Moist Tropics Research Articles

Effect of PGRs on Cucurbits: An Overview

Cucurbits, a popular vegetable, can thrive in deserts and moist tropics. The family Cucurbitaceae contains 118 genera and about 825 species. These crops are mostly grown in India during the summer and rainy seasons, with certain southern and western regions cultivating them in winter. These crops are annual and perennial. PGRs in cucurbits have shown promise in improving crop growth, productivity, and quality. PGRs affect several physiological and developmental processes in cucurbits. There are different effects of the various kind of the PGRs in cucurbits such as watermelon with 25-50 ppm GA3 yielded earlier and more fruits. Fruit count rose with 250 and 500 ppm ethrel. The 40 ppm GA3 increased vegetative growth and production. TIBA (20 ppm) increased fruit weight and yield. In case of cucumber GA3 at 20 ppm and NAA at 100 ppm increased cucumber growth and production. Application of GA3 (20 ppm) at the 4-leaf stage enhances growth, flowering, and yield in bottle gourd. In bitter gourd fruit per plant, fruit weight, and yield increased significantly with GA3 at 60 ppm. Ethrel at 50 ppm improved fruit set and weight. Gibberex treatments increased fruit quantity, weight, and yield per plant. Further study and fieldwork are needed to understand PGRs and determine the appropriate effects for cucurbits.

Development of Waterborne Heavy-Duty Anticorrosive Coatings with Modified Nanoscale Titania

The steel structures of coastal engineering in the moist tropics and subtropics are always under a C5/CX level corrosion environment with high temperature, high humidity, and high salt fog. Anticorrosive waterborne coatings with high weatherability and reliability are urgently to be developed. In this work, one kind of waterborne heavy-duty anticorrosive coatings, with the advantages of excellent corrosion resistance, self-repairing ability, self-cleaning ability, and high film compactness, was successfully achieved through modifying the side chains on the surface morphologies of the spherical nanoscale titania. The micromorphology and structure of the coating were characterized by a scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscope (AFM). The anticorrosion characteristics and forming mechanism of the modified nanoscale titania coating were analyzed. The salt spray tests showed that the neutral salt spray resistance time of the modified nanoscale titania coating was 1440 h. Its durability reached the H level and met the design requirements for 15 years of anticorrosion lifetime. The modified nanoscale titania coatings had been large-scale commercially applied at some typical steel structures under an extreme harsh corrosion environment in one coastal thermal power plant. The results showed that no rusting, peeling, or crack phenomena were observed after 3 years of service under different harsh coastal corrosion conditions.

A global evaluation of the effectiveness of voluntary REDD+ projects at reducing deforestation and degradation in the moist tropics.

Reducing emissions from deforestation and forest degradation (REDD+) projects aim to contribute to climate change mitigation by protecting and enhancing carbon stocks in tropical forests, but there have been no systematic global evaluations of their impact. We used a new data set for tropical humid forests and a standardized evaluation approach (based on pixel matching) to quantify the performance of a representative sample of 40 voluntary REDD+ projects in 9 countries certified under the Verified Carbon Standard (VCS). In the first 5 years of implementation, deforestation within project areas was reduced by 47% (95% confidence interval [CI]: 24-68) compared with matched counterfactual pixels, and degradation rates were 58% lower (95% CI: 49-63). Reductions were small in absolute terms but greater in sites located in high-deforestation settings and did not appear to be substantially undermined by leakage activities in forested areas within 10km of project boundaries. At the 26th Conference of the Parties of the United Nations Framework Convention on Climate Change, the international community renewed its commitment to tackling tropical deforestation as a nature-based solution to climate change. Our results indicate that incentivizing forest conservation through voluntary site-based projects can slow tropical deforestation and highlight the particular importance of prioritizing financing for areas at greater risk of deforestation.

Quantifying the Global Power Needed for Sap Ascent in Plants

AbstractTerrestrial photosynthesis requires the evaporation of water (transpiration) in exchange for CO2 needed to form sugars. The water for transpiration is drawn up through plant roots, stem, and branches via a water potential gradient. However, this flow of water—or sap ascent—requires energy to lift the water to the canopy and to overcome the resistance of the plant’s water transporting xylem. Here, we use a combination of field measurements of plant physiology (hydraulic conductivity) and state‐of‐the‐science global estimates of transpiration to calculate how much energy is passively harvested by plants to power the sap ascent pump across the world’s terrestrial vegetation. Globally, we find that 0.06 W/m2 is consumed in sap ascent over forest dominated ecosystems or 9.4 PWh/yr (equal to global hydropower energy production). Though small in comparison to other components of the Earth’s surface energy budget, sap ascent work in forests represents 14.2% of the energy compared to the energy consumed to create sugars through photosynthesis, with values up to 18% in temperate rainforests. The power needed for sap ascent generally increases with photosynthesis, but is moderated by both climate and plant physiology, as the most work is consumed in regions with large transpiration fluxes (such as the moist tropics) and in areas where vegetation has low conductivity (such as temperate rainforests dominated by conifer trees). Here, we present a bottom‐up analysis of sap ascent work that demonstrates its significant role in plant function across the globe.

Tropical tree mortality has increased with rising atmospheric water stress.

Evidence exists that tree mortality is accelerating in some regions of the tropics1,2, with profound consequences for the future of the tropical carbon sink and the global anthropogenic carbon budget left to limit peak global warming below 2 °C. However, the mechanisms that may be driving such mortality changes and whether particular species are especially vulnerable remain unclear3-8. Here we analyse a 49-year record of tree dynamics from 24 old-growth forest plots encompassing a broad climatic gradient across the Australian moist tropics and find that annual tree mortality risk has, on average, doubled across all plots and species over the last 35years, indicating a potential halving in life expectancy and carbon residence time. Associated losses in biomass were not offset by gains from growth and recruitment. Plots in less moist local climates presented higher average mortality risk, but local mean climate did not predict the pace of temporal increase in mortality risk. Species varied in the trajectories of their mortality risk, with the highest average risk found nearer to the upper end of the atmospheric vapour pressure deficit niches of species. A long-term increase in vapour pressure deficit was evident across the region, suggesting that thresholds involving atmospheric water stress, driven by global warming, may be a primary cause of increasing tree mortality in moist tropical forests.

A policy-driven framework for conserving the best of Earth's remaining moist tropical forests.

Tropical forests vary in composition, structure, and function such that not all forests have similar ecological value. This variability is caused by natural and anthropogenic disturbance regimes, which influence the ability of forests to support biodiversity, store carbon, mediate water yield, and facilitate human wellbeing. While international environmental agreements mandate protecting and restoring forests, only forest extent is typically considered, while forest quality is ignored. Consequently, the locations and loss rates of forests of high ecological value are unknown and coordinated strategies for conserving these forests remain undeveloped. Here, we map locations high in forest structural integrity as a measure of ecological quality, based on recently developed fine-resolution maps of three-dimensional forest structure, integrated with human pressure across the global moist tropics. Our analyses reveal that tall forests with closed-canopies and low human pressure typical of natural conditions comprise half of the global humid or moist tropical forest estate, largely limited to the Amazon and Congo basins. The vast majority of these forests have no formal protection and, given recent rates of loss, are at significant risk. With the rapid disappearance of these ‘best of the last’ forests at stake, we provide a policy-driven framework for their conservation and restoration, and recommend locations to maintain protections, add new protections, mitigate deleterious human impacts, and restore forest structure.

Role of forest regrowth in global carbon sink dynamics

Although the existence of a large carbon sink in terrestrial ecosystems is well-established, the drivers of this sink remain uncertain. It has been suggested that perturbations to forest demography caused by past land-use change, management, and natural disturbances may be causing a large component of current carbon uptake. Here we use a global compilation of forest age observations, combined with a terrestrial biosphere model with explicit modeling of forest regrowth, to partition the global forest carbon sink between old-growth and regrowth stands over the period 1981-2010. For 2001-2010 we find a carbon sink of 0.85 (0.66-0.96) Pg year-1 located in intact old-growth forest, primarily in the moist tropics and boreal Siberia, and 1.30 (1.03-1.96) Pg year-1 located in stands regrowing after past disturbance. Approaching half of the sink in regrowth stands would have occurred from demographic changes alone, in the absence of other environmental changes. These age-constrained results show consistency with those simulated using an ensemble of demographically-enabled terrestrial biosphere models following an independent reconstruction of historical land use and management. We estimate that forests will accumulate an additional 69 (44-131) Pg C in live biomass from changes in demography alone if natural disturbances, wood harvest, and reforestation continue at rates comparable to those during 1981-2010. Our results confirm that it is not possible to understand the current global terrestrial carbon sink without accounting for the sizeable sink due to forest demography. They also imply that a large portion of the current terrestrial carbon sink is strictly transient in nature.

Distinguishing the pollen of Dipterocarpaceae from the seasonally dry and moist tropics of south-east Asia using light microscopy

The Dipterocarpaceae family is ubiquitous across mainland and maritime south-east Asia. Species within the family are often so well-adapted to – and prolific within – ecologically distinct forest types, that they are used as habitat indicators within forestry and ecological research. The limited work on the classification of Dipterocarpaceae pollen under light microscopy, however, means that paleoecologists working in the region are currently unable to link fossil pollen to indicator species/assemblages with any confidence. As a consequence, ecologically meaningful and habitat-specific data are homogenized in the paleorecord.This study explores whether the classification of pollen from representative south-east Asian Dipterocarpaceae species can be resolved at a higher taxonomic resolution. This will provide paleoecologists with a tool to (1) better assess shifts in forest types (and hence climate/elevation envelopes) through time, and (2) predict species response to future regional change. Our revised classification scheme is based on an analysis of the pollen morphology of Dipterocarpaceae species from two globally significant ecoregions in south-east Asia – the seasonally dry tropical forests of mainland south-east Asia, and the Sulawesi lowland rainforests of Wallacea – under the light microscope. Results of this analysis show that, using a range of morphological metrics – notably grain size, size-to-wall thickness ratios and exine structure – mainland and Wallacean Dipterocarpaceae pollen can be readily classified into three taxonomic groups – Vatica, Dipterocarpus and Shorea/Hopea/Anisoptera/Pentacme types. At an intra-ecoregional scale, each assemblage can be further broken down into six groups. Some of these groupings appear useful for the classification of certain indicator pollen types within the family, including those taxa representative of key open – and closed – canopy dry forest habitats in the mainland seasonal tropics, the delimitation of peat/swamp forest types in both ecoregions, and the identification of lowland mafic forest types in Sulawesi. Additionally, being able to split the Dipterocarpaceae within these ecologically important ecoregions at a higher taxonomic resolution is of potential conservation significance, permitting long-term assessment of the adaptation capacity of different genera/species to climatic change or disturbance events.

Roles of non‐native species in large‐scale regeneration of moist tropical forests on anthropogenic grassland

AbstractThis paper presents a new synthesis of the role of native and non‐native species in diverse pathways and processes that influence forest regeneration on anthropogenic grassland in the moist tropics. Because of altered species composition, abiotic conditions and landscape habitat mosaics, together with human interventions, these successional pathways differ from those seen in pre‐clearing forests. However, representation of different functional life forms of plant (tree, vine, grass, herb and fern) and animal (frugivorous seed disperser, granivorous seed predator, seedling herbivore and carnivore) shows consistent global variation among areas of pasture, intact forest, and post‐grassland regrowth. Biotic webs of interaction involve complex indirect influences and feedbacks, which can account for wide observed variation in regeneration trajectories over time. Important processes include: limitation of tree establishment by dense grasses; recruitment and growth of pioneer pasture trees (shading grasses and facilitating bird‐assisted seed dispersal); and smothering of trees by vines. In these interactions, species’ functional roles are more important than their biogeographic origins. Case studies in eastern Australia show native rain forest plant species diversity in all life forms increasing over time when pioneer trees are non‐native (e.g., Cinnamomum camphora, Solanum mauritianum), concurrent with decreased grass and fern cover and increased abundance of trees and vine tangles. The global literature shows both native and non‐native species facilitating and inhibiting regeneration. However conservation goals are often targeted at removing non‐native species. Achieving large‐scale tropical forest restoration will require increased recognition of their multiple roles, and compromises about allocating resources to their removal.

Aboveground tree growth varies with belowground carbon allocation in a tropical rainforest environment.

Young secondary forests and plantations in the moist tropics often have rapid rates of biomass accumulation and thus sequester large amounts of carbon. Here, we compare results from mature forest and nearby 15–20 year old tree plantations in lowland Costa Rica to evaluate differences in allocation of carbon to aboveground production and root systems. We found that the tree plantations, which had fully developed, closed canopies, allocated more carbon belowground - to their root systems - than did mature forest. This increase in belowground carbon allocation correlated significantly with aboveground tree growth but not with canopy production (i.e., leaf fall or fine litter production). In contrast, there were no correlations between canopy production and either tree growth or belowground carbon allocation. Enhanced allocation of carbon to root systems can enhance plant nutrient uptake, providing nutrients beyond those required for the production of short-lived tissues such as leaves and fine roots, and thus enabling biomass accumulation. Our analyses support this deduction at our site, showing that enhanced allocation of carbon to root systems can be an important mechanism promoting biomass accumulation during forest growth in the moist tropics. Identifying factors that control when, where and for how long this occurs would help us to improve models of forest growth and nutrient cycling, and to ascertain the role that young forests play in mitigating increased atmospheric carbon dioxide.

Mechanisms driving an unusual latitudinal diversity gradient for grasses

AbstractAimFor most higher‐order taxa, species diversity peaks sharply in the moist tropics and declines rapidly at higher latitudes. However, the mechanisms driving this latitudinal gradient are numerous, remain uncertain and are even undocumented in some important major clades such as the grasses. Grasses are a cosmopolitan, important plant family with more than 11,000 species world‐wide. Our aims were: to investigate the latitudinal distributions of species richness for different grass lineages, and the grass family as a whole; and to test hypotheses proposed in general for the latitudinal diversity gradient or specifically as determinants of grass species richness patterns at the global scale.LocationGlobal.MethodsWe used the most comprehensive global database of grass distributions currently available to calculate species richness for 340 political regions of the world. Using generalized additive models we used these data to model the latitudinal gradients of species richness for different grass lineages and for the grass family as a whole. We constructed multiple regression models to include climatic, productivity, topographic, habitat and geographic variables.ResultsAn unusual, shallow latitudinal diversity gradient arises because different grass lineages exhibit contrasting latitudinal patterns. This reflects differential specialization of grass lineages to arid and cool environments, the legacy of historical effects, most notably the Gondwanan origin of the grasses, and a strong association between grasses and topographically heterogeneous, mountainous regions.Main conclusionsThe grasses are one of the relatively few higher‐order lineages that exhibits an atypical latitudinal gradient; this has arisen because of climatic specialization of particular grass lineages to cold and arid environments. Key additional roles have been inferred for historical biogeography and topographical heterogeneity in determining global patterns of grass species richness. These mechanisms have generally been under‐appreciated and are probably important for many other lineages.

Balancing bioenergy and biosecurity policies: estimating current and future climate suitability patterns for a bioenergy crop

AbstractIn an apparent paradox, bioenergy crops offer potential benefits to a world adjusting to the challenges of climate change and declining fossil fuel stocks, as well as potential ecological and economic threats resulting from biological invasions. In considering this paradox it is important to understand that benefits and threats may not always be apparent in equal measure throughout the potential range of each candidate biofuel species. In some environments, a species could potentially produce valuable biological materials without posing a significant invasion threat. In this study, we develop a bioclimatic niche model for a candidate biofuel crop, Millettia pinnata, and apply the model to different climatic and irrigation scenarios to estimate the current and future patterns of climate suitability for its growth and naturalization. We use Australia as a case study for interpreting the niche model in terms that may be informative for both biofuels proponents and biosecurity regulators to plan management programmes that reflect the invasive potential in different areas. The model suggests that suitable growing conditions for M. pinnata in Australia are naturally restricted to the moist and semimoist tropics. Irrigation can extend the suitable growing conditions more widely throughout the tropics, and into more arid regions. Under future climate scenarios, suitable growing conditions for M. pinnata under natural rainfall contract towards the east coast, and extend southward into the subtropics. With irrigation, M. pinnata appears to have the potential in the future to naturalize across much of Australia. The bioclimatic modelling method demonstrated here is comparatively quick and easy, and can produce a rich array of data products to inform the interests of both bioenergy proponents and biosecurity regulators. We show how this modelling can support the development of spatially explicit biosecurity policies designed to manage invasion risks in a manner that balances bioenergy and biosecurity concerns.

Climate Warming–Related Strengthening of the Tropical Hydrological Cycle

Abstract The authors estimate climate warming–related twenty-first-century changes of moisture transports from the descending into the ascending regions in the tropics. Unlike previous studies that employ time and space averaging, here homogeneous high horizontal and vertical resolution data from an Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) climate model are used. This allows for estimating changes in much greater detail (e.g., the estimation of the distribution of ascending and descending regions, changes in the vertical profile, and separating changes of the inward and outward transports). Low-level inward and midlevel outward moisture transports of the convective regions in the tropics are found to increase in a simulated anthropogenically warmed climate as compared to a simulated twentieth-century atmosphere, indicating an intensification of the hydrological cycle. Since an increase of absolute inward transport exceeds the absolute increase of outward transport, the resulting budget is positive, meaning that more water is projected to converge in the moist tropics. The intensification is found mainly to be due to the higher amount of water in the atmosphere, while the contribution of weakening wind counteracts this response marginally. In addition the changing statistical properties of the vertical profile of the moisture transport are investigated and the importance of the substantial outflow of moisture from the moist tropics at midlevels is demonstrated.

The Global Character of the Flux of Downward Longwave Radiation

Abstract Four different types of estimates of the surface downwelling longwave radiative flux (DLR) are reviewed. One group of estimates synthesizes global cloud, aerosol, and other information in a radiation model that is used to calculate fluxes. Because these synthesis fluxes have been assessed against observations, the global-mean values of these fluxes are deemed to be the most credible of the four different categories reviewed. The global, annual mean DLR lies between approximately 344 and 350 W m−2 with an error of approximately ±10 W m−2 that arises mostly from the uncertainty in atmospheric state that governs the estimation of the clear-sky emission. The authors conclude that the DLR derived from global climate models are biased low by approximately 10 W m−2 and even larger differences are found with respect to reanalysis climate data. The DLR inferred from a surface energy balance closure is also substantially smaller that the range found from synthesis products suggesting that current depictions of surface energy balance also require revision. The effect of clouds on the DLR, largely facilitated by the new cloud base information from the CloudSat radar, is estimated to lie in the range from 24 to 34 W m−2 for the global cloud radiative effect (all-sky minus clear-sky DLR). This effect is strongly modulated by the underlying water vapor that gives rise to a maximum sensitivity of the DLR to cloud occurring in the colder drier regions of the planet. The bottom of atmosphere (BOA) cloud effect directly contrast the effect of clouds on the top of atmosphere (TOA) fluxes that is maximum in regions of deepest and coldest clouds in the moist tropics.

Flooding and profuse flowering result in high litterfall in novelSpathodea campanulataforests in northern Puerto Rico

The African tulip tree, Spathodea campanulata, dominates many post-agricultural secondary forests in the moist tropics. Some consider these novel forests have no ecological value, yet they appear to restore ecosystem processes on degraded sites. This study describes the litterfall mass and seasonality, canopy phenology, and microclimate of S. campanulata forests on alluvial and karst substrates in northern Puerto Rico. These substrates have different water drainage properties and I hypothesized that (1) annual leaf fall mass and seasonality would differ between substrate types; because (2) leaf fall would be related to water availability and seasonality. I used analysis of variance to compare annual and biweekly litterfall mass across three sites on each substrate type, and multiple linear regression analysis to relate biweekly litterfall to environmental variables. Litterfall mass was high (13.8 Mg·ha−1·yr−1, n = 6, SE = 0.60) yet its components did not differ by substrate type except for reproductive part mass which was higher on karst due to more S. campanulata flowers. Leaf fall had a bimodal seasonality and was negatively related to the number of dry days indicating it occurs when water is readily available or in excess as during floods. Observations show systematic leaf senescence in this deciduous species can be caused by water and nutrient demand from flowering. Litterfall mass and seasonality of novel S. campanulata forests is similar to that of native forests in Puerto Rico, yet flower fall appears to be higher than that of tropical forests worldwide. The environmental variables that affect litterfall seasonality and canopy phenology are similar to those in tropical forests in Puerto Rico and elsewhere. Litterfall seasonality and canopy phenology regulate understory microclimate, and influence the establishment and growth of juvenile trees and other organisms within S. campanulata forests. This study illustrates how forest ecosystem processes and properties restored by novel S. campanulata forests facilitate tree species establishment, growth, and turnover in deforested, abandoned, and degraded agricultural lands in Puerto Rico.

Leaf economics traits predict litter decomposition of tropical plants and differ among land use types

1. Decomposition is a key ecosystem process that determines nutrient and carbon cycling. Individual leaf and litter characteristics are good predictors of decomposition rates within biomes worldwide, but knowledge of which traits are the best predictors for tropical species remains scarce. Also, the effect of a species’ position on the leaf economics spectrum (LES) and regeneration light requirements on decomposition rate are, until now, unknown. In addition, land use change is the most immediate and widespread global change driver, with potentially significant consequences for decomposition. 2. Here we evaluate 14 leaf and litter traits, and litter decomposition rates of 23 plant species from three different land use types (mature forest, secondary forest and agricultural field) in the moist tropics of lowland Bolivia. 3. Leaf and litter traits were closely associated and showed, in line with the LES, a slow–fast continuum ranging from species with tough, well-protected leaves (high leaf density, leaf dry matter content, force to punch and litter C : N ratio) to species with cheap, productive leaves [high specific leaf area (SLA) and nutrient concentrations in leaves and litter]. 4. Fresh green leaf traits were better predictors of decomposition rate than litter traits, and leaf nitrogen concentration (LNC) was a better predictor of decomposition than leaf phosphorus concentration, despite the widely held belief that tropical forests are P-limited. 5. Multiple regression analysis showed that LNC, SLA and chlorophyll content per unit leaf area had positive effects on decomposition, explaining together 65–69% of the variation. Species position on the LES and regeneration light requirements were also positively related to decomposition. 6. Plant communities from agricultural fields had significantly higher LNC and SLA than communities from mature forest and secondary forest. Species from agricultural fields had higher average decomposition rates than species from other ecosystems and tended to be at the fast end of the LES. 7. Both individual traits of living leaves and species’ position on the LES persist in litter, so that leaves lead influential afterlifes, affecting decomposition, nutrient and carbon cycling

Carbon stocks in above‐ground biomass of monoculture plantations, mixed species plantations and environmental restoration plantings in north‐east Australia

Summary The emergence of carbon markets has provided a potential source of funding for reforestation projects. However, there is concern amongst ecologists that these markets will promote the establishment of monoculture plantations rather than more diverse restoration plantings, on the assumption that fast‐growing monocultures are likely to store more carbon than restoration plantings. We examined the validity of this assumption for three predominantly rainforest plantation types established in the moist tropical uplands of north‐east Australia: monoculture plantations of native rainforest conifers (n = 5, mean age 13 years); mixed species plantations of rainforest cabinet timber species, rainforest conifers and eucalypts (n = 5, mean age 13 years); and, environmental restoration plantings comprised mostly of a diverse range of rainforest trees (n = 10, mean age 14 years). We found that restoration plantings stored significantly more carbon in above‐ground biomass than monoculture plantations of native conifers (on average, 106 t vs 62 t carbon per ha); and tended to store more carbon than mixed species timber plantations which were intermediate in value (86 t carbon per ha). Carbon stocks were higher in restoration plantings than in monoculture and mixed species plantations for three reasons. First, and most importantly, restoration plantings were more densely stocked than monoculture and mixed species plantations. Second, there were more large diameter trees in restoration plantings than monoculture plantations. Third, the trees used in restoration plantings had a higher average wood density than the conifers used in monoculture plantations. While, on average, wood density was higher in mixed species plantations than restoration plantings, the much higher stocking rate of restoration plantings meant they stored more carbon than mixed species plantations. We conclude that restoration plantings in the moist tropics of north‐east Australia can accumulate relatively high amounts of carbon within two decades of establishment. Comparison with reference rainforest sites suggests that restoration plantings could maintain their high stocking rates (and therefore high biomass) as they develop in future decades. However, because restoration plantings are currently much more expensive to establish than monoculture plantations, restoration plantings are unlikely to be favoured by carbon markets. Novel reforestation techniques and designs are required if restoration plantings are to both provide habitat for rainforest biota and store carbon in biomass at a cost comparable to monoculture plantations.

Predicting moisture dynamics of fine understory fuels in a moist tropical rainforest system: results of a pilot study undertaken to identify proxy variables useful for rating fire danger

*The use of fire as a land management tool in the moist tropics often has the unintended consequence of degrading adjacent forest, particularly during severe droughts. Reliable models of fire danger are needed to help mitigate these impacts. *Here, we studied the moisture dynamics of fine understory fuels in the east-central Brazilian Amazon during the 2003 dry season. Drying stations established under varying amounts of canopy cover (leaf area index (LAI) = 0 - 5.3) were subjected to a range of water inputs (5-15 mm) and models were developed to forecast litter moisture content (LMC). Predictions were then compared with independent field data. *A multiple linear regression relating litter moisture content to forest structure (LAI), ambient vapor pressure deficit (VPD(M)) and an index of elapsed time since a precipitation event (d(-1)) was identified as the best-fit model (adjusted R(2) = 0.89). Relative to the independent observations, model predictions were relatively unbiased when the LMC was <or= 50%, but consistently underestimated the LMC when the observed values were higher. *The approach to predicting fire danger based on forest structure and meteorological variables is promising; however, additional information to the LAI, for example forest biomass, may be required to accurately capture the influence of forest structure on understory microclimate.

Tall clouds from small eruptions: the sensitivity of eruption height and fine ash content to tropospheric instability

A critical factor in successfully monitoring and forecasting volcanic ash dispersion for aviation safety is the height reached by eruption clouds, which is affected by environmental factors, such as wind shear and atmospheric instability. Following earlier work using the Active Tracer High Resolution Atmospheric Model for strong Plinian eruptions, this study considered a range of eruption strengths in different atmospheres. The results suggest that relatively weak volcanic eruptions in the moist tropics can trigger deep convection that transports volcanic material to 15–20 km. For the same volcanic strength there can be ~9 km difference between eruption heights in moist tropical and dry subpolar environments (a larger height difference than previously suggested), which appears consistent with observations. These results suggest that eruption intensity should not be estimated from eruption height alone for tropospheric eruptions and also that the average height of volcanic eruptions may increase if the tropical atmospheric belt widens in a changing climate. Ash aggregation is promoted by hydrometeors (particularly liquid water), so the smaller modelled eruptions in moist atmospheres, which have a relatively small ash content for their height and water content, result in a relatively small proportion of fine ash in the dispersing cloud when compared to a dry atmosphere. This in turn makes the ash clouds much more difficult to detect using remote sensing than those in dry atmospheres. Overall, a weak eruption in the tropics is more likely to produce a plume above cruising levels for civil aviation, harder to detect and track, but with a lower concentration of fine ash than a mid-latitude or polar equivalent. There is currently no defined ‘acceptable’ concentration of ash for aircraft, but as these results suggest low-grade encounters in the tropics from undetected clouds are likely, it would be desirable to explore that issue.

The temporal evolution of pedogenic Fe–smectite to Fe–kaolin via interstratified kaolin–smectite in a moist tropical soil chronosequence

Fe-rich kaolins in soils of a terrace chronosequence in the moist tropics of Costa Rica are derived from the transformation of smectite via interstratified kaolin–smectite (K–S). The smectite is ferruginous smectite (10.1 ± 3.7% Fe 2O 3) that forms early during pedogenesis from basaltic–andesitic parent material and is the dominant mineral in < 5 ka soils. Interstratified K–S with 60–90% kaolin layers is the dominant mineral in 10 ka soils, while 125 ka soils contain a physical mixture of Fe–kaolin (6.0 ± 1.9% Fe 2O 3) and K–S with > 80% kaolin layers. K–S is indicated by X-ray diffraction (XRD) data that reveal asymmetric 001 peaks between 17.5 and 17.8 Å (ethylene glycol-solvated) and irrational 00 l peaks that exhibit Méring-like behaviour, and by transmission electron microscopy–analytical electron microscopy (TEM–AEM) data that document single crystals of K–S with compositions intermediate to those of end-member crystals of smectite and kaolin. Lattice fringe images from high resolution TEM (HRTEM) show lateral transitions from smectite layers to kaolin layers via a cell-preserved, layer-by-layer transformation mechanism, and changes in the compositions of tetrahedral and octahedral sheets, notably decreases in octahedral Fe and Mg and tetrahedral Al, indicate that the reaction is accompanied by localized dissolution of smectite 2:1 layers that proceeds laterally. Fourier transform infrared (FTIR) and XRD data indicate that kaolin layers are a mixture of kaolinite and halloysite, and differential thermal analysis–thermogravimetry (DTA–TG) combined with data from XRD and TEM–AEM indicate the development of Al-hydroxy-interlayers in K–S with time. The soil-forming factors that appear to control formation of K–S are (1) basaltic–andesitic parent that produces high concentrations of base cations and Si in young (< 5 ka) soils, and (2) moist tropical climate (3085 mm MAP, 27.3 °C MAT) with a short dry season that fosters formation of smectite in < 5 ka soils. By 125 ka, soils are dominated by Fe–kaolin and K–S, helping to constrain reaction rate. The relatively high cation exchange capacity (CEC) of the Fe-rich kaolins (18 ± 12 cmol c/kg) relative to many tropical kaolinites is attributed to small amounts of octahedral Mg and tetrahedral Al inherited from precursor ferruginous smectite layers. Accordingly, the origin (from smectite) and relatively high CEC of the K–S and Fe–kaolin has potentially important implications related to clay mineral reactions and elemental cycling in moist tropical soils.