary forests could be generalized to the chronosequence of pastures and other land use systems in the region. Modeled net increases (compared to primary forests) in total N2O and NO production after conversion from forest to pasture were 514 kg N ha−1 during the first 15 years under normal field conditions. The nitrogen loss in the form of N2O and NO during the first 15 years could range from 401 to 548 kg N ha−1, depending on the amounts of forest residue remaining on pasture sites. N2O‐N accounted for 90% of the gas fluxes, while NO‐N accounted for 10%. Sensitivity analysis indicated that the impacts of forest‐pasture conversion on N2O and NO emissions from soil into the atmosphere were complex, depending on the initial conditions of the forest‐derived pastures, management practices, soil physical and chemical conditions and their changes over time, N availability, and climate. It is therefore important to incorporate the spatial and temporal heterogeneities of those controlling factors in estimating regional and global N2O and NO emissions from soils into the atmosphere.

Calathea singularis and Stromanthe palustris, Two New Species of Neotropical Marantaceae

Calathea singularis from southeastern Brazil and Stromanthe palustris from Costa Rica are described as new. Calathea singularis, vegetatively unique within the Marantaceae, has a pulvinus of two separate parts rather than the pulvinar cells contin- uous and encircling the uppermost portion of the pet- iole. Stromanthe palustris, endemic to the Atlantic lowlands of Costa Rica, is described for inclusion in A Manual to the Plants of Costa Rica treatment.

Canopy development in tropical tree plantations: a comparison of species mixtures and monocultures

In this study, the early development of monocultures and mixtures of Cedrela odorata, Cordia alliodora, and Hyeronima alchorneoides was measured to determine what interspecific differences in structural characteristics lead to good ecological combining ability in fast-growing tropical tree species. Plantations were established at high density (2887 trees/ha) on fertile alluvial soil in the humid Atlantic lowlands of Costa Rica. The three species are native to the region and were selected to represent a range of crown structure, leaf morphology, and phenology. Height growth was rapid for all three species, with dominant heights (mean height of tallest 20% of trees) of monocultures reaching 7–8 m at age 2.0 yr, and 14–15 m at age 4.0 yr. At age 2.0 yr, monocultures had nearly reached their maximum leaf area index (LAI), with Hyeronima forming a denser canopy (LAI of 4.5) than the other two species (LAI's of 2.5). In mixtures, a partially stratified canopy developed with the tallest Cordia forming an upper canopy stratum above Cedrela and Hyeronima. Mixtures reached an LAI of 3.9 by age 3.0 yr, approaching the level of the Hyeronima monoculture. Compared to their growth in monoculture, Cordia was taller and Hyeronima was shorter in mixture, but both species developed larger mean crown size and breast-height diameters in mixture. In contrast, Cedrela was smaller in all dimensions in mixture than in monoculture. The poor growth of Cedrela in mixture likely resulted from the early onset of interspecific competition coupled with damage from attacks of the shootborer Hypsipyla grandella. Cedrela seedlings recovered their rapid growth in monoculture after early insect attacks, but were suppressed in mixtures by the competition from surrounding trees of Cordia and Hyeronima which are not susceptible to the shootborer. The compatibility of Cordia and Hyeronima was related to the formation of a stratified canopy in mixture. Cordia has more rapid seedling height growth than Hyeronima, but forms a more open canopy with low LAI and semi-deciduous foliage. This results in the interception of sufficient solar radiation in the upper canopy to allow high productivity of Cordia, and yet adequate transmission of radiation to the dense, evergreen crowns of Hyeronima for rapid growth of that species in the lower canopy. This combination of characteristics is likely to lead to compatibility in other sets of tree species.

The potential of plantations to foster woody regeneration within a deforested landscape in lowland Costa Rica

Reforestation of degraded pastures in the Atlantic lowlands of Costa Rica since 1988 has created a patchwork of small plantations that may have the potential to capture and conserve the remnant biodiversity in this largely deforested landscape. Studies of woody regeneration in 6 year old experimental plantations and on-farm plantations on abandoned pasture were conducted at the La Selva Biological Station and surrounding region. The experimental plantations include seven native and four exotic tree species, together with an abandoned pasture control. The impact of plantations on woody regeneration varied according to the plantation tree species. Some species fostered greater abundance of woody regeneration, some greater species richness, other species both abundance and richness, while another group had no effect significantly different from abandoned pasture. Abundance of woody regeneration did not appear to be related to canopy cover at the time of sampling. Nevertheless, plantation trees that established rapidly and had high growth rates did in general stimulate higher levels of woody regeneration. A comparison of this study with one conducted in the same plantations 18 months later indicates that the effect of some trees promoting greater density of regeneration may decline over time, while differences in species richness may persist or strengthen. Observations in a range of 6 year old plantations on farms showed a wide range of densities of woody regeneration. Part of this variation could be explained by the presence or absence of grazing within the plantation. Abundance and species richness of woody regeneration under plantation stands can be substantial. Nevertheless, the incidence of mature forest species was low and their ability to invade in the future will probably depend on subsequent stand management. It must not be forgotten that the motivation for reforestation is primarily economic; increased biodiversity is a by-product that, if it is to be promoted, will require adaptation of management practices.

Evidence for dispersal of fig seeds by the fruit-eating characid fish Brycon guatemalensis Regan in a Costa Rican tropical rain forest.

Studies were conducted at the La Selva Biological Station in the Atlantic lowlands of Costa Rica and in a greenhouse in California to assess the potential role of a Neotropical fish in dispersing the seeds of a rain forest tree. Feeding experiments showed that the seeds of Ficus glabrata H. B. K., a major, canopy-forming riparian tree, require approximately 18-36 h to pass through the digestive tract of Brycon guatemalensis Regan, an abundant riverine fish whose adult diet consists largely of leaves and fruits of this fig tree. The seeds were still viable after passing through the fish's gut but germinated somewhat more slowly than seeds that had been left in the fig exposed to air or floated in water. Stem elongation of seedlings from seeds that had passed through the fish's gut was faster than that of seeds in the other two treatments. Placement of seeds upstream may be more important than enhanced germination for plants such as Ficus that produce large numbers of seeds. Radio telemetry showed that five of six tagged fish had moved distances of 0.1-1 km upstream; seven other fish with transmitters, including three large males, were not relocated and may have moved into tributary streams for spawning or feeding. These findings suggest that Brycon can disperse large numbers of Ficus seeds and help maintain the upstream populations of the tree.

Effects of canopy species dominance on understorey light availability in low-elevation secondary forest stands in Costa Rica

ABSTRACTThe effect of canopy species dominance on understorey light availability in secondary forests was examined for Pentaclethra macroloba and Goethalsia meiantha, two common tree species with contrasting canopy architecture at La Selva Biological Station in the Atlantic lowlands of Costa Rica. In each of six 12- to 16-year-old successional stands dominated by one of these species, relative abundance and basal area of the focal species were determined within a 20 m × 50 m plot. Light availability at 1 m height was measured within each plot using a Licor-2000 Plant Canopy Analyzer at 27 locations and by analysing hemispherical photographs taken at 10 locations. Across stands, mean LAI values were significantly negatively correlated with canopy openness. Understorey microsites beneath Pentaclethra had significantly higher light availability than microsites beneath Goethalsia. Across stands, however, light availability was not correlated with either total basal area or stem density of trees ≥10 cm DBH. These results indicate that basal area and stem density of trees in the upper levels of the forest canopy are poor predictors of light penetration to the understorey. One hypothesis suggested here is that dense subcanopy and understorey vegetation in tropical secondary forests can strongly influence understorey light levels. Alternatively, light transmittance characteristics of the upper forest canopy may be influenced by species-specific differences in canopy architecture or foliage density that vary independently from tree diameter and density.

Species and site selection for timber production on farm boundaries in the humid Atlantic Lowlands of Costa Rica and Panama

In 1987–89 the CATIE-GTZ Agroforestry Project set up experiments with five timber tree species planted in single lines on twelve farm boundaries in cooperation with local farmers. When the trees were five years old, their height, diameter and total stem volume were: Acacia mangium 17 m, 19 cm and 67 m3 km−1, Cordia alliodora 14 m, 20 cm and 46 m3 km−1, Eucalyptus deglupta 22 m, 24 cm and 85 m3 km−1, Tectona grandis 17 m, 20 m and 64 m3 km−1, Terminalia ivorensis 18 m, 23 cm and 104 m3 km−1. Considering these excellent growth rates, planting of Cordia alliodora, Eucalyptus deglupta and Tectona grandis in lines on farm boundaries should be promoted. T. ivorensis and A. mangium are not recommendable for sites with impeded drainage because of mortality caused by root rot, mostly due to Rosellinia sp.

Leaf litter decomposition and mulch performance from mixed and monospecific plantations of native tree species in Costa Rica

An experiment with native trees was established in 1991 on degraded pasture in the Atlantic lowlands of Costa Rica to examine the influence of mixed and monospecific plantation designs on tree growth and nutrient cycling. As part of this study, leaf litter decomposition rates and mulch performance were compared among four native tree species, Callophylum brasiliense Cambess, Jacaranda copaia (Aubl.) D. Don, Vochysia guatemalensis J.D. Smith, and Strypnodendron microstachyum Poepp. et Endl. Leaf litter of V. guatemalensis, J. copaia and the mixed plantation decomposed the fastest, with less than 16% of the initial weight remaining at 12 months. C. brasiliense had the slowest decomposition rate with 23% of the leaf litter remaining at 12 months. V. guatemalensis had the greatest amount of annual leaf litter fall and accumulation. J. copaia showed high levels of annual litter fall but fluctuating forest-floor litter accumulation, and the mixture showed intermediate patterns of annual leaf litter fall and accumulation. All mulch treatments improved maize seedling performance in comparison with unmulched controls. S. microstachyum mulch was found to have the most beneficial effect on initial maize seedling height growth and N uptake. Recommendations are drawn from the results to suggest potential uses of these species in forestry and agroforestry systems.

Fluxes of nitric oxide from soils following the clearing and burning of a secondary tropical rain forest

At sites in the Atlantic Lowlands of Costa Rica, clearing and burning of a secondary tropical rain forest caused a significant increase in soil nitric oxide (NO) emissions. Soil‐atmosphere NO fluxes averaged 0.5 ng N cm−2 hr−1 prior to clearing and increased to 4.1 ng N cm−2 hr−1 following clearing and to greater than 12.0 ng N cm−2 hr−1 following burning. Soil NO emissions were elevated for a period of 3–4 months following clearing, and fluxes peaked for 1–3 days following burning. We conducted a series of experiments with intact soil cores to determine the probable mechanism responsible for elevated NO emissions from soils. In one set of experiments we added substrates for microbial nitrification (ammonium), denitrification (nitrate), and chemical denitrification (nitrite) to autoclaved (killed) and nonautoclaved (live) soil cores. Water‐only additions were used as controls. Compared to water or nitrate additions, ammonium caused a significant increase in NO emissions from live cores. Water, ammonium, and nitrate additions had no effect on emissions from autoclaved cores. Nitrite solution additions resulted in highly significant increases in NO emissions from both autoclaved and nonautoclaved soil cores. In a second set of experiments we treated intact soil cores with acetylene (1 kPa C2H2) to selectively inhibit nitrification and oxygen to inhibit denitrification. The oxygen treatment had no effect on NO production while acetylene significantly reduced NO emissions. The results from the substrate addition and inhibition experiments demonstrate that microbial denitrification is not a major pathway for NO production in these soils. In contrast, microbial nitrification appears to be a critical process responsible for NO emissions throughout the clearing and burning period. Field experiments with acetylene as an inhibitor show that immediately following burning, chemical denitriflcation of nitrite deposited in ash supports a large peak in NO fluxes.