Ecological release in lizard assemblages of neotropical savannas

We compare lizard assemblages of Cerrado and Amazonian savannas to test the ecological release hypothesis, which predicts that niche dimensions and abundance should be greater in species inhabiting isolated habitat patches with low species richness (Amazonian savannas and isolated Cerrado patches) when compared with nonisolated areas in central Cerrado with greater species richness. We calculated microhabitat and diet niche breadths with data from 14 isolated Cerrado patches and Amazon savanna areas and six central Cerrado populations. Morphological data were compared using average Euclidean distances, and lizard abundance was estimated using the number of lizards captured in pitfall traps over an extended time period. We found no evidence of ecological release with respect to microhabitat use, suggesting that historical factors are better microhabitat predictors than ecological factors. However, data from individual stomachs indicate that ecological release occurs in these areas for one species (Tropidurus) but not others (Ameiva ameiva, Anolis, Cnemidophorus, and Micrablepharus), suggesting that evolutionary lineages respond differently to environmental pressures, with tropidurids being more affected by ecological factors than polychrotids, teiids, and gymnophthalmids. We found no evidence that ecological release occurs in these areas using morphological data. Based on abundance data, our results indicate that the ecological release (density compensation) hypothesis is not supported: lizard species are not more abundant in isolated areas than in nonisolated areas. The ecology of species is highly conservative, varying little from assemblage to assemblage. Nevertheless, increases in niche breadth for some species indicate that ecological release occurs as well. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Expansion of gallery forests into central Brazilian savannas

Upland tropical forests have expanded and contracted in response to past climates, but it is not clear whether similar dynamics were exhibited by gallery (riparian) forests within savanna biomes. Because such forests generally have access to ample water, their extent may be buffered against changing climates. We tested the long‐term stability of gallery forest boundaries by characterizing the border between gallery forests and savannas and tracing the presence of gallery forest through isotopic analysis of organic carbon in the soil profile. We measured leaf area index, grass vs. shrub or tree coverage, the organic carbon, phosphorus, nitrogen and calcium concentrations in soils and the carbon isotope ratios of soil organic matter in two transitions spanning gallery forests and savanna in a Cerrado ecosystem. Gallery forests without grasses typically show a greater leaf area index in contrast to savannas, which show dense grass coverage. Soils of gallery forests have significantly greater concentrations of organic carbon, phosphorus, nitrogen and calcium than those of savannas. Soil organic carbon of savannas is significantly more enriched in ¹³C compared with that of gallery forests. This difference in enrichment is in part caused by the presence of C₄ grasses in savanna ecosystem and its absence in gallery forests. Using the ¹³C abundance as a signature for savanna and gallery forest ecosystems in 1 m soil cores, we show that the borders of gallery forests have expanded into the savanna and that this process initiated at least 3000–4000 bp based on ¹⁴C analysis. Gallery forests, however, may be still expanding as we found more recent transitions according to ¹⁴C activity measurements. We discuss the possible mechanisms of gallery forest expansion and the means by which nutrients required for the expansion of gallery forest might accumulate.     

Not all forests are expanding over central Brazilian savannas

Recently we reported on the expansion of riparian forests into savannas in central Brazil. To enlarge the scope of the earlier study we investigated whether upland deciduous and xeromorphic forests behaved similarly. We investigated past vegetation changes that occurred in forest/savanna transitions using carbon isotope ratios (δ¹³C) measured in the soil organic matter as a tracer. We analyzed the ¹⁴C activity where δ¹³C showed major shifts in vegetation. The role of soil chemical and physical attributes in defining vegetation distribution is discussed. Structural changes in vegetation were found to be associated with shifts in the isotope composition (δ¹³C) of soil organic matter. This was attributed to intrinsic differences in the biomass of trees and grasses and allowed for the determination of past shifts in vegetation by evaluating δ¹³C at different depths. The deciduous forest decreased in area approximately 980 years ago. Tree cover increased in the xeromorphic forest, but the border stayed stable through time. The deciduous forest and adjacent savanna have eutrophic soils while the xeromorphic forest and adjacent savanna have dystrophic soils. However, greater organic carbon, nitrogen and phosphorus concentrations are observed in the forests. We provide concrete evidence of deciduous forest retreat unlike the stability observed in the xeromorphic forest/savanna boundary. These results contrast with the expansion of riparian forests recently reported in the same region.     

Biodiversity of galling insects: historical, community and habitat effects in four neotropical savannas

Five hypotheses were tested to explain the pattern of galling insect species richness in four neotropical savanna physiognomies, 'canga ', 'campo sujo', 'cerrado' s. st., and 'cerradão', that occur in Minas Gerais, southeastern Brazil. We found 125 species of galling insects on 80 host plant species. The increase of plant species richness explained 35% of the variation in galling insect richness, corroborating the plant species richness hypothesis. Most of the galling species occurred on trees, followed by shrubs, and herbs. However, the difference in mean number of galls was only statistically significant between herbs and trees, corroborating partially the plant structural complexity hypothesis. A significant relationship was observed between galling species richness and density of herbs, and shrubs, corroborating partially the resource concentration hypothesis. Galling insect richness showed a negative correlation with magnesium, potassium, and zinc on soil, corroborating the soil fertility hypothesis. The content of magnesium, potassium, iron and CTC (T) explained 72% of the variation in galling insect richness. Plant family size positively influenced galling insect richness, corroborating the plant family size hypothesis. Overall, the results corroborate the hypothesis that predicts that habitat stress is the main factor generating the patterns of galling insect richness in Brazilian savannas.     

Controls on long-term root and leaf litter decomposition in neotropical forests

Litter decomposition represents one of the largest annual fluxes of carbon (C) from terrestrial ecosystems, particularly for tropical forests, which are generally characterized by high net primary productivity and litter turnover. We used data from the Long-Term Intersite Decomposition Experiment (LIDET) to (1) determine the relative importance of climate and litter quality as predictors of decomposition rates, (2) compare patterns in root and leaf litter decomposition, (3) identify controls on net nitrogen (N) release during decay, and (4) compare LIDET rates with native species studies across five bioclimatically diverse neotropical forests. Leaf and root litter decomposed fastest in the lower montane rain and moist forests and slowest in the seasonally dry forest. The single best predictor of leaf litter decomposition was the climate decomposition index (CDI), explaining 51% of the variability across all sites. The strongest models for predicting leaf decomposition combined climate and litter chemistry, and included CDI and lignin ( R ²=0.69), or CDI, N and nonpolar extractives ( R ²=0.69). While we found no significant differences in decomposition rates between leaf and root litter, drivers of decomposition differed for the two tissue types. Initial stages of decomposition, determined as the time to 50% mass remaining, were driven primarily by precipitation for leaf litter ( R ²=0.93) and by temperature for root litter ( R ²=0.86). The rate of N release from leaf litter was positively correlated with initial N concentrations; net N immobilization increased with decreasing initial N concentrations. This study demonstrates that decomposition is sensitive to climate within and across tropical forests. Our results suggest that climate change and increasing N deposition in tropical forests are likely to result in significant changes to decomposition rates in this biome.     

Adaptive strategies of perennial grasses in South American savannas

Abstract. Morpho-fimctional features of perennial grasses in South American savannas are considered as adaptive strategies to cope with stress and disturbance factors of savanna environments. The tussock growth form, annual patterns of vegetative growth and reproductive phenology, allocation of carbon and nutrients, and accumulation of standing dead phytomass at the end of the dry season, are discussed in relation to water economy, resistance to drought, photosynthetic rates, growth rhythms, regrowth after drought and fire, seasonal translocation of critical nutrients and carbohydrates, and the total nutrient budget of the grass layer. Different strategies combining various morphological patterns, phenological alternatives and mechanisms for resisting drought and fire exist within the grass flora of each savanna community. The lack of adaptive responses to grazing by large herbivores is a major distinction from African savanna grasses. Many African grasses, either introduced in pastures or colonizing disturbed savannas, do show positive responses to defoliation, including compensatory growth and enhanced photosynthetic rates. Some guidelines for further research are suggested in order to disclose the mechanisms underlying this different behaviour of native and introduced savanna grasses.     

Forest fragmentation, synergisms and the impoverishment of neotropical forests

It is well documented that the negative effects of habitat fragmentation are strong enough to promote local as well as regional extinction of canopy and emergent trees in neotropical forests. However, forest fragmentation does not occur alone, but is always associated with other human-induced threats to trees, such as logging, forest burning and hunting of key vertebrate seed dispersers within forest remnants. This association occurs because forest resources are, at least during a certain period, the main income source for local human populations. It is now possible to establish how these threats act in concert causing tree species impoverishment. Based on a predictive model, it is predicted that the most fragmented forest regions have lost or will lose an important part of their tree diversity. New integrated research must urgently test this prediction and investigate how human activities might be regulated in both old and new tropical forest frontiers to avoid species loss. If we fail to do this we will miss the opportunity of proposing sound and efficient guidelines to rescue neotropical forests from species impoverishment.     

Pterandra pyroidea: a case of pollination shift within neotropical Malpighiaceae

Background and Aims

Most Neotropical species of Malpighiaceae produce floral fatty oils in calyx glands to attract pollinating oil-collecting bees, which depend on this resource for reproduction. This specialized type of pollination system tends to be lost in members of the family that occur outside the geographic distribution (e.g. Africa) of Neotropical oil-collecting bees. This study focused on the pollination ecology, chemical ecology and reproductive biology of an oil flower species, Pterandra pyroidea (Malpighiaceae) from the Brazilian Cerrado. Populations of this species consist of plants with oil-secreting (glandular) flowers, plants with non-oil-secreting flowers (eglandular) or a mix of both plant types. This study specifically aims to clarify the role of eglandular morphs in this species.

Methods

Data on pollinators were recorded by in situ observations. Breeding system experiments were conducted by isolating inflorescences and by enzymatic reactions. Floral resources, pollen and floral oils offered by this species were analysed by staining and a combination of various spectroscopic methods.

Key Results

Eglandular flowers of P. pyroidea do not act as mimics of their oil-producing conspecifics to attract pollinators. Instead, both oil-producing and oil-free flowers depend on pollen-collecting bees for reproduction, and their main pollinators are bumble-bees. Floral oils produced by glandular flowers are less complex than those described in closely related genera.

Conclusions

Eglandular flowers represent a shift in the pollination system in which oil is being lost and pollen is becoming the main reward of P. pyroidea flowers. Pollination shifts of this kind have hitherto not been demonstrated empirically within Neotropical Malpighiaceae and this species exhibits an unusual transition from a specialized towards a generalized pollination system in an area considered the hotspot of oil-collecting bee diversity in the Neotropics. Transitions of this type provide an opportunity to study ongoing evolutionary mechanisms that promote the persistence of species previously involved in specialized mutualistic relationships.     

Influence of edaphic specialization on pteridophyte distribution in neotropical rain forests

The abundance of terrestrial pteridophytes was studied in eight sites in Western Amazonia, all representing lowland tropical rain forest in non-inundated terrain ( tierra firme ). In each site, a sample plot ranging from 0.25 ha to 1.00 ha in area was inventoried and all terrestrial pteridophytes were recorded. In total, 27,000 individuals representing sixty-four species were encountered, with each plot having between fourteen and thirty-three species. The plots represented a gradient from nutrient-poor sandy soils to relatively fertile clay soils, and it was found that their floristic compositions clearly correlated with the differences in the soils. Two-thirds of the species were restricted either to poor, intermediate or rich soils, while less than a tenth were found growing on all soil types. Analyses of species lists from another twenty neotropical localities supported the result that some pteridophyte species associate, whereas other species are never found growing together. The floristic similarities among sites showed no relationship to the geographical distances between them, but rather reflected similarities in soils. It is concluded that pteridophytes are good indicators of such ecological conditions, and that when interpreting biogeographical patterns in the neotropical rain forests, it is important to take into account the distribution of edaphically different habitats.     

Relationships among ecologically important dimensions of plant trait variation in seven neotropical forests

BACKGROUND AND AIMS: When ecologically important plant traits are correlated they may be said to constitute an ecological 'strategy' dimension. Through identifying these dimensions and understanding their inter-relationships we gain insight into why particular trait combinations are favoured over others and into the implications of trait differences among species. Here we investigated relationships among several traits, and thus the strategy dimensions they represented, across 2134 woody species from seven Neotropical forests. METHODS: Six traits were studied: specific leaf area (SLA), the average size of leaves, seed and fruit, typical maximum plant height, and wood density (WD). Trait relationships were quantified across species at each individual forest as well as across the dataset as a whole. 'Phylogenetic' analyses were used to test for correlations among evolutionary trait-divergences and to ascertain whether interspecific relationships were biased by strong taxonomic patterning in the traits. KEY RESULTS: The interspecific and phylogenetic analyses yielded congruent results. Seed and fruit size were expected, and confirmed, to be tightly related. As expected, plant height was correlated with each of seed and fruit size, albeit weakly. Weak support was found for an expected positive relationship between leaf and fruit size. The prediction that SLA and WD would be negatively correlated was not supported. Otherwise the traits were predicted to be largely unrelated, being representatives of putatively independent strategy dimensions. This was indeed the case, although WD was consistently, negatively related to leaf size. CONCLUSIONS: The dimensions represented by SLA, seed/fruit size and leaf size were essentially independent and thus conveyed largely independent information about plant strategies. To a lesser extent the same was true for plant height and WD. Our tentative explanation for negative WD-leaf size relationships, now also known from other habitats, is that the traits are indirectly linked via plant hydraulics.     

Pattern and process in neotropical secondary rain forests: the first 100 years of succession

More and more areas of deforested wet tropical lands are being handed back to nature as their erstwhile owners abandon attempts to farm them. The resulting secondary successions offer hope that some of the unique characteristics of the original rain forests may be recovered and conserved. However, most of our understanding of what secondary tropical rain forests are and how and why they develop is limited to the first decade of a process that may last for centuries. A longer-term view indicates that the causes of change in neotropical secondary successions are similar to those operating in temperate forests, but yields sobering conclusions for conservation.     

New strategies for conserving tropical forests

In an interval of just 1-2 decades, the nature of tropical forest destruction has changed. Rather than being dominated by rural farmers, tropical deforestation now is substantially driven by major industries and economic globalization, with timber operations, oil and gas development, large-scale farming and exotic-tree plantations being the most frequent causes of forest loss. Although instigating serious challenges, such changes are also creating important new opportunities for forest conservation. Here we argue that, by increasingly targeting strategic corporations and trade groups with public-pressure campaigns, conservation interests could have a much stronger influence on the fate of tropical forests.     

Seed germination requirements of Trembleya laniflora (Melastomataceae), an endemic species from neotropical montane rocky savannas

Trembleya laniflora is an endemic shrub from neotropical montane rocky savannas of southeastern Brazil. It has been indicated as a potential candidate for ecological restoration of abandoned iron‐ore mines due to heavy metal accumulation. Here, we evaluated the seed germination requirements of T. laniflora. Seeds collected in 2005 and 2008 were set to germinate under a broad range of temperature and light conditions. Seed viability was estimated by dissecting seeds under a dissecting microscope for embryo presence/absence. Seeds were photoblastic and optimum temperature range was 20–25°C, coinciding with the onset of the rainy season. Seeds were viable after 42 months of storage, which together with small seeds that easily get buried and light requirement for germination suggest formation of soil seed banks. Except the large fraction of embryoless seeds, almost all tested seeds germinated when incubated under light conditions; therefore, T. laniflora should be regarded to have nondormant seeds. Easiness of burial resulting from small seed size and positive photoblastism may both contribute to incorporation into soil seed banks. Our data suggest that the long‐term storage of T. laniflora seeds provides a useful strategy for plant reintroduction.     

Fungal community composition in neotropical rain forests: the influence of tree diversity and precipitation

Plant diversity is considered one factor structuring soil fungal communities because the diversity of compounds in leaf litter might determine the extent of resource heterogeneity for decomposer communities. Lowland tropical rain forests have the highest plant diversity per area of any biome. Since fungi are responsible for much of the decomposition occurring in forest soils, understanding the factors that structure fungi in tropical forests may provide valuable insight for predicting changes in global carbon and nitrogen fluxes. To test the role of plant diversity in shaping fungal community structure and function, soil (0-20?cm) and leaf litter (O horizons) were collected from six established 1-ha forest census plots across a natural plant diversity gradient on the Isthmus of Panama. We used 454 pyrosequencing and phospholipid fatty acid analysis to evaluate correlations between microbial community composition, precipitation, soil nutrients, and plant richness. In soil, the number of fungal taxa increased significantly with increasing mean annual precipitation, but not with plant richness. There were no correlations between fungal communities in leaf litter and plant diversity or precipitation, and fungal communities were found to be compositionally distinct between soil and leaf litter. To directly test for effects of plant species richness on fungal diversity and function, we experimentally re-created litter diversity gradients in litter bags with 1, 25, and 50 species of litter. After 6?months, we found a significant effect of litter diversity on decomposition rate between one and 25 species of leaf litter. However, fungal richness did not track plant species richness. Although studies in a broader range of sites is required, these results suggest that precipitation may be a more important factor than plant diversity or soil nutrient status in structuring tropical forest soil fungal communities.     

Nitrogen use strategies of neotropical rainforest trees in threatened Atlantic Forest

The characteristics of nitrogen acquisition, transport and assimilation were investigated in species of an Atlantic Forest succession over calcareous soil in south‐eastern Brazil. Differences in behaviour were observed within the regeneration guilds. Pioneer species showed high leaf nitrogen contents, a high capacity to respond to increased soil nitrogen availability, a high capacity for leaf nitrate assimilation and were characterized by the transport of nitrate + asparagine. At the other end of the succession, late secondary species had low leaf nitrogen contents, little capacity to respond to increased soil nitrogen availability, low leaf nitrate assimilation and were active in the transport of asparagine + arginine. The characteristics of nitrogen nutrition in some early secondary species showed similarities to those of pioneer species whereas others more closely resembled late secondary species. Average leaf δ¹⁵N values increased along the successional gradient. The results indicate that the nitrogen metabolism characteristics of species may be an additional ecophysiological tool in classifying tropical forest tree species into ecological guilds, and may have implications for regeneration programmes in degraded areas.     

Dynamics of energy and nutrient concentration and construction cost in a native and two alien C4 grasses from two neotropical savannas

In Venezuela, the alien grasses Melinis minutiflora Beauv. and Hyparrhenia rufa (Nees.) Stapf tend to displace the native savanna plant community dominated by Trachypogon plumosus (Humb. and Bonpl.) Nees. This occurs in either relatively wetter and fertile highland savannas or in drier and less fertile lowland savannas. Although the native and aliens are perennial C₄ grasses, higher net assimilation leaf biomass per plant and germination rate of the latter are some causes for their higher growth rates and for their competitive success. The objective of this study is to compare seasonal tissue energy, N, P and K concentrations and the calculated construction costs (CC) between the native grass and either one of the alien grasses from lowland and highland savannas. We predict that, in order to out-compete native plants, alien grasses should be more efficient in resource use as evidenced by lower tissue energy and nutrient concentrations and CC.Tissue energy and nutrient concentration were measured throughout the year and compared between M. minutiflora and the co-occurring local population of T. plumosus in a highland savanna and between H. rufa and its neighbor local population of T. plumosus in a lowland savanna. CC was calculated from energy, N and ash concentrations considering ammonium as the sole N source. Differences between co-occurring species, T. plumosus populations, seasons, and organs were analyzed with ANOVA.Highland and lowland grasses differed in concentration and allocation of energy and nutrients whereas the differences between alien and native grasses were specific for each pair considered. Highland grasses had higher energy, N, P and CC than lowland grasses. These variables were always lowest in the culms. In the more stressed lowland site, tissue energy and nutrient concentrations decreased significantly during the dry season except in the roots of both grasses which had the highest energy and nutrients concentrations during the drought. This seasonal response was more marked in the local lowland population of T. plumosus in which maximum CC alternated seasonally between leaves and roots. Energy and nutrient concentrations and CC were the lowest in H. rufa. In the lowland savannas, the higher efficiency of resource use in the invader grass contributes to its higher competitive success through increased growth rate. In the highlands, overall tissue energy concentration and CC, but not N nor P concentration, were lower in the fast growing M. minutiflora but seasonal differences were lacking. The higher leaf CC in T. plumosus can be attributed to the higher proportion of sclerenchyma tissue which is more expensive to construct. Considering CC, both fast growing alien grasses are more efficient in resource use than the co-occurring native grass. However, the role of CC explaining the competitive success of the former, through higher growth rates, is more evident in the more stressful environment of the lowland savanna.     

Soil factors influencing ectomycorrhizal sporome distribution in neotropical forests dominated by Pinus montezumae, Mexico

Soil factors influencing ectomycorrhizal (ECM) sporome distribution in neotropical forests dominated by Pinus montezumae were examined at Sierra Chichinautzin, Mexico. Study sites were located on three volcanoes of different ages and ECM sporomes were collected during three consecutive years. Inocybe and Laccaria species were preferentially found at the youngest site, while Tricholoma and Russula species showed a more abundant distribution at the oldest site. Total sporome richness was negatively correlated with carbon (C) and nitrogen (N) contents in the soil organic horizon. Canonical correspondence analysis (CCA) was used to investigate the relationships between sporome species and soil variables. The CCA biplot showed that Amanita rubescens, I. fastigiata, and I. geophylla had a strong positive relationship with soil C and N contents, whereas Inocybe sp.4 was negatively related to these variables. This indicates an intra-generic variability in fungal responses to soil factors. The measured soil nutrients influenced species composition patterns, and the differences in sporome distribution evidenced a large degree of community specialization along the soil quality gradient. Together, these data contribute to a better understanding of the ecology of macrofungi in neotropical forests.