Tropical Forest Productivity Research Articles

Tropical rain forest tree growth and atmospheric carbon dynamics linked to interannual temperature variation during 1984-2000.

During 1984-2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. The tree growth variations also negatively covaried with the net carbon exchange of the terrestrial tropics as a whole, as inferred from nearly pole-to-pole measurements of atmospheric carbon dioxide (CO(2)) interpreted by an inverse tracer-transport model. Strong reductions in tree growth and large inferred tropical releases of CO(2) to the atmosphere occurred during the record-hot 1997-1998 El Niño. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. As has been projected by recent process model studies, such a sensitivity of tropical forest productivity to on-going climate change would accelerate the rate of atmospheric CO(2) accumulation.

A Preliminary Study on the Net Primary Production of the Secondary Tropical Forest in Xishuangbanna

Vast areas of secondary tropical forest occur in Xishuangbanna owing to slash and burn cultivation and the destruction of tropical forest, which is a huge carbon and nutrient element bank. In order to know the dynamics of growth and bio_productivity of secondary tropical forests in different stages, “size class standard and “the dimension analysis methods were used to estimate the net primary productivity of four secondary tropical forests in different stand ages in Xishuangbanna. The results showed that the net primary productivity of four stands increases with stand age, which was 1 154.3 g·m -2·a -1 in 5_year_stand, 1 348.5 g·m -2·a -1 in 10_year_stand, 2 212.1 g·m -2·a -1 in 14_year_stand, and 2 660.1 g·m -2·a -1 in 22_year_stand respectively. The allocation of net primary productivity in four stands showed that annual net growth had the highest net productivity, which was 93.82%, 92.83%, 91.72% and 92.09% in four stands respectively, leaf grazed ranked the second and represented 4.97%, 3.92%, 5.23% and 3.96% respectively, and reproductive parts contributed 1.20%, 3.25%, 3.05% and 3.95% respectively. In the allocation of net primary productivity among different parts of plants in four stands, the net primary productivity of stem ranked the first, which occupied 35.79% of the total in 5_year_stand, 38.96% in 10_year_stand, 38.82% in 14_year_stand and 35.94% in 22_year_stand, and that of root was the second which made 23.03%, 21.38% and 23.18% of the total in 5_year_stand, 10_year_stand and 14_year_stand respectively except that of 19.26% of the total in 22_year_stand. Branch productivity increased with the stand age from 15.54% of the total in 5_year_stand to 23% of the total in 22_year_stand, while leaf contributed 23.53%, 17.81%, 18.50% and 18.49% of the total in four stands respectively which gradually decreased with the stand age. Although reproductive parts contributed only a little, it also increased from 1.20% of the total in 5_year_stand to 3.95% of the total in 22_year_stand. Concerning the allocation of net primary productivity in different layers of four stands, the net primary productivity of the tree layer increased with the stand age which was 69.07%, 74.52%, 66.07% and 93.74% of the total in four stands respectively, and net primary productivity of shrub layer did not increase until 14_year_stand, and then sharply dropped, which was 22.05% of the total in 5_year_stand, 19.49% in 10_year_stand and 32.24% in 14_year_stand, but only 2.96% in 22_year_stand, while the net primary productivity of the herb layer decreased from 8.01% in 5_year_stand to 4.41% in 10_year_stand, and that of liana plants increased with the stand age from 0.87% in 5_year_stand to 3.30% in 22_year_stand. The index of leaf area (LAI) in four stands increased with the stand age from 2.70 in 5_year_stand to 7.41 in 22_year_stand, which was lower than that of other forests. The optimum regression models of the organic growth of the dominant plants and the sample trees in arborous layer for four stands were built. These models can be used to estimate net primary productivity of the same secondary tropical forests in Xishuangbanna. Compared with other tropical forests, net primary productivity of 22_year_stand was higher than that of tropical seasonal rainforest (2 576.4 g·m -2·a -1) in Xishuangbanna, and net primary productivity of aboveground parts in 22_year_stand was also higher than that of a 20_year_stand of secondary tropical forest in Northeastern India. From the results, we can conclude that secondary tropical forest which regenerated from tropical rain forest after slash and burn cultivation in Xishuangbanna can grow fast under protection. So taking protective measures for vast secondary tropical forest is an effective way to speed recovery.

A model analysis of N and P limitation on carbon accumulation in Amazonian secondary forest after alternate land-use abandonment

Productivity and carbon (C) storage in many mature tropical forests are considered phosphorus (P) limited because of advanced soil weathering. However, disturbance can shift limitation away from P and toward nitrogen (N) because of disproportionately large N losses associated with its mobility relative to P in ecosystems. This shift was illustrated by model analyses in which large disturbances including timber extraction and slash-burn were simulated in a P-limited tropical forest. Re-accumulation of ecosystem C during secondary forest growth was initially N-limited, but long term limitation reverted to P. Mechanisms controlling shifts between N and P limitation included: (1) N volatility during slash combustion produced ash that increased soil solution P more than N, (2) a wide N:P ratio in residual fuel and belowground necromass relative to soil organic matter (SOM) N:P produced a simultaneous P sink and N source during decomposition, (3) a supplemental (to aerosol deposition) external N source via biological N fixation. Redistribution of N and P from low C:nutrient SOM to high C:nutrient vegetation was the most important factor contributing to the resilience of ecosystem C accumulation during secondary growth. Resilience was diminished when multiple harvest and re-growth cycles depleted SOM. Phosphorus losses in particular resulted in long-term reductions of C storage capacity because of slow re-supply rates via deposition and the absence of other external sources. Sensitivity analyses limiting the depth of microbially active SOM in soil profiles further illustrated the importance of elements stored in SOM to ecosystem resilience, pointing to a need for better knowledge on the functioning of deeply buried SOM.

Ecosystem structure and productivity of tropical rain forests along altitudinal gradients with contrasting soil phosphorus pools on Mount Kinabalu, Borneo

Summary We measured above‐ground net primary productivity (ANPP) and ecosystem structure and processes in eight rain forest stands at four elevations (700, 1700, 2700 and 3100 m) and on two geological substrates (sedimentary vs. ultrabasic rock) on Mount Kinabalu, Borneo. All ultrabasic sites had smaller pools of total soil phosphorus (P) and of labile inorganic P than did the sedimentary sites at the same altitudes. We predicted that the magnitude of altitudinal changes in ANPP would be less on ultrabasic than on sedimentary substrates, reflecting lower temperature dependency of ANPP under stronger P limitation. Although ANPP declined with increasing altitude on both substrates, the slopes of the two regression lines were similar. The intercept was, however, marginally greater on sedimentary than on ultrabasic substrate. Stand‐level nutrient‐use efficiencies (the ratio of litterfall mass to nutrient return) for N and P were only affected by altitude on ultrabasic substrate where they increased exponentially. Mean foliar N and P contents per unit leaf area of the canopy species increased with altitude on both substrates, but differed between substrates at the same altitude only for P (lower on ultrabasic). Leaf area index (LAI) decreased upslope on both substrates. We assumed that half of primary production was allocated below‐ground in order to evaluate stand level net assimilation rate (NAR). This was nearly constant on sedimentary substrate, but declined linearly with increasing altitude on ultrabasic substrate, where it may have to be added to LAI to explain ANPP patterns. We suggest that on sedimentary substrate trees may be able to maintain NAR under colder environments by increasing foliar N and P per unit leaf area, but P deficiency prevents them from adjusting on ultrabasic substrate.

An Ecological and Economic Assessment of the Nontimber Forest Product Gaharu Wood in Gunung Palung National Park, West Kalimantan, Indonesia

Abstract: Ecological and economic data are essential to the identification of tropical nontimber forest products with the potential for sustainable and profitable extraction in a managed system. We studied the demographic effect and economic returns of harvesting aromatic gaharu wood from fungus‐infected trees of Aquilaria malaccensis Lam. at Gunung Palung National Park, Indonesia, to evaluate the management potential of gaharu wood. Aquilaria malaccensis trees openface> 20 cm in diameter occurred at low preharvest densities (0.16–0.32 ha) but were distributed across five of six forest types surveyed. During a recent harvest, 75% of trees were felled, with harvest intensities ranging from 50% to 100% among forest types. Overall, 50% of trees contained gaharu wood, but trees at higher elevations contained gaharu wood more frequently ( 73%) than trees at lower elevation (27%). The mean density of regeneration ( juveniles> 15 cm in height) near adult trees (3–7 m away) was 0.2/m2, 200 times greater than at random in the forest (10/ha), but long‐term data on growth and survivorship are needed to determine whether regeneration is sufficient for population recovery. Gaharu wood extraction from Gunung Palung was very profitable for collectors, generating an estimated gross financial return per day of US $8.80, triple the mean village wage. Yet, the estimated sustainable harvest of gaharu wood at natural tree densities generates a mean net present value of only $10.83/ha, much lower than that of commercial timber harvesting, the dominant forest use in Kalimantan. Returns per unit area could be improved substantially, however, by implementing known silvicultural methods to increase tree densities, increase the proportion of trees that produce gaharu wood, and shorten the time interval between successive harvests. The economic potential of gaharu wood is unusual among nontimber forest products and justifies experimental trials to develop small‐scale cultivation methods.

Errata: Net Primary Production in Tropical Forests: An Evaluation and Synthesis of Existing Field Data

Errata: Net Primary Production in Tropical Forests: An Evaluation and Synthesis of Existing Field Data

Net Primary Production in Tropical Forests: An Evaluation and Synthesis of Existing Field Data

Net Primary Production in Tropical Forests: An Evaluation and Synthesis of Existing Field Data

NET PRIMARY PRODUCTION IN TROPICAL FORESTS: AN EVALUATION AND SYNTHESIS OF EXISTING FIELD DATA

Information on net primary production in tropical forests is needed for the development of realistic global carbon budgets, for projecting how these ecosystems will be affected by climatic and atmospheric changes, and for evaluating eddy covariance mea- surements of tropical forest carbon flux. However, a review of the database commonly used to address these issues shows that it has serious flaws. In this paper we synthesize the data in the primary literature on NPP in old-growth tropical forests to produce a consistent data set on NPP for these forests. Studies in this biome have addressed only a few NPP com- ponents, all aboveground. Given the limited scope of the direct field measurements, we sought relationships in the existing data that allow estimation of unmeasured aspects of production from those that are more easily assessed. We found a predictive relationship between annual litterfall and aboveground biomass increment. For 39 diverse tropical forest sites, we then developed consistent, documented estimates of the upper and lower bounds around total NPP to serve as benchmarks for calibrating and validating biogeochemical models with respect to this biome. We developed these estimates based on existing field measurements, current understanding of aboveground consumption and biogenic volatile organic carbon emissions, and our judgment that belowground production is bounded by the range 0.2-1.2 3 ANPP (aboveground NPP). Across this broad spectrum of tropical forests (dry to wet, lowland to montane, nutrient-rich to nutrient-poor soils), our estimates of lower and upper bounds on total NPP range from 1.7 to 11.8 Mg C·ha 21 ·yr 21 (lower bounds) and from 3.1 to 21.7 Mg C·ha 21 ·yr 21 (upper bounds). We also showed that two relationships that have been used for estimating NPP (the Bray-Gorham relationship based on leaf litterfall and the Miami model based on temperature or precipitation) are not valid for the tropical forest biome.

Can Timber Production Be Compatible with Conservation of Forest Biodiversity?–Two Case Studies of Plant Biodiversity in Managed Neotropical Forests

SUMMARY.A variety of practices can greatly diminish the impacts of forest management on biodiversity. A case study from a subtropical humid forest reserve in Misiones, Argentina is presented to illustrate a “uniform spacing” method of forest harvest. In this method, trees are selected for extraction or marked for retention according to their scarcity, their horizontal distribution, and their characteristics to serve as seed trees. Three years after harvesting, the forest cut by uniform spacing had the highest density of seedlings of total as well as commercial species, and it also exhibited high diversity of understory plants other than trees.A case study of a silvicultural experiment carried out by CATIE in Costa Rica is also presented, focusing on the effects of logging and post-harvest silvicultural treatments on forest species richness and composition during the first 6-7 yr following logging and 5 yr following the application of treatments. The forest studied was shown to exhibit marked compositional variation in relation to a topographical gradient after the implementation of the experiment; such □ or ecosystem diversity should be taken into account in all future evaluations of the effect of forest management on plant biodiversity. Post-harvest silvicultural treatments caused an immediate reduction in species-richness = 10 cm dbh in 1.0 ha due to the chance elimination of species represented by one or a small number of individuals in the plots, and may affect overall species composition in the medium term as they are directed to favoring commercial species by decreasing competition from non-commercial species. Species richness declined under both silvicultural treatments, however, no changes of species-richness or composition as a result of silvicultural treatment were evident in the forest understory (individuals 2.5-9.9 cm dbh). The only detectable changes in understory plant biodiversity were caused by the felling and extraction of timber, and these changes were restricted to the very localized areas disturbed by these management operations.The application of low-intensity logging methods with selection of remnant trees requires detailed pre- and post-harvest forest inventories and follow up of treatments to ensure successful regeneration and growth of desired species. The Costa Rican study shows that different forest management operations (logging and silviculture in this case) may have different effects on plant biodiversity; this perspective should be applied to future studies in order to better understand the balance between production and conservation in tropical forests. Long-term follow up is needed to assess the true effects of forest management on biodiversity, including natural regeneration as well as growth dynamics of trees, until a whole harvest cycle is completed.

Effects of light availability and rainfall on leaf production in a moist tropical forest in central Panama

New leaf production in seasonal tropical forests may result from changes in water or light availability. In this study, the relationship between leaf flushing, photosynthetically active radiation (PAR) and rainfall was examined for understorey saplings in a moist tropical forest over one year. During the wet season, weeks with greater PAR were correlated with a greater proportion of saplings flushing new leaves during subsequent weeks in nine out of ten species. Rainfall was negatively correlated with subsequent leafing during the wet season for six of ten species. However, during the dry season, rainfall was positively correlated with leafing during the following weeks for six species, but the relationship was much weaker. PAR in the dry season was negatively correlated with flushing in eight species. These results support the hypothesis that under well-watered conditions, light limits leaf production, and peaks in insolation result in greater leaf production.

Regional application of an ecosystem production model for studies of biogeochemistry in Brazilian Amazonia

AbstractThe degree to which primary production, soil carbon, and trace gas fluxes in tropical forests of the Amazon are limited by moisture availability and other environmental factors was examined using an ecosystem modelling application for the country of Brazil. A regional geographical information system (GIS) serves as the data source of climate drivers, satellite images, land cover, and soil properties for input to the NASA Ames‐CASA (Carnegie‐Ames‐Stanford Approach) model over a 8‐km grid resolution. Simulation results lead us to hypothesize that net primary production (NPP) is limited by cloud interception of solar radiation over the humid north‐western portion of the region. Peak annual rates for NPP of nearly 1.4 kg C m–2 year–1 are localized in the seasonally dry eastern Amazon in areas that we assume are primarily deep‐rooted evergreen forest cover. Regional effects of forest conversion on NPP and soil carbon content are indicated in the model results, especially in seasonally dry areas. Comparison of model flux predictions along selected eco‐climatic transects reveal moisture, soil, and land use controls on gradients of ecosystem production and soil trace gas emissions (CO2, N2O, and NO). These results are used to formulate a series of research hypotheses for testing in the next phase of regional modelling, which includes recalibration of the light‐use efficiency term in NASA‐CASA using field measurements of NPP, and refinements of vegetation index and soil property (texture and potential rooting depth) maps for the region.

EXPERIMENTAL INVESTIGATION OF NUTRIENT LIMITATION OF FOREST GROWTH ON WET TROPICAL MOUNTAINS

This article reviews literature and summarizes experiments to investigate the extent to which productivity of tropical montane rain forests is constrained by low nutrient supply. On any one mountain, with increase in altitude foliar N decreases, and P and K usually decrease, but Ca and Mg show no consistent trend. However for a wide range of sites N, P, K, Mg, and Ca show no trends. Litterfall contents of N and P and often K, Ca, and Mg are lower in montane forests than in lowland forests, mainly because of reduced litterfall mass, but N and P concentrations are also lower in forests above 1500 m. Tropical montane soils usually have more soil organic matter per unit ground area; N mineralization levels are lower at higher altitudes in Costa Rica, and extractable and total soil P are lower in sites with lower litterfall P concentrations. We speculate that many lowland forests are limited by P and many montane forests by N. Fertilization studies on ash-derived montane soils in Hawai‘i showed a trend for a switch from N limitation on young soils to P, or N and P, limitation on soils over older substrates. Jamaican montane trees were limited by N and by P separately. Venezuelan montane trees were limited by N. The sites in Jamaica and Venezuela have soils of indeterminate age. Taken together these results show that nutrient limitation is widespread in montane soils (all sites have responded to at least one nutrient) and that the particular nutrient(s) that limit(s) production may differ for explicable reasons. First results from lowland forests on sandy soils in Kalimantan show N or simultaneous N and P limitation. Many more experiments, especially in lowland forests, are needed to test our speculation that P usually limits productivity in tropical lowland rain forests and that N limits productivity in tropical montane rain forests.

Experimental Investigation of Nutrient Limitation of Forest Growth on Wet Tropical Mountains

This article reviews literature and summarizes experiments to investigate the extent to which productivity of tropical montane rain forests is constrained by low nutrient supply. On any one mountain, with increase in altitude foliar N decreases, and P and K usually decrease, but Ca and Mg show no consistent trend. However for a wide range of sites N, P, K, Mg, and Ca show no trends. Litterfall contents of N and P and often K, Ca, and Mg are lower in montane forests than in lowland forests, mainly because of reduced litterfall mass, but N and P concentrations are also lower in forests above 1500 m. Tropical montane soils usually have more soil organic matter per unit ground area; N mineralization levels are lower at higher altitudes in Costa Rica, and extractable and total soil P are lower in sites with lower litterfall P concentrations. We speculate that many lowland forests are limited by P and many montane forests by N. Fertilization studies on ash-derived montane soils in Hawai‘i showed a trend for a switch from N limitation on young soils to P, or N and P, limitation on soils over older substrates. Jamaican montane trees were limited by N and by P separately. Venezuelan montane trees were limited by N. The sites in Jamaica and Venezuela have soils of indeterminate age. Taken together these results show that nutrient limitation is widespread in montane soils (all sites have responded to at least one nutrient) and that the particular nutrient(s) that limit(s) production may differ for explicable reasons. First results from lowland forests on sandy soils in Kalimantan show N or simultaneous N and P limitation. Many more experiments, especially in lowland forests, are needed to test our speculation that P usually limits productivity in tropical lowland rain forests and that N limits productivity in tropical montane rain forests.

Approaches to Developing Sustainable Extraction Systems for Tropical Forest Products

There are few if any examples of the demonstrably sustainable extraction of either timber or non—timber forest products. Even well—known products such as brazil nuts and mahogany lack a sufficient knowledge base to design a sustainable extraction system. Potential extraction systems for timber and non—timber forest products from tropical forests should be evaluated both in terms of their sustainability and their impact at the ecosystem level. The impact of forest product harvest on the demographics of the target species can be explored with the use of mathematical models, although we still lack an adequate understanding of some of the basic processes that are structuring tropical tree communities. Matrix models are relatively quick to construct, and they may be appropriate for modeling the dynamics of populations that are harvested without introducing large changes to the ecosystem, while individual—based mechanistic models are more appropriate for modeling the effects of harvest that cause large changes in population and ecosystem structure. Once the maximum sustainable level of harvest has been identified with the use of models, an economic analysis of the range of harvest intensities between zero and maximum sustainable yield should be conducted, with the goal of identifying the range of possible harvest intensities that are both sustainable and economically viable. This range of harvest intensities should then be analyzed in terms of its impact on the ecosystem, so that the harvest intensity that is chosen will not result in impacts to the forest that are unacceptably high.

Ecología de las poblaciones de plantas en una selva húmeda de México

Se presenta una revisión y análisis de los estudios de ecología de poblaciones de plantas realizados en selvas húmedas de México, particularmente en la Estación de Biología Tropical Los Tuxtlas (UNAM); se abordan los siguientes temas: (i) patrones estructurales y demográficos, (ii) análisis de la dinámica de poblaciones, (iii) exploración de la evolución de historias de vida y (iv) demografía, conservación y manejo de recursos naturales. Los estudios muestran un considerable avance en la descripción de patrones, en el análisis de la dinámica de poblaciones y en la detección de algunos aspectos demográficos que pueden ser claves para los propósitos de la conservación y manejo de recursos. Sin embargo, el nivel de entendimiento de las causas que originan tales patrones y dinámicas es aún pobre. Los estudios han centrado su interés principalmente en poblaciones de árboles que son abundantes; otras formas de vida y especies que son raras han recibido una atención virtualmente nula. Después de sintetizar los puntos emergentes de la presente revisión, indicamos algunas perspectivas que creemos que serán importantes de seguir en el futuro.

The effect of income on the extraction of non-timber tropical forest products: Model, hypotheses, and preliminary findings from the Sumu Indians of Nicaragua

We use microeconomic theory to frame hypotheses about the effects of income on the use of non-timber rain forest products. We hypothesize that an increase in income: (a) encourages foraging specialization, resulting in the extraction of fewer goods; (b) increases the share of household income from occupations besides foraging; (c) produces a yearly value from the extraction of nontimber forest goods of about $50 per hectare; and (d) produces depletion of forest goods entering commercial channels and sustainable extraction of goods facing cheaper industrial substitutes. To examine these hypotheses we present worldwide ethnographic information and preliminary findings from field work carried out among the Sumu Indians of Nicaragua. Field work suggests that higher income produces: (a) foraging specialization with animals rather than with plants; (b) a decline in the economic importance of forest goods in household income; (c) and a rise in the value of non-timber goods removed from the forest to about $35/ha/year. We did not have time to test hypothesis “d.”

Climate-Induced Annual Variation in Canopy Tree Growth in a Costa Rican Tropical Rain Forest

1 Annual growth was measured over eight consecutive years (1984-92) for saplings and adults of a diverse group of nonpioneer tree species in a lowland neotropical rain forest (La Selva Biological Station, Costa Rica). The study species included five emergents and one canopy species. 2 Adult diameter increments varied markedly from year to year for all species. In the year of highest growth for individual species, mean increments were 25-112% greater than in the year of least growth. These among-year differences were significant for five of the six species. 3 The variation in adult growth rates showed strong temporal concordance across species. For all six species, mean adult diameter increments were higher than average in 1985 and were lower than average in 1986 and 1988. 4 Marked year to year growth differences were also shown by juvenile trees of these species. In the class > 50 cm tall and < 1 cm in diameter, mean diameter increments were 3-10 times greater in the year of highest growth than in the year of least growth. In each of the other two juvenile size classes (1-10 cm and 10-30 cm in diameter), two species showed highly significant among-year growth variation. 5 Within all three juvenile size classes, the year to year growth variation showed significant temporal concordance across species. As for adults, 1985 and/or 1984 were among the two highest growth years. 6 That these patterns of interyear growth differences were shared across tree species and between saplings and adults demonstrates significant impacts of climatic variability on the productivity of this 'equable' ecosystem. 7 Annual rainfall was not correlated with the annual growth patterns. Although the two years of highest overall growth, 1984 and 1985, were exceptionally dry, adult trees of two species showed their greatest growth in one of the highest rainfall years (1990). 8 Recent studies suggest that photosynthetically active radiation (PAR) controls productivity of tropical wet forests. Recent data from La Selva show strong year to year variation in monthly PAR. Demonstration of a causal relationship will, however, require concurrent measurement of tree growth, PAR, and other climatic factors. 9 Marked temporal variation in tree growth in tropical wet forests will have pervasive consequences for many processes in these complex ecosystems. Moreover, the degree of climatic sensitivity found in this study indicates that tropical rain forests could be strongly affected by global climate change.

Mineral nutrient status of coastal hill dipterocarp forest and adinandra belukar in Singapore: bioassays of nutrient limitation

ABSTRACTBioassays of nutrient limitation were carried out for Melastoma malabathricum growing on soil from primary lowland dipterocarp rainforest in Singapore, and for Dillenia suffruticosa on soil from adinandra belukar, a nutrient-poor secondary forest type dominated by Adinandra dumosa. Three questions were addressed. 1. What is the nutrient most limiting to growth in primary forest? 2. What is the nature of nutrient limitation under conditions of adequate P supply? 3. Is there a qualitative difference in the nature of nutrient limitation under primary forest and adinandra belukar? Results showed that there was a strong limitation by P availability in both primary forest and adinandra belukar under the experimental conditions used. Once plants had an adequate P supply, all other nutrients became limiting to growth in primary forest soil. These findings are interpreted as support for the hypothesis that P availability would limit the productivity of moist tropical forests in general in the absence of mycorrhizas; tentative conclusions are drawn on the assumption that most woody tropical plants are mycorrhizal. It is argued that limitation by major cations may be common on old, highly leached tropical rainforest soils.

The importance of nutrient pulses in tropical forests

Recent research shows that nutrient fluxes are often pulsed In tropical forests, and that pulsed versus gradual inputs have different effects on the fates of nutrients in the ecosystem. Synchrony of nutrient mineralization with plant uptake can lower competition between microbes and plants for limiting nutrients while maintaining tight nutrient cycling, whereas asynchrony can lead to losses of nutrients from the system. Thus, nutrient pulses may play a critical role in maintaining productivity in tropical forests with tight nutrient cycling.

Zonation and seasonality of benthic primary production and community respiration in tropical mangrove forests.

Benthic oxygen consumption and primary production were measured using the bell jar technique in deltaic and fringing mangrove forests of tropical northeastern Australia. In a deltaic forest, rates of sediment respiration ranged from 197 to 1645 μmol O2 m-2 h-1 (mean=836), but did not vary significantly with season or intertidal zone. Gross primary production varied among intertidal zones and seasons, ranging from -281 to 1413 μmol O2 m-2 h-1 (mean=258). Upon tidal exposure, rates of gross primary production increased, but respiration rates did not change significantly. In a fringing mangrove forest, benthic respiration and gross primary production exhibited strong seasonality. In both forests, rates of oxygen consumption and production were low compared to salt marshes, but equivalent to rates in other mangrove forests. The production:respiration (P/R) ratio varied greatly over space and time (range:-0.61 to 1.76), but most values were «1 with a mean of 0.15, indicating net heterotrophy. On a bare creek bank and a sandflat, rates of gross primary production and P/R ratios were generally higher than in the adjacent mangroves. Low microalgal standing stocks, low light intensity under the canopy, and differences in gross primary production between mangroves and tidal flats, and with tidal status, indicate that benthic microalgae are light-limited and a minor contributor to primary productivity in these tropical mangrove forests.