Fire-mediated dieback and compositional cascade in an Amazonian forest

The only fully coupled land-atmosphere global climate model predicts a widespread dieback of Amazonian forest cover through reduced precipitation. Although these predictions are controversial, the structural and compositional resilience of Amazonian forests may also have been overestimated, as current vegetation models fail to consider the potential role of fire in the degradation of forest ecosystems. We examine forest structure and composition in the Arapiuns River basin in the central Brazilian Amazon, evaluating post-fire forest recovery and the consequences of recurrent fires for the patterns of dominance of tree species. We surveyed tree plots in unburned and once-burned forests examined 1, 3 and 9 years after an unprecedented fire event, in twice-burned forests examined 3 and 9 years after fire and in thrice-burned forests examined 5 years after the most recent fire event. The number of trees recorded in unburned primary forest control plots was stable over time. However, in both once- and twice-burned forest plots, there was a marked recruitment into the 10-20cm diameter at breast height tree size classes between 3 and 9 years post-fire. Considering tree assemblage composition 9 years after the first fire contact, we observed (i) a clear pattern of community turnover among small trees and the most abundant shrubs and saplings, and (ii) that species that were common in any of the four burn treatments (unburned, once-, twice- and thrice-burned) were often rare or entirely absent in other burn treatments. We conclude that episodic wildfires can lead to drastic changes in forest structure and composition, with cascading shifts in forest composition following each additional fire event. Finally, we use these results to evaluate the validity of the savannization paradigm.     

Litterfall and nitrogen turnover in an Amazonian blackwater inundation forest

In 1976/77 energy flow and nutrient cycling in an Amazonian blackwater inundation forest were studied. The major part of the litter biomass turnover occurred during the emersion phase. 95% decomposition rate for nitrogen was measured with 15 mm mesh litter bags and was 4.7 years. Over 30 per cent of the annual leaf-fall was decomposed by soil-dwelling arthropods.     

Abundance and cover of ground herbs in an Amazonian rain forest

In a 1-ha plot in an unflooded moist tropical forest in Reserva de Producción Faunística Cuy abeno in Amazonian Ecuador, 96 species of vascular herbaceous plants were found rooted in the ground; they were all perennials; 25 species were pteridophytes, representing 11 families, 71 species (14 families) were angiosperms. Araceae, Marantaceae, and Poaceae were the most important angiosperm families. The total abundance of the ground herbs was 10 960 individuals, the total cover was 250 m², or 2.5% of the 1-ha plot. Species of ground herbs exhibit two major life-form strategies: the obligate terrestrial species (59%) are restricted to the ground; the facultative terrestrial species (41 %) have climbing and epiphytic individuals as well. Difference in life-form strategy as well as difference in edaphic specialization along a topographic gradient are two factors that may enhance the number of coexisting species within the 1-ha sample plot.     

Breeding-habitat use by five Amazonian frogs at forest edge

Understanding patterns of speciesl habitat use and their response to dramatic changes in habitat constitutes a basis for sound conservation practice. This study examined use of breeding habitat by two rainforest-breeding frog species near primary forest edge. Nine artificial breeding pools were established along each of three transects perpendicular to continuous primary forest near Manaus, Brazil. Five frog species bred in the pools over the six-month monitoring period. For the two most abundant species, patterns of habitat use were independent of proximity to forest edge. Also,Epipedobates femoralis used pools outside the forest, in secondary growth, at the same frequency as pools inside the forest.Osteocephalus taurinus did not breed outside primary forest. These results have clear implications for conservation biology: (i) buffer zones around isolated reserves, to counter possible negative edge effects on habitat, are unnecessary for these frog species, and (ii) secondary growth habitat, which is not natural breeding habitat forE. femoralis, could serve as a source of colonists for emigration into new areas and does not represent a barrier to dispersal for this species. These results argue against generalization of edge effects for different taxa.     

Predation by vertebrates and invertebrates on the seeds of five canopy tree species of an Amazonian forest

We studied the pre-germination loss of seeds to invertebrate and vertebrate seed predators of 5 species of Amazonian trees (Astrocaryum macrocalyx—Palmae; Bertholletia excelsa—Lecithydaceae; Calatola venezuelana—Icacinaceae; Dipteryx micrantha—Leguminosae (Papilionoidae); Hymenaea courbaril-Leguminosae (Caesalpinoidae)). These five species were selected from a large tree flora on several criteria. All possess large (3–10 cm) well-protected seeds that might plausibly be attractive to mammalian seed predators. The reproductive biology of three of the species, or close congeners, had been studied elsewhere in the Neotropics (Astrocaryum, Dipteryx, Hymenaea); one is important to the economy of southeastern Peru (Bertholletia); and one, despite large and apparently edible seeds, appeared to suffer no pre-germination loss to predators (Calatola). We conducted the research in mature forests in the Manu National Park of southeastern Peru where mammal densities are unperturbed by human activities. Densities of adult trees of the five species in our area range from very high (>30 per ha: Astrocaryum) to very low (1 per ha: Hymenaea).Loss of seeds to all causes, and to mammalian seed predators in particular, was determined for seeds placed in 2-square meter mammal exclosures and in open controls located at 10 m (near) and 50 m (far) from a large mature individual of the target species (with minor variations in the design for Astrocaryum and Calatola). The exclosures were of two types: impermeable—designed to exclude all mammals, but not invertebrate seed predators, and semipermeable—designed to admit small (<500 g), but not large mammals. Experimental and control plots were stocked with apparently viable seeds during the dry-wet transition period (October–November) and scored one year later.A significant distance effect (higher predation near vs far from a large conspecific adult) was found in only one of the species (Astrocaryum), the only one to be attacked with high frequency by invertebrate seed predators. The absence of any detectable distance effect attributable to mammals suggests that mammals, over the course of a year, thoroughly search the forest floor for seeds. Invertebrates may thus be responsible for most pre-germination distance (density) effects. With respect to the treatments, we found three qualitatively distinct results: seeds of three species (Astrocaryum, Bertholletia, Dipteryx) were significantly protected by the impermeable, but not semipermeable exclosures, implicating small mammals in seed loss; the seeds of one species (Hymenaea) were significantly protected by exclosures of both types, implicating large mammals; and the seeds of one species (Calatola) exhibited 100% survival, whether or not protected by exclosures.The importance of large mammals as seed predators is generally underestimated in these experiments because semipermeable exclosures may serve as foraging reserves for small mammals. Finally, we noted no relationship between the intensity of mammalian seed predation (as suggested by the survival of unprotected seeds) and the abundance of adults of the five species in the environment. The diversity of results obtained for the five species reveals that large-seeded tropical trees may display a wide range of demographic patterns, and points to the likely importance of post-germination bottlenecks in the population biology of many species, even those that may experience severe pre-germination seed loss.     

Phenol metabolism by two microorganisms isolated from Amazonian forest soil samples

Two microorganisms isolated from Amazonian forest soil samples and identified as Candida tropicalis and Alcaligenes faecalis were capable of degrading phenol (16 and 12 mM, respectively) at high salt concentrations (15% and 5.6%, respectively). Chromatographic and enzymatic studies revealed that each microorganism cleaved phenol at the ortho position with total phenol mineralization. ¹⁴C-phenol mineralization assays showed that both microorganisms assimilated about 30% of the total label. No phenol degradation metabolite (i.e., catechol, cis, cis-muconic acid) was detected in the intercellular medium. The presence of phenol hydroxylase (EC 1.14.13.7) and catechol 1,2-dioxygenase (EC 1.13.11.1) extracellular activity suggested that these microorganisms may secrete these enzymes into the extracellular medium. Journal of Industrial Microbiology & Biotechnology (2000) 24, 403–409. Received 02 November 1999/ Accepted in revised form 08 March 2000     

Spatial distribution and floristic composition of trees and lianas in different forest types of an Amazonian rainforest

A quantitative inventory of trees and lianas was conducted (1) to compare floristic composition, diversity and stem density variation between three different forest types (tierra firme, floodplain and swamp), and (2) to analyse the relationships between floristic similarity and forest structure in two regions ~60 km apart in Yasuní National Park, Amazonian Ecuador. A total of 1,087 species with a diameter at breast height ≥ 2.5 cm were recorded in 25 0.1-ha plots. Tierra firme was the habitat with the highest number of species and stem density for trees and lianas, followed by floodplain and swamp in both regions. Two hypotheses that have been independently proposed to describe plant distribution in tropical rain forests, together explain species spatial distribution in this study. The fact that the 30 most important species per forest type (totalling 119 species) accounted for 48.2% of total individuals supports the oligarchy hypothesis. Likewise, 28 out of these 119 species are reported as restricted to a single forest type, which supports the environmental-determinism hypothesis. In general, both canopy and understorey trees and lianas showed rather similar floristic patterns across different forest types and regions.     

Rainfall exclusion in an eastern Amazonian forest alters soil water movement and depth of water uptake

Deuterium-labeled water was used to study the effect of the Tapajós Throughfall Exclusion Experiment (TTEE) on soil moisture movement and on depth of water uptake by trees of Coussarea racemosa, Sclerolobium chrysophyllum, and Eschweilera pedicellata. The TTEE simulates an extended dry season in an eastern Amazonian rainforest, a plausible scenario if the El Ni?o phenomenon changes with climate change. The TTEE excludes 60% of the wet season throughfall from a 1-ha plot (treatment), while the control 1-ha plot receives precipitation year-round. Mean percolation rate of the label peak in the control plot was greater than in the treatment plot during the wet season (0.75 vs. 0.07 m/mo). The rate was similar for both plots during the dry season (ca. 0.15 m/mo), indicative that both plots have similar topsoil structure. Interestingly, the label peak in the control plot during the dry season migrated upward an average distance of 64 cm. We show that water probably moved upward through soil pores-i.e., it did not involve roots (hydraulic lift)-most likely because of a favorable gradient of total (matric + gravitational) potential coupled with sufficient unsaturated hydraulic conductivity. Water probably also moved upward in the treatment plot, but was not detectable; the label in this plot did not percolate below 1 m or beyond the depth of plant water uptake. During the dry season, trees in the rainfall exclusion plot, regardless of species, consistently absorbed water significantly deeper, but never below 1.5-2 m, than trees in the control plot, and therefore may represent expected root function of this understory/subcanopy tree community during extended dry periods.     

Roots of pioneer trees in the Amazonian rain forest

. On low fluvial terraces of the Caquetá River, Araracuara region, Colombia, tree root systems were examined with regard to their structure, underground stratification and biomass distribution. Excavations of skeleton roots and microscopic observation of terminal fine roots included ten species belonging to the Cecropia, Vismia, Miconia, Goupia, Clathrotropis and Brosimum genera. Roots of particular species varied in the external features of their periderm and structure of freshly cut slash. Coarse skeleton roots differed in the shape and position of the plagiotropic branches and positively geotropic taproots or sinkers. None of the observed species could be identified as a shallow-rooted tree, in spite of the general fine root concentration in the upper soil horizon. In the two seasons of study, the terminal roots displayed prevailingly secondary anatomical structure and did not maintain much primary anatomical tissue in their apices. Neither ectomycorrhizas nor endomycorrhizas were detected in the samples. In a set of regeneration stages the amount of tree roots linearly increased with the age of growth. However, fine roots below 2 mm in diameter shared 80-90% of all roots in the upper 20 cm layer of all sample plots. In an old-growth forest, the total tree root biomass amounted to 39 ton/ha, thus being comparable to the underground biomass observed in similar tropical forests.     

Litterflow chemistry and nutrient uptake from the forest floor in northwest Amazonian forest ecosystems

Samples of the fraction of net rainfall passing through the forest floor collected at monthly intervals in four pristine forests in Colombian Amazonia, during the period between 1995–1997 were analysed for solute concentrations to estimate the element fluxes from the forest floor into the mineral soil and root nutrient uptake from these forest floors. Results were compared with inputs by throughfall, stemflow, litterfall and fine root decay. Element concentrations were tested for their relationship with litterflow amounts, rainfall intensity and length of the antecedent dry period and differences in element fluxes between ecosystems were assessed. Concentrations of elements in litterflow followed a similar pattern as those in throughfall, which indicates that element outputs from the forest floor are strongly related to those inputs in throughfall. In the forests studied, the average concentrations of elements as K, Mg, orthoP and the pH of the litterflow decreased relative to that in throughfall in most events, while the concentration of elements such as dissolved organic carbon, H, SO₄ and Si increased in litterflow from these forests. Element concentrations in litterflow showed a poor correlation with variables such as litterflow amounts, rainfall intensity and antecedent dry period, except for K which showed a significant correlation (p>0.95) with analysed variables in all forests. Outputs were significantly different between forests (p>0.95); these fluxes, which particularly concerned cations, being the largest in the flood plain, while for anions outputs increased from the flood plain to the sedimentary plain. After adding the nutrient contributed by litter decomposition and fine root decay, the net outputs of main elements from the forest floors were still smaller than inputs by net precipitation (throughfall+stemflow) indicating that the litter layers clearly acted as a sink for most nutrients. Accordingly, the element balances confirm that the forest floors acted as a sink for nutrients coming in by throughfall, stemflow, litterfall and fine root decomposition. P, Mg and N appeared to be the most limiting nutrients and the forests studied efficiently recycled these nutrients.     

Thresholds of riparian forest use by terrestrial mammals in a fragmented Amazonian deforestation frontier

Species persistence in fragmented landscapes is intimately related to the quality, structure, and context of remaining habitat remnants. Riparian vegetation is legally protected within private landholdings in Brazil, so we quantitatively assessed occupancy patterns of terrestrial mammals in these remnants, examining under which circumstances different species effectively use them. We selected 38 riparian forest patches and five comparable riparian sites within continuous forest, at which we installed four to five camera-traps per site (199 camera-trap stations). Terrestrial mammal assemblages were sampled for 60 days per station during the dry seasons of 2013 and 2014. We modelled species occupancy and detection probabilities within riparian forest remnants, and examined the effects of patch size, habitat quality, and landscape structure on occupancy probabilities. We then scaled-up modelled occupancies to all 1915 riparian patches throughout the study region to identify which remnants retain the greatest potential to work as habitat for terrestrial vertebrates. Of the ten species for which occupancy was modelled, six responded to forest quality (remnant degradation, cattle intrusion, palm aggregations, and understorey density) or structure (remnant width, isolation, length, and area of the patch from which it originates). Patch suitability was lower considering habitat quality than landscape structure, and virtually all riparian remnants were unsuitable to maintain a high occupancy probability for all species that responded to forest patch quality or structure. Beyond safeguarding legal compliance concerning riparian remnant amount, ensuring terrestrial vertebrate persistence in fragmented landscapes will require curbing the drivers of forest degradation within private landholdings.     

Gene flow across tribal barriers and its effect among the Amazonian Içana river Indians

Demographic information was obtained from 622 individuals of five communities of primarily Baniwa Amerindians living near the Içana river in Brazil. Four of these populations, plus another from the same area, were also studied genetically. The latter investigation included the blood and, in some cases, saliva of 531 subjects, variously tested in relation to 40 genetic systems. Demographically these groups are characterized by young age, high intertribal admixture, low non-Indian admixture, high exogamy but low marital distance and high inbreeding, high fertility but low variance in offspring number, and relatively low mortality. Their gene pool shows a peptidase B variant (PEPB^2BAN1) and “private” polymorphism of carbonic anhydrase₂ (CA₂^BAN1) until now observed only among them. Other distinctive characteristics are the low frequencies of L^NS (0.08), L^Ns (0.09), R^z (0.01), R^O or r (0.02), ACP^A (0.08), GALT^D (0.01), and the relatively high prevalences of Gm^1;11,13,16 (0.05) and Gc¹ (0.82). Tf^Dehi occurs with a low prevalence (0.01). Genetic distance analysis reveals that the one Baniwa sample by history comprised of minimally admixed individuals is quite similar genetically to the Wapishana, another Arawak-speaking tribe some 900 km to the east, and that the genetic distances between the Baniwa communities reflect the amount of historical admixture in a way that indicates which should be excluded from considerations of intertribal genetic distances. Finally, the genetic relation of the Baniwa to the nearby tribes is examined.     

Composition and structure of a species rich Amazonian rain forest obtained by two different sample methods

Inventory data for trees ≥ 10 cm DBH from a hectare plot are compared to data obtained by the Point-Centered Quarter Method along a line transect from the same locality in Anangu, Amazonian Ecuador. The one-hectare quadrat plot of 100 × 100 m had 734 individuals, 153 species, 46 families, a total basal area of 22.2 m², and an estimated above ground tree volume of 240.5 m³. The line transect had a calculated density of 728 individuals per hectare, which included 239 species, 51 families, a total basal area of 34.1 m², and an estimated above ground tree volume of 409.6 m³. Of the 20 species with the highest IVI, only four were shared by the two samples. The most important species were Quararibea ochrocalyx on the hectare plot and Iriartea deltoidea on the line transect, constituting 26.6 and 13.3% of the individuals, respectively. The five families with the highest FIV on the hectare plot (Bombacaceae, Arecaceae, Moraceae, Caesalpinaceae, and Lauraceae) and on the line transect (Arecaceae, Moraceae, Meliaceae, Mimosaceae and Caesalpinaceae) constitute 40.4% and 35.4% of the Family Importance Values of the samples, respectively. The Point-Centered Quarter Method used along a line transect reflects maximum diversity and provides average values of density and tree size in the area. The quadrat plot reflects the local structure and composition of the forest within the plot.     

Soil acidity and nutrient deficiency in central Amazonian heath forest soils

Experiments were carried out to test the effects of liming and nutrient additions on plant growth and soil processes such as C and N mineralisation in three contrasting forest types in central Amazonia: the stunted facies of heath forest (SHF), the tall facies of heath forest (THF) and the surrounding lowland evergreen rain forest (LERF). Calcium-carbonate additions increased soil respiration in the field plots in the SHF; in laboratory incubations, soil respiration was higher in the SHF when soils were fertilised with N, and in THF and LERF after S additions. The addition of N alone or in different combinations generally induced a net immobilisation of soil N. Net nitrification increased during the incubation in SHF and THF soils fertilised with N+P, and in LERF soils fertilised with either N, or P, or CaCO₃. In a field experiment using ingrowth bags, a higher fine root production was observed in all forest types when bags were fertilised with CaCl₂ or CaCO₃, suggesting that Ca may be a limiting nutrient in these soils. Calcium-carbonate addition in a glasshouse bioassay experiment with rice showed an overall positive effect on the survival and growth of the seedlings. In other treatments where soil pH was not raised, the rice showed acute toxicity symptoms, poor root and shoot growth and high mortality. Similar results were yielded in a field experiment, using naturally established seedlings in the field plots in SHF, THF and LERF. It is concluded that the acute H⁺ ion toxicity is a major growth-limiting factor for non-adapted plants in heath forest soils in central Amazonia.     

Extremely long-distance seed dispersal by an overfished Amazonian frugivore

Throughout Amazonia, overfishing has decimated populations of fruit-eating fishes, especially the large-bodied characid, Colossoma macropomum. During lengthy annual floods, frugivorous fishes enter vast Amazonian floodplains, consume massive quantities of fallen fruits and egest viable seeds. Many tree and liana species are clearly specialized for icthyochory, and seed dispersal by fish may be crucial for the maintenance of Amazonian wetland forests. Unlike frugivorous mammals and birds, little is known about seed dispersal effectiveness of fishes. Extensive mobility of frugivorous fish could result in extremely effective, multi-directional, long-distance seed dispersal. Over three annual flood seasons, we tracked fine-scale movement patterns and habitat use of wild Colossoma, and seed retention in the digestive tracts of captive individuals. Our mechanistic model predicts that Colossoma disperses seeds extremely long distances to favourable habitats. Modelled mean dispersal distances of 337-552 m and maximum of 5495 m are among the longest ever reported. At least 5 per cent of seeds are predicted to disperse 1700-2110 m, farther than dispersal by almost all other frugivores reported in the literature. Additionally, seed dispersal distances increased with fish size, but overfishing has biased Colossoma populations to smaller individuals. Thus, overexploitation probably disrupts an ancient coevolutionary relationship between Colossoma and Amazonian plants.     

Growth and mortality of trees in Amazonian tropical rain forest in Ecuador

Abstract. Structural changes are analysed in four samples representing 4 ha, two line transects and two hectare plots, of Amazonian tropical lowland rain forest in northern Ecuador. Only trees with a DBH ≥ 10 cm were included. A sample of floodplain forest in Añangu represents the largest turnover found in tropical forests (stand half-life = 23 yr). The line transect and hectare plot both of tierra firme forest in Añangu have the same turnover (37 yr) and were balanced for death and in-growth of both individuals and wood (basal area). The 1-ha tierra firme sample in Cuyabeno had a turnover of 67 yr and was in a growing phase. The floodplain line transect in Añangu was in a phase of structural breakdown. However, the floodplain line transect had the largest growth of basal area per tree (23.4 cm²/yr). The tierra firme samples had a growth of 9.6, 10.1, and 13.6 cm²/yr. Most of the dead trees fell with some uprooting in three of the four samples. However, no significant difference in the distribution of mode of death was found between the four samples. Death was independent of topography and the dead trees were randomly distributed. As the trees grow up they occupy more space and larger trees (DBH ≥ 15 cm) become more uniformly distributed, whereas smaller trees (DBH ≤ 15 cm) were randomly distributed. Our study confirms that plots of 1 ha are not sufficient to include representative samples of different stages of forest structure.