Blackwater Floodplain Research Articles

Macrolobium paulobocae (Leguminosae, Detarioideae), a new species from seasonally inundated black-water floodplain forests in the Brazilian Amazon.

Macrolobium paulobocae is presented as a new species of the legume subfamily Detarioideae. It is restricted to seasonally flooded igapó forests in the Central Amazon. We provide a description, illustration, photographs, and a distribution map of the new species, as well as a table of comparative morphology with similar, likely phylogenetically related species. The epithet is in honor of Paulo Apóstolo Costa Lima Assunção, or Paulo Boca, a great Amazonian botanist, victim of COVID-19 in January 2021.

Microbial metabolic activity in Amazon floodplain forest and agricultural soils.

Microorganisms play an essential role in ecosystem functions. An increasingly used method for conducting functional analyses of a soil microbial community is based on the physiological profile at the community level. This method allows the metabolic capacity of microorganisms to be assessed based on patterns of carbon consumption and derived indices. In the present study, the functional diversity of microbial communities was assessed in soils from seasonally flooded-forest (FOR) and -traditional farming systems (TFS) in Amazonian floodplains flooded with black, clear, and white water. The soils of the Amazon floodplains showed differences in the metabolic activity of their microbial communities, with a general trend in activity level of clear water floodplain > black water floodplain > white water floodplain. The redundancy analysis (RDA) indicated that soil moisture (flood pulse) was the most important environmental parameter in determining the metabolic activity of the soil microbial communities in the black, clear, and white floodplains. In addition, the variance partitioning analysis (VPA) indicated that the microbial metabolic activity of the soil was more influenced by water type (41.72%) than by seasonality (19.55%) and land use type (15.28%). The soil microbiota of the white water floodplain was different from that of the clear water and black water floodplains in terms of metabolic richness, as the white water floodplain was mainly influenced by the low substrate use during the non-flooded period. Taken together, the results show the importance of considering soils under the influence of flood pulses, water types, and land use as environmental factors when recognizing functional diversity and ecosystem functioning in Amazonian floodplains.

Fish on Fire: Shifts in Amazonian fish communities after floodplain forest fires

Abstract Severe droughts can lead to fires that cause massive tree mortality in even the wettest and most isolated Amazonian forests. After repeated fires, blackwater floodplain forests can remain in an open vegetation state of arrested succession that facilitates a transition towards a white‐sand ‘savanna‐like’ vegetation. These vegetation shifts, from closed‐canopy floodplain forests to open fire scars and eventually white‐sand savannas, may have profound implications for fish communities that depend on floodplain forests for food and recruitment. In turn, changes in fish communities may contribute to the arrested forest recovery after fires as fish are important seed dispersal agents for many floodplain tree species. To explore the impacts of floodplain forest fires on fish communities, we sampled fish in unburnt forests, fire scars, and white‐sand savannas in the middle Rio Negro basin (Brazil) during two consecutive flooding seasons. We compared the abundance, species richness, and the taxonomic and trophic composition of fish assemblages across the three habitat types. We found significant shifts in fish assemblages in fire scars compared to unburnt floodplain forests. Also, as fire scars increased in size, total fish biomass decreased strongly. Fish communities in unburnt floodplain forests seem to be characterized by a higher proportion of smaller and omnivorous fish species than fish communities in burned floodplain forests and white sand savannas. Fish assemblages in fire scars and white‐sand savanna were not significantly different. Synthesis and Applications. Amazonian fish communities change after floodplain forest fires. Unburnt forests have diverse fish communities, with a large proportion of unique and small omnivorous species. In contrast, carnivores and detritivores become more common in fire scars and white‐sand savannas formed after forest fires. Less omnivore fish after forest fires can reduce tree seed dispersal and forest regeneration, affecting ecosystem functioning and the services provided by Amazonian forests. Fish are also a primary source of food and income for people living in Amazonian floodplains. Preventing forest fires is therefore of fundamental importance for the conservation of Amazonian aquatic and terrestrial ecological communities as well as for the livelihood of people.

Seasonal flooding decreases fruit-feeding butterfly species dominance and increases spatial turnover in floodplain forests of central Amazonia.

The seasonal flood pulse in Amazonia can be considered a primary driver of community structure in floodplain environments. Although this natural periodic disturbance is part of the landscape dynamics, the seasonal inundation presents a considerable challenge to organisms that inhabit floodplain forests. The present study investigated the effect of seasonal flooding on fruit-feeding butterfly assemblages in different forest types and strata in central Amazonia. We sampled fruit-feeding butterflies in the canopy and the understory using baited traps in adjacent upland (unflooded forests-terra firme), white and blackwater floodplain forests (várzea and igapó, respectively) during the low- and high-water seasons. Butterfly abundance decreased in the high-water season, especially of dominant species in várzea, but the number of species was similar between seasons in the three forest types. Species composition differed between strata in all forest types. However, the flood pulse only affected butterfly assemblages in várzea forest. The β-diversity components also differed only in várzea. Species replacement (turnover) dominated the spatial β-diversity in igapó and terra firme in both seasons and várzea in the high-water season. Nonetheless, nestedness was relatively higher in várzea forests during the low-water season, mainly due to the effect of dominant species. These results emphasize the importance of seasonal flooding to structure butterfly assemblages in floodplain forests and reveal the idiosyncrasy of butterfly community responses to flooding in different forest types. Our results also suggest that any major and rapid changes to the hydrological regime could severely affect floodplain communities adapted to this natural seasonal hydrological cycle, threatening the existence of these unique environments.

Fires in Amazonian Blackwater Floodplain Forests: Causes, Human Dimension, and Implications for Conservation

The Amazon basin is being increasingly affected by anthropogenic fires, however, most studies focus on the impact of fires on terrestrial upland forests and do not consider the vast, annually inundated floodplains along the large rivers. Among these, the nutrient-poor, blackwater floodplain forests (igapós) have been shown to be particularly susceptible to fires. In this study we analyzed a 35-year time series (1982/1983–2016/2017) of Landsat Thematic Mapper from the Jaú National Park (Central Amazonia) and its surroundings. Our overall objective was to identify and delineate fire scars in theigapófloodplains and relate the resulting time series of annual burned area to the presence of human populations and interannual variability of regional hydroclimatic factors. We estimated hydroclimatic parameters for the study region using ground-based instrumental data (maximum monthly temperature–Tmax, precipitation–P, maximum cumulative water deficit–MCWD, baseflow index–BFI, minimum water level–WLmin90of the major rivers) and large-scale climate anomalies (Oceanic Niño Index–ONI), considering the potential dry season of the non-flooded period of theigapófloodplains from September to February. Using a wetland mask, we identified 518,135 ha ofigapófloodplains in the study region, out of which 17,524 ha (3.4%) burned within the study period, distributed across 254 fire scars. About 79% of the fires occurred close to human settlements (<10 km distance), suggesting that human activities are the main source of ignition. Over 92.4% of the burned area is associated with El Niño events. Non-linear regression models indicate highly significant relationships (p< 0.001) with hydroclimatic parameters, positive withTmax(R2adj. = 0.83) and theONI(R2adj. = 0.74) and negative withP(R2adj. = 0.88),MCWD(R2adj. = 0.90),WLmin90(R2adj. = 0.61) andBFI(R2adj. = 0.80). Hydroclimatic conditions were of outstanding magnitude in particular during the El Niño event in 2015/2016, which was responsible for 42.8% of the total burned floodplain area. We discuss these results under a historical background of El Niño occurrences and a political, demographic, and socioeconomic panorama of the study region considering the past 400 years, suggesting that disturbance ofigapósby fires is not a recent phenomenon. Concluding remarks focus on current demands to increase the conservation to prevent and mitigate the impacts of fire in this vulnerable ecosystem.

Flood-pulse disturbances as a threat for long-living Amazonian trees.

The long-lived tree species Eschweilera tenuifolia (O. Berg) Miers is characteristic of oligotrophic Amazonian black-water floodplain forests (igapó), seasonally inundated up to 10months per year, often forming monodominant stands. We investigated E.tenuifolia' growth and mortality patterns in undisturbed (Jaú National Park - JNP) and disturbed igapós (Uatumã Sustainable Development Reserve - USDR, downstream of the Balbina hydroelectric dam). We analysed age-diameter relationships, basal area increment (BAI) through 5-cm diameter classes, growth changes and growth ratios preceding death, BAI clustering, BAI ratio, and dated the individual year of death (14 C). Growth and mortality patterns were then related to climatic or anthropogenic disturbances. Results were similar for both populations for estimated maximum ages (JNP, 466yr; USDR, 498yr, except for one USDR tree with an estimated age of 820yr) and slightly different for mean diameter increment (JNP: 2.04mm; USDR: 2.28mm). Living trees from JNP showed altered growth post-1975 and sparse tree mortality occurred at various times, possibly induced by extreme hydroclimatic events. In contrast with the JNP, abrupt growth changes and massive mortality occurred in the USDR after the dam construction began (1983). Even more than 30yr after dam construction, flood-pulse alteration continues to affect both growth and mortality of E.tenuifolia. Besides its vulnerability to anthropogenic disturbances, this species is also susceptible to long-lasting dry and wet periods induced by climatic events, the combination of both processes may cause its local and regional extinction.

Impact of climatic and hydrological disturbances on blackwater floodplain forests in Central Amazonia

AbstractExtreme climatic events and anthropic disturbances affect the hydrological regime of Amazonian rivers and connected floodplain forests. This study aims to investigate the impacts of the Balbina hydroelectric power plant on the floodplain forests of the Uatumã River, in the Central Amazon. For this, tree age and diameter increment from the most abundant tree species of three different topographic levels were obtained and analyzed in the affected area downstream of the Balbina dam (Uatumã River) and compared to an undisturbed site (Abacate River, affluent) considering age structure and mean diameter increments between the topographic levels and the two systems. The occurrence of old trees is much lower at the disturbed site compared to the undisturbed system. Especially at the middle topography of the Uatumã site, we observed tree species with high mean diameter increment indicating a strong disturbance. We suggest that the disturbances may be associated with extreme hydro‐climatic events, such as extreme droughts that occurred during the El Niño years 1925/26 and 1982/83 and that these events may increase vulnerability of igapó floodplains to wildfires.Abstract in Portuguese is available with online material.

Trait divergence and habitat specialization in tropical floodplain forests trees.

Habitat heterogeneity of tropical forests is thought to lead to specialization in plants and contribute to the high diversity of tree species in Amazonia. One prediction of habitat specialization is that species specialized for resource-rich habitats will have traits associated with high resource acquisition and fast growth while species specialized for resource-poor habitats will have traits associated with high resource conservation and persistence but slow growth. We tested this idea for seven genera and for twelve families from nutrient-rich white-water floodplain forest (várzea) and nutrient-poor black-water (igapó) floodplain forest. We measured 11 traits that are important for the carbon and nutrient balance of the trees, and compared trait variation between habitat types (white- and black-water forests), and the effect of habitat and genus/family on trait divergence. Functional traits of congeneric species differed between habitat types, where white-water forest species invested in resource acquisition and productive tissues, whereas black-water forest species invested in resource conservation and persistent tissues. Habitat specialization is leading to the differentiation of floodplain tree species of white-water and black-water forests, thus contributing to a high diversity of plant species in floodplain forests.

Massive tree mortality from flood pulse disturbances in Amazonian floodplain forests: The collateral effects of hydropower production

Large dams built for hydroelectric power generation alter the hydrology of rivers, attenuating the flood pulse downstream of the dam and impacting riparian and floodplain ecosystems. The present work mapped black-water floodplain forests (igapó) downstream of the Balbina Reservoir, which was created between 1983 and 1987 by damming the Uatumã River in the Central Amazon basin. We apply remote sensing methods to detect tree mortality resulting from hydrological changes, based on analysis of 56 ALOS/PALSAR synthetic aperture radar images acquired at different flood levels between 2006 and 2011. Our application of object-based image analysis (OBIA) methods and the random forests supervised classification algorithm yielded an overall accuracy of 87.2%. A total of 9800 km2 of igapó forests were mapped along the entire river downstream of the dam, but forest mortality was only observed below the first 49 km downstream, after the Morena rapids, along an 80-km river stretch. In total, 12% of the floodplain forest died within this stretch. We also detected that 29% of the remaining living igapó forest may be presently undergoing mortality. Furthermore, this large loss does not include the entirety of lost igapó forests downstream of the dam; areas which are now above current maximum flooding heights are no longer floodable and do not show on our mapping but will likely transition over time to upland forest species composition and dynamics, also characteristic of igapó loss. Our results show that floodplain forests are extremely sensitive to long-term downstream hydrological changes and disturbances resulting from the disruption of the natural flood pulse. Brazilian hydropower regulations should require that Amazon dam operations ensure the simulation of the natural flood-pulse, despite losses in energy production, to preserve the integrity of floodplain forest ecosystems and to mitigate impacts for the riverine populations.

Response of black-water floodplain (igapó) forests to flood pulse regulation in a dammed Amazonian river

The monomodal flood pulse of major Amazonian rivers is a seasonal phenomenon that determines ecological and biogeochemical processes in adjacent floodplain forests. River damming transforms the pattern of downstream flood pulses and provides a natural disturbance to which the native biota might be poorly adapted. Severe modifications of the flood pulse were recorded in the Uatumã River after the installation of the Balbina dam, Central Amazonia. Flood pulse regulation increased mortality of flood-adapted species in the black-water floodplain (igapó) forest. No previous studies have investigated impacts of flood pulse regulation on the species composition and forest structure of igapó forests. Therefore, we examined species composition and forest structure of igapó forests along a regulated river in comparison to a pristine tributary, the Abacate River, evaluating soil texture characteristics and flood duration. In order to assess potential encroachment of species less sensitive to flood alteration, we also inventoried adjacent non-flooded upland forest in each river section. A quantitative inventory of all trees with diameter at breast height (DBH) ≥5 cm was carried out in low-igapó, high-igapó and adjacent upland forests, totaling one half-hectare in each river. In both rivers investigated, the clay fraction of the soil was significantly related to tree height. Flood duration was correlated to DBH and basal area, with the largest trees found in low-igapó forests which are exposed to long-term flooding. Species composition, richness and diversity significantly responded to flood duration. Species richness was highest in upland forests and lowest in low-igapó forest. In the pristine river, tree species composition exhibited a turnover of species along the flooding gradient. In the regulated river, flood intensification in the low-igapó forest increased dominance of a few flood-adapted species, which produced floristic dissimilarity to all forest types investigated. On the other hand, high-igapó forest showed higher floristic similarity with upland forest due to flood suppression that contributed to encroachment of species commonly described in secondary upland forests. Our results emphasize the urgent need for Brazilian environmental regulatory agencies to incorporate downstream impacts in the environmental assessments of dam projects in the Amazon Basin.

Species richness, composition, and spatial distribution of vascular epiphytes in Amazonian black-water floodplain forests

This study examines the occurrence of vascular epiphytic species in Central Amazonian black-water floodplain forests (igapo) and considers whether their horizontal and vertical distribution is influenced by the flood pulse, as is the case with tree species (phorophytes). Research was conducted in sixteen forest plots the Jau National Park. In these, epiphytes on all phorophytes with DBH ≥ 10 cm were identified. We measured flood height using the watermark left by the last high-water period, then estimated the height relative to the ground of every epiphytic individual. We recorded 653 individuals in 37 species, distributed on 109 phorophytes. Igapo floodplain forests have much lower richness and abundance of vascular epiphyte species than do other Amazonian forests. This may reflect the limitation of available sites for colonization (only 24.9% of studied trees were occupied by epiphytes). Holoepiphytes predominated, and the combined presence of a flood-pulse, linked to the nutrient-poor soil poor seems to limit the occurrence of nomadic vines. Horizontal distribution of epiphytes followed the distribution of phorophytes, which in turn followed the flood-level gradient. Also flooding interacted strongly with vertical zonation to determine species richness. As already well-reported for trees, and unlike reports of epiphytes in other floodplains, flooding strongly influenced richness and distribution of vascular epiphytes in the studied igapo forests.

Composition, diversity and structure of vascular epiphytes in two contrasting Central Amazonian floodplain ecosystems

ABSTRACT Research focusing on assemblages of vascular epiphytes in the Amazon are scarce. This is especially true for Amazonian floodplain forests, for which only two previous studies have been published. We compared composition, richness and structure of epiphyte assemblages in white-water and black-water floodplains (varzea and igapo) in Central Amazonia in order to close knowledge gaps concerning the distribution and richness of epiphytes. We established sixteen 25x25 m plots in each forest type, and counted and identified all species of vascular epiphytes occurring on trees with a diameter at breast height (DBH) ≥10 cm. We observed a clear distinction in epiphytic species composition (r2=0.83, p=0.001) and diversity (t=3.24, P=0.003) between the two environments, with 61.5 % of species being restricted to varzea, 22.9 % restricted to igapo and only 15.6 % common to both ecosystems. The floodplains were also structurally different for the most abundant species and those with the highest Epiphytic Importance Value (IVe). The diversity of trees did not influence the epiphyte diversity in either ecosystem. The forests were found to differ in the composition, diversity and structure of their epiphytic assemblages, which must be taken into account when designing conservation action plans for these ecosystems and for their vascular epiphytes.

Repeated fires trap Amazonian blackwater floodplains in an open vegetation state

Summary Climate change may increase the occurrence of droughts and fires in the Amazon. Most of our understanding on how fire affects tropical ecosystems is based on studies of non‐flooded forest–savanna ecotones. Nonetheless, tropical floodplain forests in the Amazon can burn severely during extreme droughts. The mechanisms slowing down forest regeneration in these ecosystems remain poorly understood and have never been assessed in the field. We studied the recovery of Amazonian blackwater floodplain forests after one and two fire events. We used Landsat images to map fire history and conducted field surveys to measure forest structure, tree species richness, tree seed bank and post‐fire invasion of herbaceous plants. Sites burnt once had on average 0% trees, 6% tree seed abundance, 23% tree seed species richness and 8% root mat thickness compared to unburnt forests. In contrast, herbaceous cover increased from 0 to 72%. Nonetheless, forest structure and diversity recovered slowly towards pre‐burn levels, except for tree seed banks that remained depleted even 15 years after fire. Sites burnt twice had on average 0% trees, 1% tree seed abundance, 3% tree seed species richness and 1% root mat thickness compared to unburnt forests. Herbaceous cover increased to 100%. Mean recovery of tree basal area was 50% slower and of root mat thickness 93% slower compared to recovery in sites burnt once. Tree seed banks did not recover at all, and herbaceous cover persisted close to 100% for more than 20 years after the second fire. Synthesis and applications. Our results indicate that after a second fire event, Amazonian blackwater floodplain forests lose their recovery capacity, and persist in a non‐forested state dominated by herbaceous vegetation. Such fragility implies that preventing human ignited fires during drought episodes is a particularly important conservation strategy for these ecosystems.

Effects of the hydroedaphic gradient on tree species composition and aboveground wood biomass of oligotrophic forest ecosystems in the central Amazon basin

Amazonian forests in black-water floodplains (igapo) and upon hydromorphic white-sand soils (campinarana) cover at least 500,000 km2 of the area of the Amazon basin, but are poorly investigated ecosystems. We compared variation in tree species richness and composition (≥ 10 cm diameter at breast height), as well as forest structure and aboveground wood biomass (AGB) along hydroedaphic gradients in an igapo and a campinarana in the central Brazilian Amazon, in an area totalling 6 ha. Inundation height (igapo) and groundwater level oscillations (campinarana) were monitored during a one year period. Soil grain sizes and chemical variables were analysed. Variation in tree species composition was assessed using non-metric multidimensional scaling, and soil parameters using principal component analysis. The influence of hydroedaphic gradients on tree species richness, composition and AGB was investigated using partial and multiple regression analyses. Significant differences in soil texture, soil chemical variables, and tree species richness and composition were detected between both forest types, while AGB amounted to similar values, ranging from 141 ± 62 Mg·ha−1 in the igapo to 164 ± 121 Mg·ha−1 in the campinarana. Although both forest types were floristically distinct, inundations in the igapo and groundwater table oscillations in the campinarana influenced patterns of species richness and forest structure in similar ways, indicating decreasing species richness, forest stature and AGB in plots subjected to higher inundations and/or groundwater levels. Given the comparatively low AGB in the ecosystems studied, we call attention to the need for more studies in oligotrophic ecosystems of the Amazon basin with emphasis on their contribution to global carbon cycles.

A classification of the major habitats of Amazonian black-water river floodplains and a comparison with their white-water counterparts

The Amazon River and its large tributaries are bordered by floodplains covering tens of thousands of square kilometers. Studies on the structure, function, and species composition have allowed a classification of the macrohabitats of Amazonian white-water floodplains, rich in suspended matter and nutrients and of neutral pH (varzea). Here we describe the use of a similar approach to classify the macrohabitats of the black-water floodplains, rich in humic substances, poor in nutrients and acidic (igapo) of the Negro River and its black-water tributaries. With 12 subclasses and 25 macrohabitats, the igapo is less complex than the varzea. Although white-water and black-water rivers are subjected to similar flood regimes, the low sediment load and shallower declivity of the Negro River lead to reduced sedimentation and erosion processes. Differences in nutrient levels between both ecosystems influence species composition, richness, and growth rates of higher plant communities. Species richness is lower in igapo than in varzea, and wood increment and litter production of igapo trees is about half that reported for varzea trees. In addition, igapo lacks highly productive herbaceous plant communities that are common in varzea. The classification of igapo macrohabitats provides a valuable tool for the elaboration of sustainable management strategies and conservation. While many varzea macrohabitats are suitable for small-scale agriculture, animal husbandry, forestry and commercial fisheries, the carrying capacity of igapo is limited and allows only for subsistence-level fishery and agriculture, the capture of ornamental fishes, and ecotourism. We argue that the biota of most igapo macrohabitats is highly sensitive to changes in hydrological cycles as caused by river damming and/or by climate change.

Current Distribution, Relative Abundance, and Landscape-Level Habitat Associations of the Cerulean Warbler (Setophaga cerulea) Along the Lower Roanoke River in North Carolina

Setophaga cerulea (Cerulean Warbler) has been inadequately monitored along the Roanoke River in North Carolina since a breeding population was discovered there in 1972. Our objectives were to estimate the Cerulean Warbler's current population size and distribution along the river, and evaluate landscape habitat characteristics in the Roanoke River Basin among areas used and unused by the same species. In May 2001 and 2011, we surveyed for singing male Cerulean Warblers, primarily by boat, along approximately 160 km of the Roanoke River from Weldon to Williamston in northeast North Carolina. We found Cerulean Warblers in three distinct groups along the Roanoke River during both survey years; however, we detected at least 32.4% fewer males in 2011 (n = 23) than in 2001 (n = 34). The landscape within 500 m of areas used by Cerulean Warblers had significanlty less crop cover, blackwater floodplain (i.e., swamp) forest, and variation in mean canopy height than unused landscapes we surveyed. These same differences existed at distances up to 1 km, but several additional dissimilarities became evident at this scale, including presence of more evergreen plantations and a greater fragmentation of the dominant forested land cover at used versus unused landscapes. We recommend continued monitoring of the Cerulean Warbler along the Roanoke River, increased habitat protection, and encourage an in-depth investigation into management strategies to sustain this population.

What drives the dynamics of a soil mite population under seasonal flooding? A null model analysis

Floods can inflict high mortality on terrestrial organisms, but may also promote adaptive evolution. In seasonal floodplains, several taxa show flood-related traits that may be important for their long-term persistence, but the available evidence is conflicting. Here, we used a simulation approach to investigate the interplay between seasonal floods and submersion resistance in driving the population dynamics of the parthenogenetic soil mite Rostrozetes ovulum in an Amazonian blackwater floodplain. First, we gathered data from two flood cycles to estimate field survival rate. Next, we used further data from a submersion survival laboratory experiment and a historical flood record to build a null model for R. ovulum's survival rate under seasonal flooding, and then tested it against field survival estimates. Floods caused marked density declines, but the two estimates of field survival rate were statistically equivalent, suggesting relatively constant survival across years. Submersion survival time varied tenfold among individuals, but its variability was within the range known for life history traits of other asexual invertebrates. Both field survival rates were consistent with the null model, supporting seasonal flooding as the main mortality factor. Surprisingly, though, average flood duration was actually larger than the average mite could survive, suggesting that population persistence relies on relatively rare, super-resistant phenotypes. Overall, the studied R. ovulum population appears to have a mainly density-independent dynamics across years, with its viability depending on mechanisms that buffer flood survival rate against temporal oscillations.

Leaf decomposition and fine fuels in floodplain forests of the Rio Negro in the Brazilian Amazon

Abstract:Despite being inundated for up to 9 mo of the year, black-water floodplain forests in the Brazilian Amazon are susceptible to fire. Post-fire tree mortality is higher and fire spreads further in the floodplain, compared with adjacent upland forest. To understand these differences between the two forest types, we compared how leaf decomposition and fine-fuel loads change with inundation and soil texture. Litterbags containing leaves of Clitoria fairchildiana were placed on upland forest floor and submerged at two depths in a backwater of the Rio Negro. We used 80 bags per treatment and retrieved subsets every ~16 d from which the contents were cleaned, dried, weighed and discarded. Over the 81-d experiment, upland leaves decomposed two to three times faster than submerged leaves. Fine-fuel biomass (litter + root mat) was measured at 28 upland forest sites and 29 floodplain forest sites of the middle Rio Negro. Floodplain forests held about twice the fine fuel (25.9 ± 10.6 Mg ha−1) of uplands (10.9 ± 2.3 Mg ha−1). Upland soils had more sand but a carpet of fine apogeotropic tree roots was more common and thicker in floodplains. We infer that slow decomposition of submerged leaves leads to high tree mortality from fire in black-water floodplains by (1) increasing fire intensity due to high fine-litter fuel load and (2) making tree roots more vulnerable to burning because they form a peat-like mat to absorb nutrients from the thick litter.

Floristic variation across 600 km of inundation forests (Igapó) along the Negro River, Central Amazonia

We inventoried 10 ha of late-successional and seasonally inundated black-water floodplain (igapo) forest along four river sections of the Negro River, Central Amazonia, Brazil. The aim of the study was to test if tree species composition and diversity changes along the river course, and whether these changes reflect the different geological formations of the Negro River. On a continental-wide scale, we assessed alpha-diversity patterns of black-water flooded forests across the Amazon and Orinoco basins. Phytosociological analyses include family and species importance, species similarity, and Fisher’s alpha-diversity, as well as Detrended Correspondence Analysis. A total of 6.126 individuals were recorded, belonging to 243 tree species. Only few tree species occurred in more than one river section, and floristic composition changed abruptly from one section to the other. Tree species richness ranged from 57 to 79 species ha−1, and alpha-diversity was highest (27.24) in the lower river section upon sediments of Pliocene–Pleistocene origin. We found a gradual decrease in species diversity with increasing age of the geological formations. The igapo forest is relatively species-poor, which we interpret to be the result of general low nutrient availability in alluvial substrates of the Negro River.

Sediment provenance and geomorphic development of soils in a blackwater ecosystem, South Carolina Coastal Plain

Mineral soil is a conundrum on southeastern U.S. blackwater floodplains because these freshwater ecosystems are known for slow-flowing streams bearing little suspended sediment. Investigations on soils along this riparian system revealed two distinct geomorphic surfaces, the modern swamp floodplain (classified as Inceptisols according to Soil Taxonomy) and slightly elevated landforms (intra-swamp terraces) with more developed soils (Alfisols and Ultisols). This study examined soil characteristics and sediment provenance on the Black River floodplain and its tributaries in the Atlantic Coastal Plain of South Carolina to determine the properties and pedogenic relationship between these two geomorphic surfaces and to propose a theory for the geomorphic evolution for soils on these landforms. Spatial characterization of the soils will benefit ecologists researching the biological aspects of blackwater ecosystems such as the concentration and flux of dissolved organic carbon (DOC). Results showed terrace soils had aquic moisture regimes while floodplain soils had peraquic moisture regimes. Floodplain surface horizons were organic carbon-rich (5% to 25% OC) but seldom with enough to qualify as organic soil material. Discontinuities occurred within the soils of both landforms indicating two distinct sedimentological environments. The fine-earth fraction of soil profiles on both landforms generally had upper strata composed mainly of silt and clay indicative of wash load sediments underlain with sandy strata indicative of bed load deposits. Fine clay distribution and fine clay:total clay ratios (FC/TC) showed argillic horizon development in the terrace subsoils but not in the floodplain soils. Micromorphology supported argillic horizons for the terraces, but also revealed modest clay accumulation in pores and channels of the floodplain soils. These floodplain clay features are interpreted to have formed during a period of subaerial weathering. The sandy strata were likely deposited during the beginning of the transgressions following the down cutting that occurred during the regressions and sea level lows. Silt fraction comparisons between the blackwater soils and soils on other landscapes in the region suggest the silts and clays likely were deposited in an estuarine environment late in the transgressions.