Intraspecific trait variability facilitates tree species persistence along riparian forest edges in Southern Amazonia.

Tropical forest fragmentation from agricultural expansion alters the microclimatic conditions of the remaining forests, with effects on vegetation structure and function. However, little is known about how the functional trait variability within and among tree species in fragmented landscapes influence and facilitate species' persistence in these new environmental conditions. Here, we assessed potential changes in tree species' functional traits in riparian forests within six riparian forests in cropland catchments (Cropland) and four riparian forests in forested catchments (Forest) in southern Amazonia. We sampled 12 common functional traits of 123 species across all sites: 64 common to both croplands and forests, 33 restricted to croplands, and 26 restricted to forests. We found that forest-restricted species had leaves that were thinner, larger, and with higher phosphorus (P) content, compared to cropland-restricted ones. Tree species common to both environments showed higher intraspecific variability in functional traits, with leaf thickness and leaf P concentration varying the most. Species turnover contributed more to differences between forest and cropland environments only for the stem-specific density trait. We conclude that the intraspecific variability of functional traits (leaf thickness, leaf P, and specific leaf area) facilitates species persistence in riparian forests occurring within catchments cleared for agricultural expansion in Amazonia.

The Ground-Dwelling Ant Fauna from a Cerrado Reserve in Southeastern Brazil: Vegetation Heterogeneity as a Promoter of Ant Diversity.

Ants represent one of the most diverse and ecologically important group of insects in tropical ecosystems, including in highly threatened ones such as the Brazilian Cerrado. Yet, a detailed understanding of the species diversity and composition of local Cerrado ant assemblages is lacking in many cases. Here we present the results of a comprehensive ant inventory performed within a region of the Cerrado (in São Paulo state) where most of the original vegetation has already been lost and where few conservation units exist. We performed consecutive surveys of the ant fauna that forage on the ground in replicated plots established in open savanna (campo sujo), dense savanna (cerrado sensu stricto), and forest (cerradão). Our surveys, with an estimated sample coverage of 99.4%, revealed a total of 219 species of ants from 60 genera, of which 36.1% were found in all the three vegetation types and 29.7% in just one. Rarefied species richness did not differ between vegetation types, but species composition differed markedly, especially between the two savannas in one hand and the forest in the other. Several species (60.1% of the 128 species analyzed) were significant "indicator" species due to their strong association with a given vegetation type. Overall, our findings reinforce the idea that habitat heterogeneity enhances ant diversity and that the mosaic of vegetation types that characterizes the Cerrado biome is one of the main factors explaining the elevated number of species that can be found at relatively small scales.

The neglected tropical grasslands: first record of campo com murundus and its plant communities in the state of São Paulo, Brazil / Campos tropicais negligenciados: primeiro registro de campo com murundus e suas comunidades vegetais no estado de São Paulo, Brasil

Abstract Tropical grasslands have been systematically neglected worldwide in maps, conservation policies, and ecological studies. After eradicating invasive pine trees from a Cerrado reserve in southeastern Brazil, an unprecedented grassy ecosystem arose in recent satellite images. In the field, we confirmed the first record of wet grasslands with termite mounds - locally named campo com murundus ˗ beyond 21°S in the São Paulo state. Besides reporting this occurrence, we sampled the two plant communities forming this peculiar vegetation type (the mounds and the waterlogged matrix around them) to investigate if they are floristically and functionally distinct. We also explored how these two communities relate to those of the surrounding open vegetation types (savanna, dry and wet grassland). Woody plants were recorded on the mounds but not in the matrix, although the two communities share some ground layer species. Compared to the adjacent vegetation types, the mounds were floristically distinct and functionally more balanced in growth forms, dispersal syndrome, and tolerance to waterlogging. We hope this borderline record of campo com murundus can stimulate the search for other unnoticed remnants out of their known occurrence region, triggering efforts for their conservation and studies to improve comprehension of these iconic ecosystems.

Resumo Os campos naturais tropicais têm sido sistematicamente negligenciados em todo o planeta, seja nos mapas, nas políticas de conservação ou em estudos ecológicos. Após controle da invasão por Pinus em unidade de conservação do Cerrado no sudeste do Brasil, um ecossistema graminoso sem registro regional surgiu nas imagens de satélite recentes. Observação em campo resultou no primeiro registro de campo com murundus em latitude superior a 21°S, no estado de São Paulo. Além de relatar essa ocorrência, amostramos as duas comunidades vegetais que formam esse tipo de vegetação peculiar (ocorrendo sobre os murundus e na matriz alagadiça ao redor deles) para investigar se são florística e funcionalmente distintas. Também exploramos como essas duas comunidades estão relacionadas com os tipos de vegetação aberta circundantes (savana, campo seco e campo úmido). Plantas lenhosas foram registradas nos murundus, mas não na matriz, embora as duas comunidades compartilhem algumas espécies do estrato rasteiro. Em comparação com os outros tipos de vegetação adjacentes, os murundus se mostraram floristicamente distintos e funcionalmente mais equilibrados em relação às formas de crescimento, síndrome de dispersão e tolerância ao encharcamento. Esperamos que esse registro limítrofe de campo com murundus possa estimular a busca por outros remanescentes despercebidos fora da região de ocorrência já conhecida, desencadeando esforços para sua conservação e estudos que possam melhorar a compreensão desses ecossistemas icônicos.

Effects and response of the Cerrado ground-layer to frost along the canopy cover gradient.

Frost effects on savanna plant communities have been considered as analogous to those from fire, both changing community structure and filtering species composition. However, while frost impacts have been well-studied for the woody component of savannas, it is still poorly explored for the ground-layer community. Here, we investigated effects of frost in the Cerrado along a gradient of tree cover, focusing on ground-layer plant species, near the southern limit of the Cerrado in Brazil. We aimed to elucidate if the pattern already described for the tree layer also extends to the ground layer in terms of mimicking the effects of fire on vegetation structure and composition. We assessed how damage severity differs across species and across the tree-cover gradient, and we examined the recovery process after frost in terms of richness and community structure along the canopy cover gradient. Frost caused immediate and widespread dieback of the perennial ground-layer, with greatest impact on community structure where tree cover was lowest. However, frost did not reduce the number of species, indicating community resilience to this natural disturbance. Although frost mimicked the effects of fire in some ways, in other ways it differed substantially from fire. Unlike fire, frost increases litter cover and decreases the proportion of bare soil, likely hindering crucial processes for recovery of plant populations, such as seed dispersal, seed germination and plant resprouting. This finding calls attention to the risk of misguided conclusions when the ground layer is neglected in ecological studies of tropical savannas and grasslands.

The distinct roles of water table depth and soil properties in controlling alternative woodland-grassland states in the Cerrado.

Open grassy vegetation and forests share riparian zones across the Neotropical savannas, characterizing alternative stable states. However, factors determining the occurrence and maintenance of each vegetation type are yet to be elucidated. To disentangle the role of environmental factors (soil properties and groundwater depth) constraining tree colonization of wet grasslands in the Cerrado, we assessed tree establishment during the early seedling and sapling stages and the influence of these factors on leaf gas exchange and leaf water potential of tree saplings. Three functionally distinct tree species were studied: (1) flood-tolerant species characteristic of gallery forests, (2) flood-intolerant species characteristic of seasonally dry savannas, and (3) generalist species found in both gallery forests and seasonally dry savannas. Savanna species was constrained by waterlogging, especially at the sapling stage, with restricted stomatal conductance and leaf water potential, resulting in low carbon assimilation, decreased plant size, and high mortality (above 80%). The gallery forest and the generalist species, however, were able to colonize the wet grasslands and survive, despite the low seedling emergence (below 30%) and sapling growth constrained by low gas exchange rates. Soil waterlogging is, therefore, an effective environmental filter that prevents savanna trees from expanding over wet grasslands. However, colonization by trees adapted to a shallow water table cannot be constrained by this or other soil properties, turning the wet grasslands dependent on natural disturbances to persist as an alternative state, sharing the waterlogged environments with the gallery forests in the Cerrado region.

Flammability thresholds or flammability gradients? Determinants of fire across savanna-forest transitions.

Vegetation-fire feedbacks are important for determining the distribution of forest and savanna. To understand how vegetation structure controls these feedbacks, we quantified flammability across gradients of tree density from grassland to forest in the Brazilian Cerrado. We experimentally burned 102 plots, for which we measured vegetation structure, fuels, microclimate, ignition success and fire behavior. Tree density had strong negative effects on ignition success, rate of spread, fire-line intensity and flame height. Declining grass biomass was the principal cause of this decline in flammability as tree density increased, but increasing fuel moisture contributed. Although the response of flammability to tree cover often is portrayed as an abrupt, largely invariant threshold, we found the response to be gradual, with considerable variability driven largely by temporal changes in atmospheric humidity. Even when accounting for humidity, flammability at intermediate tree densities cannot be predicted reliably. Fire spread in savanna-forest mosaics is not as deterministic as often assumed, but may appear so where vegetation boundaries are already sharp. Where transitions are diffuse, fire spread is difficult to predict, but should become increasingly predictable over multiple fire cycles, as boundaries are progressively sharpened until flammability appears to respond in a threshold-like manner.

Comment on "The global tree restoration potential".

Bastin et al's estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration.

Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands.

Despite growing recognition of the conservation values of grassy biomes, our understanding of how to maintain and restore biodiverse tropical grasslands (including savannas and open-canopy grassy woodlands) remains limited. To incorporate grasslands into large-scale restoration efforts, we synthesised existing ecological knowledge of tropical grassland resilience and approaches to plant community restoration. Tropical grassland plant communities are resilient to, and often dependent on, the endogenous disturbances with which they evolved - frequent fires and native megafaunal herbivory. In stark contrast, tropical grasslands are extremely vulnerable to human-caused exogenous disturbances, particularly those that alter soils and destroy belowground biomass (e.g. tillage agriculture, surface mining); tropical grassland restoration after severe soil disturbances is expensive and rarely achieves management targets. Where grasslands have been degraded by altered disturbance regimes (e.g. fire exclusion), exotic plant invasions, or afforestation, restoration efforts can recreate vegetation structure (i.e. historical tree density and herbaceous ground cover), but species-diverse plant communities, including endemic species, are slow to recover. Complicating plant-community restoration efforts, many tropical grassland species, particularly those that invest in underground storage organs, are difficult to propagate and re-establish. To guide restoration decisions, we draw on the old-growth grassland concept, the novel ecosystem concept, and theory regarding tree cover along resource gradients in savannas to propose a conceptual framework that classifies tropical grasslands into three broad ecosystem states. These states are: (1) old-growth grasslands (i.e. ancient, biodiverse grassy ecosystems), where management should focus on the maintenance of disturbance regimes; (2) hybrid grasslands, where restoration should emphasise a return towards the old-growth state; and (3) novel ecosystems, where the magnitude of environmental change (i.e. a shift to an alternative ecosystem state) or the socioecological context preclude a return to historical conditions.

Phylogenetic classification of the world's tropical forests.

Knowledge about the biogeographic affinities of the world's tropical forests helps to better understand regional differences in forest structure, diversity, composition, and dynamics. Such understanding will enable anticipation of region-specific responses to global environmental change. Modern phylogenies, in combination with broad coverage of species inventory data, now allow for global biogeographic analyses that take species evolutionary distance into account. Here we present a classification of the world's tropical forests based on their phylogenetic similarity. We identify five principal floristic regions and their floristic relationships: (i) Indo-Pacific, (ii) Subtropical, (iii) African, (iv) American, and (v) Dry forests. Our results do not support the traditional neo- versus paleotropical forest division but instead separate the combined American and African forests from their Indo-Pacific counterparts. We also find indications for the existence of a global dry forest region, with representatives in America, Africa, Madagascar, and India. Additionally, a northern-hemisphere Subtropical forest region was identified with representatives in Asia and America, providing support for a link between Asian and American northern-hemisphere forests.

Comment on "The extent of forest in dryland biomes".

Bastin et al (Reports, 12 May 2017, p. 635) infer forest as more globally extensive than previously estimated using tree cover data. However, their forest definition does not reflect ecosystem function or biotic composition. These structural and climatic definitions inflate forest estimates across the tropics and undermine conservation goals, leading to inappropriate management policies and practices in tropical grassy ecosystems.

The biodiversity cost of carbon sequestration in tropical savanna.

Tropical savannas have been increasingly viewed as an opportunity for carbon sequestration through fire suppression and afforestation, but insufficient attention has been given to the consequences for biodiversity. To evaluate the biodiversity costs of increasing carbon sequestration, we quantified changes in ecosystem carbon stocks and the associated changes in communities of plants and ants resulting from fire suppression in savannas of the Brazilian Cerrado, a global biodiversity hotspot. Fire suppression resulted in increased carbon stocks of 1.2 Mg ha-1 year-1 since 1986 but was associated with acute species loss. In sites fully encroached by forest, plant species richness declined by 27%, and ant richness declined by 35%. Richness of savanna specialists, the species most at risk of local extinction due to forest encroachment, declined by 67% for plants and 86% for ants. This loss highlights the important role of fire in maintaining biodiversity in tropical savannas, a role that is not reflected in current policies of fire suppression throughout the Brazilian Cerrado. In tropical grasslands and savannas throughout the tropics, carbon mitigation programs that promote forest cover cannot be assumed to provide net benefits for conservation.

Savanna woody encroachment is widespread across three continents.

Tropical savannas are a globally extensive biome prone to rapid vegetation change in response to changing environmental conditions. Via a meta-analysis, we quantified savanna woody vegetation change spanning the last century. We found a global trend of woody encroachment that was established prior the 1980s. However, there is critical regional variation in the magnitude of encroachment. Woody cover is increasing most rapidly in the remaining uncleared savannas of South America, most likely due to fire suppression and land fragmentation. In contrast, Australia has experienced low rates of encroachment. When accounting for land use, African savannas have a mean rate annual woody cover increase two and a half times that of Australian savannas. In Africa, encroachment occurs across multiple land uses and is accelerating over time. In Africa and Australia, rising atmospheric CO2 , changing land management and rainfall are likely causes. We argue that the functional traits of each woody flora, specifically the N-fixing ability and architecture of woody plants, are critical to predicting encroachment over the next century and that African savannas are at high risk of widespread vegetation change.

Woody encroachment and its consequences on hydrological processes in the savannah.

Woody encroachment due to changes in climate or in the disturbance regimes (fire and herbivory) has been observed throughout the savannah biome over the last century with ecological, hydrological and socioeconomic consequences. We assessed changes in tree density and basal area and estimated changes in rain interception by the canopies across a 5-year period over a biomass gradient in Cerrado vegetation protected from fire. We modelled throughfall, stemflow and net rainfall on the basis of tree basal area (TBA). Tree density increased by an average annual rate of 6.7%, basal area at 5.7% and rain interception by the canopies at 0.6% of the gross rainfall. Independent of the vegetation structure, we found a robust relationship of 0.9% less rainfall reaching the ground as TBA increases by 1 m(2) ha(-1) Increases in tree biomass with woody encroachment may potentially result in less water available for uptake by plants and to recharge rivers and groundwater reserves. Given that water is a seasonally scarce resource in all savannahs, woody encroachment may threaten the ecosystem services related to water resources.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

An estimate of the number of tropical tree species.

The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher's alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between ∼ 40,000 and ∼ 53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼ 19,000-25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼ 4,500-6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.

Influence of Removal of a Non-native Tree Species Mimosa caesalpiniifolia Benth. on the Regenerating Plant Communities in a Tropical Semideciduous Forest Under Restoration in Brazil.

Exotic species are used to trigger facilitation in restoration plantings, but this positive effect may not be permanent and these species may have negative effects later on. Since such species can provide a marketable product (firewood), their harvest may represent an advantageous strategy to achieve both ecological and economic benefits. In this study, we looked at the effect of removal of a non-native tree species (Mimosa caesalpiniifolia) on the understory of a semideciduous forest undergoing restoration. We assessed two 14-year-old plantation systems (modified "taungya" agroforestry system; and mixed plantation using commercial timber and firewood tree species) established at two sites with contrasting soil properties in São Paulo state, Brazil. The experimental design included randomized blocks with split plots. The natural regeneration of woody species (height ≥0.2 m) was compared between managed (all M. caesalpiniifolia trees removed) and unmanaged plots during the first year after the intervention. The removal of M. caesalpiniifolia increased species diversity but decreased stand basal area. Nevertheless, the basal area loss was recovered after 1 year. The management treatment affected tree species regeneration differently between species groups. The results of this study suggest that removal of M. caesalpiniifolia benefited the understory and possibly accelerated the succession process. Further monitoring studies are needed to evaluate the longer term effects on stand structure and composition. The lack of negative effects of tree removal on the natural regeneration indicates that such interventions can be recommended, especially considering the expectations of economic revenues from tree harvesting in restoration plantings.

Germinação de 15 espécies vegetais do cerrado sob diferentes condições de luz / Germination of 15 cerrado plant species under different light conditions

A ausência de conhecimento sobre a germinação de plantas do Cerrado tem sido apontada como importante obstáculo à propagação dessas espécies e, consequentemente, à restauração da vegetação. Assim, o objetivo deste estudo foi investigar o comportamento de quinze espécies herbáceas, arbustivas e arbóreas do Cerrado, semeadas em diferentes condições de luz. Realizamos um experimento em viveiro, onde as sementes foram expostas a pleno sol e sob sombreamento a 75%, com cinco repetições por espécie e 10 tubetes por repetição, totalizando 1500 sementes. Acompanhamos a germinação diariamente durante nove meses. Das quinze espécies estudadas doze germinaram, sendo três apenas na sombra e quatro apenas a pleno sol. De modo geral, a germinação foi baixa, sendo que seis espécies tiveram no máximo 10% de sementes germinadas. Somente duas espécies apresentaram germinação superior a 50%. Três espécies apresentaram diferenças significativas entre os dois tratamentos para a germinação: Cyrtocymura scorpioides e Fimbristylis autumnalis, típicas de fisionomias campestres, cuja germinação foi maior a pleno sol, e Styrax pohlii, espécie de ambiente florestal, que germinou mais à sombra, características essas que podem explicar os resultados observados. Algumas espécies permaneceram viáveis no substrato por muito tempo e continuaram germinando mesmo cinco meses após a semeadura. Esses resultados confirmam que a luz pode influenciar diferentemente a germinação das plantas do Cerrado. As baixas taxas de germinação e a variabilidade no tempo para germinar são importantes desafios a serem superados pela ciência e pela tecnologia para a propagação das espécies do Cerrado, seja por meio da produção de mudas ou semeadura direta. Informações sobre as respostas das espécies a diferentes fatores abióticos são essenciais para aumentar as chances de sucesso dessas técnicas, permitindo ampliar o número de espécies a serem utilizados na restauração.

The lack of knowledge about germination of Brazilian Savanna species has been pointed out as an important constrain to their propagation and, as a consequence, to ecological restoration. The aim of this study was to assess the germination of fifteen herb, shrub and tree Savanna species under different light conditions. We carried out a nursery experiment, sowing the seeds under full sunlight and 75% shade. The experimental design comprised five replicates per species and 10 pots per replicate, in a total of 1500 seeds. We monitored germination daily during nine months. Twelve out of the fifteen species germinated, being three exclusively under 75% shade and four exclusively under full sun. The overall germination was low; six species showed no more than 10% of germination and only two species germinated more than 50%. Three species differed significantly between treatments: Cyrtocymura scorpioides and Fimbristylis autumnalis, which are frequent in open grassland savannas and showed higher germination under full sun, and Styrax pohlii, which is more common in wooded savannas and had higher germination under shade. The natural occurrence of those species in such environments might help to explain our results. Some species remained viable for a long time in the soil, and still germinated five months after sowing. Our results agree with other studies showing that light may differently affect germination of savanna species. The low germination rates and the variability in the time to germinate are important challenges to be overcome by science and technology for the propagation of the Cerrado species, either through the production of seedlings or direct sowing. Information on species response to different abiotic factors are essential to increase the chances of success of such techniques, and to increase the number of species to be used in restoration projects.

Savanna vegetation-fire-climate relationships differ among continents.

Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.

Ecological restoration of Xingu Basin headwaters: motivations, engagement, challenges and perspectives.

Over the past two decades, the headwaters of the Xingu Basin in the Amazon have been subjected to one of the highest deforestation rates in Brazil, with negative effects on both terrestrial and aquatic systems. The environmental consequences of forest land conversion have concerned the indigenous people living downstream, and this was the first motivation for the Y Ikatu Xingu campaign--'save the good water of the Xingu'. Among the objectives of the initiative was to restore riparian forests on private land across the basin. For a region where the rivers, rainstorms, forest remnants, distances and farms are huge, the challenges were equally large: crossing the biotic and abiotic thresholds of degradation, as well as addressing the lack of technology, know-how, seeds, forest nurseries, trained personnel and roads, and the lack of motivation for restoration. After 6 years, despite the remarkable advances in terms of technical innovation coupled with a broad and effective social involvement, the restored areas represent only a small portion of those aimed for. The still high costs of restoration, the uncertainties of legislation and also the global economy have been strong forces constraining the expansion of restored forests. Additional efforts and strategies are necessary to overcome these barriers.