Tropical Sites Research Articles

Changes in tropical leafing behaviour with climate change over nine decades: A case study from the Singapore Botanic Gardens

Societal Impact StatementLong‐term phenology data is crucial to elucidate the effects of climate change on plants, but such efforts are lacking in the tropics. Historical records at the Singapore Botanic Gardens and ongoing phenological monitoring have allowed us to study leafing behaviour and its association with changing rainfall patterns over nine decades in equatorial Singapore. We found that the leafing behaviour of trees in our tropical site changed between 1927 and 2022. This study was made possible by assiduous past and present record‐keeping, and demonstrates the potential of botanical gardens as sentinels of environmental change, which is especially important in the tropics.Summary Understanding tropical phenology and the potential impacts of a changing climate requires long‐term monitoring but such efforts are rare in the tropics. We took advantage of the availability of historical phenological and weather records at the Singapore Botanic Gardens and ongoing phenological monitoring to ask whether leafing behaviour has changed over an exceptionally long period of about nine decades, and whether these changes are associated with changing rainfall patterns in supposedly aseasonal Singapore. Records of leaf flush intervals from 1927 to 1939 in the Singapore Botanic Gardens were compared against leaf flush intervals calculated from present‐day phenological monitoring conducted since 2016. Daily rainfall was modelled as a predictor of present‐day leaf flushing events using moving‐window analyses. Leaf flush intervals have shortened considerably for most of the species that were monitored in the two periods. Mean daily rainfall over a window of 1 to 4 months before was negatively associated with probability of leaf flushing for five species and positively associated with four out of a total of 23 species analysed. Long‐term rainfall records from 1929 to 2022 show that February has become drier and May has become wetter since the 1960s. The results consistently support that more distinctive dry and wet periods today compared to nine decades ago have resulted in more frequent leaf flushing, which may have implications for plant‐herbivore interactions, nutrient cycling and consequently plant health and ecosystem resilience. We also demonstrate the potential and importance of supporting long‐term research in botanical gardens in the tropics.

Applying and Improving Pyranometric Methods to Estimate Sunshine Duration for Tropical Site

Applying and Improving Pyranometric Methods to Estimate Sunshine Duration for Tropical Site

Hot Tropical Temperatures During the Paleocene‐Eocene Thermal Maximum Revealed by Paired In Situ δ13C and Mg/Ca Measurements on Individual Planktic Foraminifer Shells

AbstractThe Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of 13C‐depleted carbon into the ocean‐atmosphere system, making it an informative analogue for future climate change. However, uncertainty still exists regarding tropical sea‐surface temperatures (SSTs) in open ocean settings during the PETM. Here, we present the first paired δ13C:Mg/Ca record derived in situ from relatively well‐preserved subdomains inside individual planktic foraminifer shells taken from a PETM record recovered in the central Pacific Ocean at Ocean Drilling Program Site 865. The δ13C signature of each individual shell was used to confirm calcification during the PETM, thereby reducing the unwanted effects of sediment mixing that secondarily smooth paleoclimate signals constructed with fossil planktic foraminifer shells. This method of “isotopic screening” reveals that shells calcified during the PETM have elevated Mg/Ca ratios reflecting exceptionally warm tropical SSTs (∼33–34°C). The increase in Mg/Ca ratios suggests ∼6°C of warming, which is more congruent with SST estimates derived from organic biomarkers in PETM records at other tropical sites. These extremely warm SSTs exceed the maximum temperature tolerances of modern planktic foraminifers. Important corollaries to the findings of this study are (a) the global signature of PETM warmth was uniformly distributed across different latitudes, (b) our Mg/Ca‐based SST record may not capture peak PETM warming at tropical Site 865 due to the thermally‐induced ecological exclusion of planktic foraminifers, and (c) the record of such transitory ecological exclusion has been obfuscated by post‐depositional sediment mixing at Site 865.

Eucalyptus pellita harvest residue management in sandy tropical soils – nutrient content, decomposition and potential emissions from burning

Plantation forestry on sandy, nutrient poor soils has been developed over the last few decades in seasonally dry tropical monsoon environments. Despite the known benefits of harvest residue conservation between rotations in temperate climes, there are very few studies around the world on Eucalyptus harvest residue management in tropical areas. In Northern Australia on Melville Island 30,000-hectares of Acacia mangium is planned to be converted to Eucalyptus pellita. E. pellita harvest residues comprise high loads of stringy bark, leaves and twigs, posing a fire risk throughout the annual 6-to-8-month dry season in Australia's monsoonal tropics. In this environment, quantifying and demonstrating the anticipated benefits of residue retention is important for determining appropriate responses to fire risk.A 4-hectare experimental trial of 12-year-old E. pellita was cut with trees moved intact to the coupe edge and processed for woodchips. The tree residues from the roadside chipper were pushed back over the site. A total of 35 Mg ha−1 of harvest residue was left on the site, mainly comprised of bark (24.5 Mg ha−1 or 60 % of the total residue mass). Although the leaf and branch residues contained higher concentrations of N, Ca, Cu, K, Mg, Mn, P, and Zn than bark, stem and woodchips, bark N (51.5 kg ha−1) was twice foliage N (26.9 kg ha−1), and contributed to more than 50 % of residue N.Harvest residue bark, leaves and twigs were sampled for a litterbag decomposition study of mass loss on the harvested site. After 10 months about 80 % of bark original weight remained on site (20 % mass loss), compared to about 55 % of the leaves/twigs remaining (45 % mass loss).The potential non-CO2 emissions from the burning of E. pellita harvest residues were estimated at 12.3 Mg CO2-eq ha−1 or 3.35 Mg C ha−1, with the majority of emission arising from the smoldering of bark residues. This study discusses the implications of either retaining or burning harvest residues – for plantation long-term sustainability on sandy tropical sites, and for greenhouse gas emissions.

Comparison of satellite and ground-based measurements of erythemal dose from two tropical sites: Belo Horizonte (20° S, 44° W, 858 m) and Saint-Denis La Reunion (21° S, 55° E, 85 m)

Comparison of satellite and ground-based measurements of erythemal dose from two tropical sites: Belo Horizonte (20° S, 44° W, 858 m) and Saint-Denis La Reunion (21° S, 55° E, 85 m)

Improving plot-level above ground biomass estimation in tropical Indian forests

Accurate plot-level estimates of above ground biomass (AGB) are crucial for reducing uncertainty in spatial AGB quantification, a key requirement for Reducing Emissions from Deforestation and Forest Degradation framework (REDD+) and achieving Intended Nationally Determined Contributions (INDC) goals. This study explores the impact of plot size, selection and use of height-diameter (H–D) model, and allometric models on plot-level AGB estimation in tropical Indian forests. We analyze a dataset of 8179 tree measurements from 51 one-hectare plots across four tropical sites to investigate these factors and improve estimation accuracy. Our findings highlight the importance of locally derived H–D models for superior accuracy. We propose two H–D models for improved plot-level AGB estimation accuracy in tropical Indian forests: a general model with a residual standard error (RSE) of 4.18 m and a forest type-specific model with a RSE of 3.65 m. These models outperform global models, which significantly underestimate tree height by up to 51.3% and overestimate by 43.4% across our sites. The developed H–D models were evaluated using spatial cross-validation to ensure broader applicability across Indian tropical forests. Further, we evaluated the impact of the choice of the allometric model on plot-level AGB estimation using the prevalent models (local destructive models by the Forest Survey of India and global pantropical models). Our results indicate minimal variability between the local destructive models and the pantropical model when height measurements are derived from this study's developed H–D models. We have also investigated the effect of plot size on plot-level AGB estimation using synthetic plots generated from 1-ha plot data. Notably, the relative error diminishes from approximately 22% to about 5% as plot size increases from 0.04 ha (20x20m) to 0.49 ha (70x70m) across all sites, and we recommend field plots of size ≥0.5 ha for AGB mapping over Tropical Indian Forests. By making this data publicly available, this study paves the way for more accurate spatial AGB mapping in tropical Indian forests, ultimately contributing to better forest management.

Soil water percolation and nutrient fluxes as a function of topographical, seasonal and soil texture variation in Central Amazonia, Brazil

AbstractUnderstanding soil water dynamics and transport of nutrients is challenging in tropical rainforests due to the uniqueness of several properties related to soils, vegetation and seasonality that make relating patterns found in temperate environments to tropical sites difficult. We address the need for better edaphic characterization in tropical environments by investigating soil water percolation rates and chemistry across topographic, soil texture and seasonal gradients in a mature tropical rainforest in Central Amazonia, Brazil. We utilized a passive wick flux meter (e.g., drainage lysimeter) to directly measure real‐time percolation fluxes at 60‐cm depth, and to sample a suite of chemical species across plateau, slope and valley topographic positions. We found percolation flux volume and chemical exports generally increase with decreasing elevation and clay content, which was lowest in the valley. Daily percolation flux was observed to be 2.39 ± 0.44 in plateau, 3.01 ± 0.50 in slope and 6.16 ± 0.83 mm in valley. Most solutes were present in small amounts of <1 mg L−1, such as PO₄3−, Fe2+/Fe3+ and Mn2+; however, NO3− concentrations were >20 mg L−1, even exceeding 100 mg L−1 in the valley. Based on additional isotopic analysis, we speculate high NO3− concentrations are partially an artefact of root decomposition following installation of the flux meters. The empirical relationships we show among percolation volume and nutrient exports under varying topographies and soil textures can improve Earth System Model performance by better constraining ecohydrological relationships to nutrient fluxes, which can in‐turn better illuminate the important factors that govern their behaviour.

Investigation of rainfall disaggregation with flexible timescales based on point process models

Analyzing high-temporal-resolution rainfall data with disaggregation tools is vital for understanding precipitation variability, especially in climate change studies with limited access to such data. To facilitate such study for tropical region, we proposed a Point-Process Modelling and Rainfall Disaggregation (PPMRD) framework for rainfall disaggregation with flexible time scales. The framework was tested on two tropical sites in Singapore, using six point-process models and four disaggregation schemes (24 h to 5 min, 3 h to 5 min, 24 h to 1 h, and 6 h to 1 h). Most models excelled fundamental statistics and extreme value analysis. The Bartlett-Lewis Rectangular Pulse Seven parameter (BLRP7) model was found to be the best performer for simulating and disaggregating tropical rainfall. The study findings emphasized the utility of disaggregation models in generating fine-resolution time series for analyzing rainfall extremes. However, it is crucial to underscore the importance of employing ensembles to comprehensively address uncertainties arising from the stochastic nature of rainfall. The study also emphasized the importance of model selection in the proposed framework, as the disaggregation method depends on the generator’s performance. The PPMRD surpasses traditional methods by handling subdaily upper-level series disaggregation to minute-level rainfall. This advancement offers greater capabilities and flexibility, promising benefits for generating high-temporal-resolution rainfall data in hydrological modeling and climate change impact studies.

The Variation in Atmospheric Turbidity over a Tropical Site in Nigeria and Its Relation to Climate Drivers

Atmospheric turbidity exhibits substantial spatial–temporal variability due to factors such as aerosol emissions, seasonal changes, meteorology, and air mass transport. Investigating atmospheric turbidity is crucial for climatology, meteorology, and atmospheric pollution. This study investigates the variation in atmospheric turbidity over a tropical location in Nigeria, utilizing the Ångström exponent (α), the turbidity coefficient (β), the Linke turbidity factor (TL), the Ångström turbidity coefficient (βEST), the Unsworth–Monteith turbidity coefficient (KAUM), and the Schüepp turbidity coefficient (SCH). These parameters were estimated from a six-month uninterrupted aerosol optical depth dataset (January–June 2016) and a one-year dataset (January–December 2016) of solar radiation and meteorological data. An inverse correlation (R = −0.77) was obtained between α and β, which indicates different turbidity regimes based on particle size. TL and βEST exhibit pronounced seasonality, with higher turbidity during the dry season (TL = 9.62 and βEST = 0.60) compared to the rainy season (TL = 0.48 and βEST = 0.20) from May to October. Backward trajectories and wind patterns reveal that high-turbidity months align with north-easterly air flows from the Sahara Desert, transporting dust aerosols, while low-turbidity months coincide with humid maritime air masses originating from the Gulf of Guinea. Meteorological drivers like relative humidity and water vapor pressure are linked to turbidity levels, with an inverse exponential relationship observed between normalized turbidity coefficients and normalized water vapor pressure. This analysis provides insights into how air mass origin, wind patterns, and local climate factors impact atmospheric haze, particle characteristics, and solar attenuation variability in a tropical location across seasons. The findings can contribute to environmental studies and assist in modelling interactions between climate, weather, and atmospheric optical properties in the region.

Sharp declines in observation and capture rates of Amazon birds in absence of human disturbance

Across the globe, unprecedented declines have been reported across a range of taxa. Among the most well documented are declines in bird populations, with most declines attributable to human activities such as deforestation and other alterations to habitats. There is increasing evidence that bird populations also have declined at sites within large expanses of relatively undisturbed lowland tropical forest, such as in Amazonia. Causes of such declines likely are varied and may be related to direct or indirect effects of climate change. Here, we use mist-net data and direct observations to examine changes in bird numbers over a 22-yr period on two 100-ha study plots in an Amazonian lowland forest of eastern Ecuador. As previously reported, capture rates experienced large declines from 2009 on; observation rates also showed large declines through 2013 but subsequently have shown no consistent trend up or down. Overall capture rates and observation rates are now approximately half the rates during the first decade. Most foraging guilds also declined over time, with declines particularly pronounced among insectivorous groups. With few exceptions, patterns of change were similar between plots suggesting a more general response rather than responses to local factors on each plot. Capture rates and observation rates of ∼90% of the most common species in both understory and canopy were lower during the latter part of the study, with only a few species (∼10%) showing increases. Similarly, large declines were seen in both canopy and understory mixed-species flocks. Large declines in bird numbers have been noted in other tropical sites that have conducted long-term population studies, including Panama and Brazil, suggesting a response to large-scale factors, such as climate change. While not yet documented, the consequences of losing approximately 50% of population abundance are likely substantial and could result in local extinction of rare species, altered species interactions and social organization, as well as declining ecological function and biotic integrity of these tropical forest ecosystems.

Clay larvae do not accurately measure biogeographic patterns in predation

AbstractAimSpatial variation in predation can shape geographic patterns in ecology and evolution, but testing how predation varies across ecosystems is challenging as differing species compositions and defensive adaptations can mask underlying patterns. Recently, biogeography has borrowed a tool from ecology: clay prey models. But clay models have not been adequately tested for geographic comparisons, and a well‐known problem –that clay prey only appeal to a subset of potential predators– could bias detected geographic patterns whenever the relative importance of predator guilds varies among sites. Here, we test whether clay larvae accurately capture geographic differences in predation on real larvae.Location90° of latitude and >2000 m elevation across the Americas.TaxonVertebrate and invertebrate predation on ‘superworms’ (Zophobas larvae).MethodsAcross six sites that vary dramatically in latitude, elevation, and biome, we quantified predation on live, dead, and clay larvae. We physically excluded vertebrate predators from some larvae to distinguish total predation and invertebrate‐only predation.ResultsPredation on live superworms almost doubled from our high‐elevation high‐latitude site to our low‐elevation tropical site. Geographic patterns were consistent among live and dead larvae, but clay larvae missed extremely high predation at some sites and therefore mis‐measured true geographic patterns. Clay larvae did a particularly bad job at capturing geographic patterns in predation by invertebrates, although sample sizes for invertebrate predation were small.Main ConclusionsClay larvae are inappropriate for comparing predation rates across sites. They should be abandoned for biogeographic studies and reserved for comparisons within, rather than across, predator communities.

Quantitative biostratigraphy and paleoecology of Neogene tropical dinoflagellate cysts

Understanding the biostratigraphy of tropical areas during the Neogene period can be improved by studying dinoflagellate cyst events. These events have traditionally been identified through qualitative analysis. Nevertheless, to obtain consistent data on this subject, we used quantitative and probabilistic analysis of palynological data from the Southern Gulf of Mexico to identify dinoflagellate cyst events in the Neogene era. We analyzed a database of six wells, including 1,005 core and cuttings samples, a change from past qualitative analysis methods. With this quantitative approach with the RASC program, we obtained the Ranking Optimum Sequence (ROS) and compared it with ten published databases from Neogene tropical sites. The ROS identified eleven dinoflagellate species as age markers, we propose a most likely sequence of dinoflagellate cysts bioevents and define three assemblages: “Cantharellus” (Oligocene to early Miocene), “Hyalina” (Miocene), and “Patulum” (middle Miocene to Pleistocene), that can help recognize most Neogene stratigraphic stages in tropical areas. The comparison with the previous databases shows that the Gulf of Mexico and the tropical sites share 60% of the species, which enables age calibration of the optimal sequence and allows us to infer a possible provincial affinity. Additionally, we found a predominance of autotrophic dinocysts during the Neogene in tropical oceans. This trend mirrors the current abundance of gonyaulacoid cysts in tropical and subtropical areas, which has persisted since the Jurassic era.

Site-specific transfer parameters for the tritium transport model at a tropical site, Tarapur, west coast of India

The study of the fate of atmospheric release and transport of Tritium requires an understanding of the exchange/transfer of Tritium (HTO) from within the compartments; air, soil, and plant systems. The Tritium transfer parameters within these compartments are used in specific activity models to compute TFWT and OBT in terrestrial plants and are highly dependent on the site-specific conditions. Hence a study was carried out to generate site-specific Tritium transport model parameters in the tropical environment of Tarapur which receives the atmospheric releases of Tritium from nuclear facilities operating at this site, on the West coast of India. The paper presents the details of the study carried out in Tarapur, India. The transfer parameters generated through this study are soil water to air moisture HTO transfer ratio: CRs, plant water to air moisture HTO transfer ratio: Rpa, and plant water to soil water HTO transfer ratio: Rpw. The geometric mean and geometric standard deviation (GSD) of the transfer ratios, CRs, Rpa and Rpw determined were found to be 0.50 (GSD: 3.52), 0.53 (GSD: 2.51) and 1.12 (GSD: 2.58), respectively. In addition, the Tritium scavenging ratio, surface loading, and wet deposition velocity were also derived and were found to be 1.59 ± 1.32, 1.21 × 10−2 Bq m−2 s−1, and 7.91 × 10−2 m s−1, respectively. The study has generated a set of site-specific model input parameters for a tropical coastal site of Tarapur, India, and details are discussed in the manuscript.

Palaeoenvironments and hominin evolutionary dynamics in southeast Asia

Secure environmental contexts are crucial for hominin interpretation and comparison. The discovery of a Denisovan individual and associated fauna at Tam Ngu Hao 2 (Cobra) Cave, Laos, dating back to 164–131 ka, allows for environmental comparisons between this (sub)tropical site and the Palearctic Denisovan sites of Denisova Cave (Russia) and Baishiya Karst Cave (China). Denisovans from northern latitudes foraged in a mix of forested and open landscapes, including tundra and steppe. Using stable isotope values from the Cobra Cave assemblage, we demonstrate that, despite the presence of nearby canopy forests, the Denisovan individual from Cobra Cave primarily consumed plants and/or animals from open forests and savannah. Using faunal evidence and proxy indicators of climates, results herein highlight a local expansion of rainforest at ~ 130 ka, raising questions about how Denisovans responded to this local climate change. Comparing the diet and habitat of the archaic hominin from Cobra Cave with those of early Homo sapiens from Tam Pà Ling Cave (46–43 ka), Laos, it appears that only our species was able to exploit rainforest resources.

Wave Spectral Analysis for designing Wave Energy Converters

When designing a Wave Energy Converter (WEC) it is inappropriate to consider overall bulk parameters, like for instance, the typical scatter diagrams Hs-Tp. The reason is that in most of the world wave conditions are complex and involve several long-term wave systems, each with different characteristics (Fig. 1a). This is true even for apparently simple basins like the Mediterranean or the North Sea, in the open ocean this is certainly the case with swells arriving from remote places. Naturally, each Wave System (WS) has different characteristics, like their seasonal variability, typical wave height, but most importantly, the wave period and the spectral width are fundamental design parameters. As WECs must resonate on the exciting force to effectively convert energy, that resonance depends on the wave period. On the other hand, the spectral width indicates how disperse is energy in frequency and direction, the broader the spectrum (more entropy) the more difficult to tap its energy. Therefore, a smart WEC’s design cannot aim to the full spectrum, but to the particular WS with the more favorable characteristics (high energy, narrow spectral band, low frequency). For the present application we focus on a site in the Galapagos Islands. Apart from the aforementioned spectral conditions, this tropical site offers a couple of additional advantages, namely a small seasonal variability, and the absence of high extremes (to be withstood by the device even if idle), with still an interesting average resource (~20 kW/m). At this site we find three main WSs (see Fig. 1, a and b). WS1 belongs to the southerly swells, from the Antarctic extra-tropical storms. WS2 corresponds to swells from the northern hemisphere. WS3 is due to the southern trade-winds, active locally in the area (wind-sea). Clearly, the target for energy conversion is WS1, but for a non-directional device (e.g., point absorber), WS2 is also interesting due to the similar peak frequency. In addition, WS1 and WS2 are seasonally complementary, as they are mainly active in the austral and boreal winters respectively. The identification and separation of these different WSs allows us to determine their characteristics and to obtain specific design parameters. The modulation strategy for the WEC design is the subject of a parallel paper.
 
 Fig. 1. a) (left) Spectral wave conditions in the Galapagos Islands. WS1: Antarctic swells, WS2: northern swells, WS3: southern trade-winds, WS4: Central America wind jets. The colorbar indicates number of occurrences within 1979-2015. b) (right) Average one dimensional spectra for the WSs in panel a. Data obtained from GLOSWAC: https://modemat.epn.edu.ec/nereo/.

Latitudinal and elevational variation in the reproductive biology of house wrens, Troglodytes aedon.

While cross-species comparisons of birds suggest that as latitude decreases or elevation increases, clutch size decreases and the duration of developmental stages and parental attentiveness increases, studies comparing populations of the same species are rare. We studied populations of house wrens, Troglodytes aedon, at high and low elevations in California and Costa Rica, collecting data on clutch size, the duration of incubation and nestling periods, parental attentiveness, nestling growth rate, and nesting success. Our data support results from cross-species comparisons, but also revealed unanticipated results from low elevation temperate zone house wrens in the southwest. This population had prolonged incubation and nestling periods similar to those found in the tropics. We also found that temperate zone females, especially those at our higher elevation site, spent more of their day incubating than did tropical females. Nest temperature at our high elevation temperate zone site was higher than that at all other tropical sites. Age at fledging did not differ between sites. Total feeding rates per chick and male feedings per chick did not vary between sites. Nest success rates showed the predicted effect of latitude, but not the predicted effects of elevation. Our results extend low elevation house wren research into the southwestern US and contribute the first intraspecific elevational comparison in the Neotropics. Data from our low elevation southwestern site present a unique suite of life history traits that align more with tropical house wrens, although with a larger clutch size, and point to food limitation and/or high predation pressure as being possible drivers of some of these differences. These results highlight the need for additional studies of house wrens and other broadly distributed species at a more diverse array of sites to better understand which forces drive the evolution of different life history strategies across major biogeographical gradients.

Estimation of Diffuse Solar Radiation Models for a Tropical Site in Nigeria

Estimation of Diffuse Solar Radiation Models for a Tropical Site in Nigeria

Recent growth increase in endemic Juglans boliviana from the tropical Andes

The spatial coverage of tree-ring chronologies in tropical South America is low compared to the extratropics, particularly in remote regions. Tree-ring dating from such tropical sites is limited by the generally weak temperature seasonality, complex coloration, and indistinct anatomical morphology in some tree species. As a result, there is a need to complement traditional methods of dendrochronology with innovative and independent approaches. Here, we supplement traditional tree-ring methods via the use of radiocarbon analyses to detect partial missing rings and/or false rings, and wood anatomical techniques to precisely delineate tree-ring boundaries. In so doing we present and confirm the annual periodicity of the first tree-ring width (TRW) chronology spanning from 1814 to 2017 for Juglans boliviana (‘nogal’), a tree species growing in a mid-elevation tropical moist forest in northern Bolivia. We collected 25 core samples and 4 cross-sections from living and recently harvested canopy-dominant trees, respectively. The sampled trees were growing in the Madidi National Park and had a mean age of 115 years old, with certain trees growing for over 200 years. Comparison of (residual and standard) TRW chronologies to monthly climate variables shows significant negative relationships to prior year May-August maximum temperatures (r = −0.54, p < 0.05) and positive relationships to dry season May-October precipitation (r = 0.60, p < 0.05) before the current year growing season. Additionally, the radial growth of Juglans boliviana shows a significant positive trend since 1979. Our findings describe a new and promising tree species for dendrochronology due to its longevity and highlight interdisciplinary techniques that can be used to expand the current tree-ring network in Bolivia and the greater South American tropics.

Ground-based measurements of cloud and aerosol optical depths in the UV-B at an urban tropical site

Cloud and aerosol optical depths (CODλ and AODλ respectively) at wavelength λ in the terrestrial atmosphere have been obtained from space and ground-based remote experiments. Generally, they are for the ultraviolet A (UV-A), visible, and infrared ranges of the electromagnetic spectrum. Despite the significant effects of the ultraviolet B (UV-B) on the biosphere, fewer optical depth studies have aimed at this range. The weaker irradiance of the solar UV-B in relation to the three other ranges, hampering UV-B direct Sun measurements through clouds and aerosol plumes, would be a reason for that. Nonetheless, for 16 months, CODλ and AODλ from ground-based measurements in the UV-B were obtained for an urban tropical site, where biomass burning aerosols are potential contributors in addition to anthropogenic pollutants and cumulus and stratus cloud genera as predominant cloud cover. CODλ and AODλ are drawn from the reduction of the optical depths of both ozone and molecular scattering (τO3λ and τMSλ respectively) from the atmospheric optical depth (τAtλ) obtained from Beer's Law. The daily averages of CODUV-B ranged from 0.205 to 1.101 ± 0.916 (1 standard deviation) for low and middle altitude clouds mainly, being the maximum CODUV-B ≈ 3.2. The daily averages of AODUV-B ranged from 0.09 ± 0.02 to 1.75 ± 0.07, being augmented by the presence of clouds.

Potential of Bioassays to Assess Consequences of Cultivation of Acacia mangium Trees on Nitrogen Bioavailability to Eucalyptus Trees: Two Case-Studies in Contrasting Tropical Soils.

We hypothesized that the nitrogen-fixing tree Acacia mangium could improve the growth and nitrogen nutrition of non-fixing tree species such as Eucalyptus. We measured the N-mineralization and respiration rates of soils sampled from plots covered with Acacia, Eucalyptus or native vegetation at two tropical sites (Itatinga in Brazil and Kissoko in the Congo) in the laboratory. We used a bioassay to assess N bioavailability to eucalypt seedlings grown with and without chemical fertilization for at least 6 months. At each site, Eucalyptus seedling growth and N bioavailability followed the same trends as the N-mineralization rates in soil samples. However, despite lower soil N-mineralization rates under Acacia in the Congo than in Brazil, Eucalyptus seedling growth and N bioavailability were much greater in the Congo, indicating that bioassays in pots are more accurate than N-mineralization rates when predicting the growth of eucalypt seedlings. Hence, in the Congo, planting Acacia mangium could be an attractive option to maintain the growth and N bioavailability of the non-fixing species Eucalyptus while decreasing chemical fertilization. Plant bioassays could help determine if the introduction of N2-fixing trees will improve the growth and mineral nutrition of non-fixing tree species in tropical planted forests.