Addressing Conservation Needs: Genetic Diversity and Population Ecology of the Endemic Tree Spondias tuberosa Arruda

Spondias tuberosa Arruda (Anacardiaceae), popularly known as umbuzeiro or imbuzeiro, is a fruit tree native to the semiarid region of Brazil. The extractive harvesting of its fruits contributes significantly to the economy, generating an annual revenue of approximately $4,2 million. The present study aimed to assess the spatial pattern, allometric variations, fruit measurements, and genetic diversity of trees within a remaining forest of the Caatinga biome, with a focus on intrapopulation analysis. We used intersimple repeated sequence markers and the second-order function density of neighbours to determine the genetic and spatial structure. The density of neighbours was highest within a 10-meter radius. Biometric analyses revealed average fruit lengths of 31.12 mm (±0.22), diameters of 28.68 mm (±0.25), and fresh masses of 15.56 g (±0.33). Diaspores exhibited an average length, diameter, and thickness of 19.27 mm, 13.95 mm, and 11.14 mm, respectively, with a fresh mass of 2.28 g. Notably, the fresh mass demonstrated the highest coefficient of variation. Ten molecular markers were selected, generating 103 highly polymorphic loci (99.03%) with an average informative content of 0.45. Nei's diversity index (0.37) and Shannon's index (0.55) indicated moderate genetic diversity. Furthermore, Bayesian analysis revealed a population structure with two distinct genetic groups. The Infinite Allele and Mutation Step Models suggested a significant historical decline in population size, indicative of a genetic bottleneck. As a result, proactive in situ conservation strategies, including establishing protected natural areas, become essential, considering the socioeconomic significance of the species. Additionally, it is recommended to establish germplasm banks for ex situ conservation and the development of managed cultivation initiatives to reduce the pressure on native populations of S. tuberosa caused by extraction.

Allometric relationships and trade-offs in 11 common Mediterranean-climate grasses

Biomass allocation in plants is the foundation for understanding dynamics in ecosystem carbon balance, species competition, and plant-environment interactions. However, existing work on plant allometry has mainly focused on trees, with fewer studies having developed allometric equations for grasses. Grasses with different life histories can vary in their carbon investment by prioritizing the growth of specific organs to survive, outcompete co-occurring plants, and ensure population persistence. Further, because grasses are important fuels for wildfire, the lack of grass allocation data adds uncertainty to process-based models that relate plant physiology to wildfire dynamics. To fill this gap, we conducted a greenhouse experiment with 11 common California grasses varying in photosynthetic pathway and growth form. We measured plant sizes and harvested above- and belowground biomass throughout the life cycle of annual species, while for the establishment stage of perennial grasses to quantify allometric relationships for leaf, stem, and root biomass, as well as plant height and canopy area. We used basal diameter as a reference measure of plant size. Overall, basal diameter is the best predictor for leaf and stem biomass, height, and canopy area. Including height as another predictor can improve model accuracy in predicting leaf and stem biomass and canopy area. Fine root biomass is a function of leaf biomass alone. Species vary in their allometric relationships, with most variation occurring for plant height, canopy area, and stem biomass. We further explored potential trade-offs in biomass allocation across species between leaf and fine root, leaf and stem, and allocation to reproduction. Consistent with our expectation, we found that fast-growing plants allocated a greater fraction to reproduction. Additionally, plant height and specific leaf area negatively influenced the leaf-to-stem ratio. However, contrary to our hypothesis, there were no differences in root-to-leaf ratio between perennial and annual or C4 and C3 plants. Our study provides species-specific and functional-type-specific allometry equations for both above- and belowground organs of 11 common California grass species, enabling nondestructive biomass assessment in California grasslands. These allometric relationships and trade-offs in carbon allocation across species can improve ecosystem model predictions of grassland species interactions and environmental responses through differences in morphology.

Disentangling the effect of growth from development in size-related trait scaling relationships

In plant ecology, the terms growth and development are often used interchangeably. Yet these constitute two distinct processes. Plant architectural traits (e.g. number of successive forks) can estimate development stages. Here, we show the importance of including the effect of development stages to better understand size-related trait scaling relationships (i.e. between height and stem diameter). We focused on one common savanna woody species (Senegalia nigrescens) from the Greater Kruger Area, South Africa. We sampled 406 individuals that experience different exposure to herbivory, from which we collected four traits: plant height, basal stem diameter, number of successive forks (proxy for development stage), and resprouting. We analysed trait relationships (using standardized major axis regression) between height and stem diameter, accounting for the effect of ontogeny, exposure to herbivory, and resprouting. The number of successive forks affects the scaling relationship between height and stem diameter, with the slope and strength of the relationship declining in more developed individuals. Herbivory exposure and resprouting do not affect the overall height-diameter relationship. However, when height and stem diameter were regressed separately against number of successive forks, herbivory exposure and resprouting had an effect. For example, resprouting individuals allocate more biomass to both primary and secondary growth than non-resprouting plants in more disturbed conditions. We stress the need to include traits related to ontogeny so as to disentangle the effect of biomass allocation to primary and secondary growth from that of development in plant functional relationships.

Blocking then stinging as a case of two-step evolution of defensive cage architectures in herbivore-driven ecosystems

Dense branching and spines are common features of plant species in ecosystems with high mammalian herbivory pressure. While dense branching and spines can inhibit herbivory independently, when combined, they form a powerful defensive cage architecture. However, how cage architecture evolved under mammalian pressure has remained unexplored. Here we show how dense branching and spines emerged during the age of mammalian radiation in the Combretaceae family and diversified in herbivore-driven ecosystems in the tropics. Phylogenetic comparative methods revealed that modern plant architectural strategies defending against large mammals evolved via a stepwise process. First, dense branching emerged under intermediate herbivory pressure, followed by the acquisition of spines that supported higher speciation rates under high herbivory pressure. Our study highlights the adaptive value of dense branching as part of a herbivore defence strategy and identifies large mammal herbivory as a major selective force shaping the whole plant architecture of woody plants.

Solutions to fire and shade: resprouting, growing tall and the origin of Eurasian temperate broadleaved forest

Tree species of Eurasian broadleaved forest possess two divergent trait syndromes with contrasting patterns of resource allocation adapted to different selection environments: short-stature basal resprouters that divert resources to a bud bank adapted to frequent and severe disturbances such as fire and herbivory, and tall trees that delay reproduction by investing in rapid height growth to escape shading. Drawing on theory developed in savanna ecosystems, we propose a conceptual framework showing that the possession of contrasting trait syndromes is essential for the persistence of broadleaved trees in an open ecosystem that burns. Consistent with this hypothesis, trees of modern Eurasian broadleaved forest bear a suite of traits that are adaptive to surface and crown-fire regimes. We contend that limited opportunities in grassland restricts recruitment to disturbance-free refugia, and en masse establishment creates a wooded environment where shade limits the growth of light-demanding savanna plants. Rapid height growth, which involves investment in structural support and the switch from a multi-stemmed to a monopodial growth form, is adaptive in this shaded environment. Although clustering reduces surface fuel loads, these establishment nuclei are vulnerable to high-intensity crown fires. The lethal effects of canopy fire are avoided by seasonal leaf shedding, and aerial resprouting enhances rapid post-fire recovery of photosynthetic capacity. While these woody formations satisfy the structural definition of forest, their constituents are clearly derived from savanna. Contrasting trait syndromes thus represent the shift from consumer to resource regulation in savanna ecosystems. Consistent with global trends, the diversification of most contemporary broadleaved taxa coincided with the spread of grasslands, a surge in fire activity and a decline in wooded ecosystems in the late Miocene-Pliocene. Recognition that Eurasian broadleaved forest has savanna origins and persists as an alternative state with adjacent grassy ecosystems has far-reaching management implications in accordance with functional rather than structural criteria. Shade is a severe constraint to the regeneration and growth of both woody and herbaceous growth forms in consumer-regulated ecosystems. However, these ecosystems are highly resilient to disturbance, an essential process that maintains diversity especially among the species-rich herbaceous component that is vulnerable to shading when consumer behaviour is altered.

Canopy plant composition and structure of Cape subtropical dune thicket are predicted by the levels of fire exposure

Background

The subtropical dune thicket (hereafter "dune thicket") of the Cape Floristic Region experiences a wide range of fire exposure throughout the landscape, unlike other dry rainforest formations that rarely experience fire. We sought to determine how fire exposure influences species composition and the architectural composition of dune thicket.

Methods

We used multivariate analysis and diversity indices based on cover abundance of species to describe the species composition, architectural guild composition and structure of dune thicket sites subject to different levels of fire exposure, namely low (fire return interval of >100 years), moderate (fire return interval of 50-100 years), and high (fire return interval of 10-50 years).

Results

The diversity, cover abundance and architectural guild cover abundance of dune thicket canopy species were strongly influenced by the level of fire exposure such that each level was associated with a well-circumscribed vegetation unit. Dune thickets subject to low fire exposure comprises a floristically distinct, low forest characterized by shrubs with one-to-few upright stems (ca. 4-8 m tall) and a relatively small canopy spread (vertical growers). Of the 25 species in this unit, 40% were restricted to it. Dune thickets subject to moderate fire exposure had the highest abundance of lateral spreaders, which are multi-stemmed (ca. 3-6 m tall) species with a large canopy spread and lower stature than vertical growers. None of the 17 species found in this unit was restricted to it. Dune thickets subject to high fire exposure had the highest abundance of hedge-forming shrubs, these being low shrubs (ca. 0.6-1.4 m tall), with numerous shoots arising from an extensive system of below-ground stems. Of the 20 species in this unit, 40% were restricted to it. Multivariate analysis identified three floristic units corresponding to the three fire exposure regimes. Compositional structure, in terms of species and architectural guilds, was most distinctive for dune thickets subject to high and low fire exposure, while the dune thicket subject to moderate fire exposure showed greatest compositional overlap with the other units.

Conclusion

Fire exposure profoundly influenced the composition and structure of dune thicket canopy species in the Cape Floristic Region. In the prolonged absence of fire, the thicket is invaded by vertical-growing species that overtop and outcompete the multi-stemmed, laterally-spreading shrubs that dominate this community. Regular exposure to fire selects for traits that enable thicket species to rapidly compete for canopy cover post-fire via the prolific production of resprouts from basal buds below- and above-ground. The trade-off is that plant height is constrained, as proportionately more resources are allocated to below-ground biomass.

Tree-to-Shrub Shift Benefits the Survival of Quercus mongolica Fisch. ex Ledeb. at the Xeric Timberline

Woody species are either trees or shrubs, with the exception of approximately 9.2% “trub” species exhibiting both tree and shrub growth forms. Little is known thus far about the ecological importance of plant growth-form plasticity under a drying climate. Quercus mongolica Fisch. ex Ledeb., a dominant tree species of temperate forests in East Asia, typically shows tree-to-shrub growth-form shifts at the xeric timberline and is suitable to test whether growth-form plasticity can physiologically benefit plant drought acclimation. We quantified the tree architecture, drought stress, physiological drought tolerance, and nonstructural carbohydrates of Q. mongolica trees and shrubs on the opposite slope of the same mountain at the xeric timberline in July 2018. Compared with Q. mongolica trees on shady slopes, the shrubs on sunny slopes had less available water to use and were more severely threatened by drought. Moreover, the shrubs had stronger tolerance to drought but still tended to have less nonstructural carbohydrate accumulation. The tree-to-shrub shift effectively benefits the survival of Q. mongolica under a dry climate and may strongly contribute to forest dynamics and even fire regimes under climate drying, especially for sensitive ecosystems such as the xeric timberline, which will be vulnerable under future climate change scenarios.

Differences in Allometric Relationship of Two Dominant Woody Species Among Various Terrains in a Desert Region of Central Asia

The allometric relationship among different functional traits is an ecological strategy for plants to promote resource utilization, which indicates the ability of plants to adapt to environmental changes coordinately. In this study, we conducted a field survey on Haloxylon ammodendron and H. persicum among different terrains (dune crest, eastern slope, western slope and inter-dune) in the Gurbantunggut Desert, obtained their quantitative and morphological characteristics, and analyzed their allometric relationships between plant height and canopy radius, plant height and basal diameter by using standardized major axis estimation. We found that: (1) The dominated terrains of H. ammodendron and H. persicum were different; (2) The individual morphology of the two Haloxylon species changed significantly with the terrains (p < 0.05), with the largest and smallest ones growing on the eastern slope and the inter-dune lowland, respectively; (3) Fixed allometric patterns were observed in the above-ground parts of the two Haloxylon species, as the growth of canopy and basal stem was preferentially to plant height; (4) These allometric relationships were significantly affected by the terrain, and exhibited discrepancy between two species, they both invested less in plant height in windy habitats, such as the dune crest and western slope, but H. ammodendron growing on the western slope and H. persicum growing on the eastern slope invested more in basal diameter for strengthening mechanical support and resources acquisition, respectively. These results indicated that both studied species adopted an ecological strategy that allocating more resources to horizontal expansion rather than vertical growth, the terrain has an important influence on the allometric relationship of their above-ground parts, and the trade-off mechanism of main components investing was different for these two species due to habitat heterogeneity and ecological adaptability.

Disentangling biotic and abiotic drivers of intraspecific trait variation in woody plant seedlings at forest edges

In fragmented forests, edge effects can drive intraspecific variation in seedling performance that influences forest regeneration and plant composition. However, few studies have attempted to disentangle the relative biotic and abiotic drivers of intraspecific variation in seedling performance. In this study, we carried out a seedling transplant experiment with a factorial experimental design on three land-bridge islands in the Thousand Island Lake, China, using four common native woody plant species. At different distances from the forest edge (2, 8, 32, 128 m), we transplanted four seedlings of each species into each of three cages: full-cage, for herbivore exclusion; half-cage, that allowed herbivore access but controlled for caging artifacts; and no-cage control. In the 576 cages, we recorded branch architecture, leaf traits, and seedling survival for each seedling before and after the experimental treatment. Overall, after one full growing season, edge-induced abiotic drivers and varied herbivory pressure led to intraspecific variation in seedling performance, including trade-offs in seedling architecture and resource-use strategies. However, responses varied across species with different life-history strategies and depended on the driver in question, such that the abiotic and biotic effects were additive across species, rather than interactive. Edge-induced abiotic variation modified seedling architecture of a shade-tolerant species, leading to more vertical rather than lateral growth at edges. Meanwhile, increased herbivory pressure resulted in a shift toward lower dry matter investment in leaves of a light-demanding species. Our results suggest that edge effects can drive rapid directional shifts in the performance and intraspecific traits of some woody plants from early ontogenetic stages, but most species in this study showed negligible phenotypic responses to edge effects. Moreover, species-specific responses suggest the importance of interspecific differences modulating the degree of trait plasticity, implying the need to incorporate individual-level responses when understanding the impact of forest fragmentation on plant communities.

What drives grassland-forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture

Fire and frost represent two major hurdles for the persistence of trees in open grassy biomes and have both been proposed as drivers of grassland-forest boundaries in Africa.We assess the response of young tree seedlings, which represent a vulnerable stage in tree recruitment, to traumatic fire and frost disturbances.In a greenhouse experiment, we investigated how seedling traits predicted survival and resprouting ability in response to fire versus frost; we characterized survival strategies of seedlings in response to the two disturbances, and we documented how the architecture of surviving seedlings is affected by fire versus frost injury.Survival rates were similar under both treatments. However, different species displayed different levels of sensitivity to fire and frost. Seedling survival was higher for older seedlings and seedlings with more basal leaves. Survivors of a fire event lost more biomass than the survivors of a frost event. However, the architecture of recovered fire- and frost-treated seedlings was mostly similar. Seedlings that recovered from fire and frost treatments were often shorter than those that had not been exposed to any disturbance, with multiple thin branches, which may increase vulnerability to the next frost or fire event. Synthesis. Fire caused more severe aboveground damage compared with a single frost event, suggesting that fire is an important driver of tree distribution in these open grassland systems. However, the impact of repeated frost events may be equally severe and needs to be investigated. Also, woody species composition may be influenced by phenomena that affect the timing and frequency of seedling exposure to damage, as mortality was found to be dependent on seedling age. Therefore, changes in fire regime and climate are likely to result in changes in the composition and the structure of the woody components of these systems.

Drought-modulated allometric patterns of trees in semi-arid forests

Tree allometry in semi-arid forests is characterized by short height but large canopy. This pattern may be important for maintaining water-use efficiency and carbon sequestration simultaneously, but still lacks quantification. Here we use terrestrial laser scanning to quantify allometry variations of Quercus mongolica in semi-arid forests. With tree height (Height) declining, canopy area (CA) decreases with substantial variations. The theoretical CA-Height relationship in dynamic global vegetation models (DGVMs) matches only the 5^th percentile of our results because of CA underestimation and Height overestimation by breast height diameter (DBH). Water supply determines Height variation (P = 0.000) but not CA (P = 0.2 in partial correlation). The decoupled functions of stem, hydraulic conductance and leaf spatial arrangement, may explain the inconsistency, which may further ensure hydraulic safety and carbon assimilation, avoiding forest dieback. Works on tree allometry pattern and determinant will effectively supply tree drought tolerance studying and support DGVM improvements.

A unified framework for plant life-history strategies shaped by fire and herbivory

Fire and herbivory both remove aboveground biomass. Environmental factors determine the type and intensity of these consumers globally, but the traits of plants can also alter their propensity to burn and the degree to which they are eaten. To understand plant life-history strategies associated with fire and herbivory we need to describe both response and effect functional traits, and how they sort within communities, along resource gradients, and across evolutionary timescales. Fire and herbivore functional traits are generally considered separately, but there are advances made in understanding fire that relate to herbivory, and vice versa. Moreover, fire and herbivory interact: the presence of one consumer affects the type and intensity of the other. Here, we present a unifying conceptual framework to understand plant strategies that enable tolerance and persistence to fire and herbivory. Using grasses as an example, we discuss how flammability and fire tolerance, palatability, and grazing tolerance traits might organize themselves in ecosystems exposed to these consumers, and how these traits might have evolved with reference to other strong selective processes, like aridity. Our framework can be used to predict both the diversity of life-history strategies and plant species diversity under different consumer regimes.

The Variation Driven by Differences between Species and between Sites in Allometric Biomass Models

Background and Objectives: It is commonly assumed that allometric biomass models are species-specific and site-specific. However, the magnitude of species and site dependency in these models is not well-known. This study aims to investigate the variation in allometric models (i.e., aboveground biomass predicted by diameter at breast height and tree height) that has originated from the differences between tree species and between sites, thereby contributing to a better understanding of species and site-specificity issue in these models. Materials and Methods: The study is based on two large biomass datasets of 4921 and 5199 trees, from Eurasia and Canada. Using a nested ANOVA model on relative aboveground biomass residuals (with species and site as random effects), the proportion of variance explained by species or site was assessed by means of Variance Partition Coefficient (VPC). Results: The proportion of variance explained by species (VPCspecies = 42.56%, SE = 6.10% for Dataset 1 and VPCspecies = 47.54%, SE = 6.07% for Dataset 2) was larger than that explained by site (VPCsite = 20.08%, SE = 3.35% for Dataset 1 and VPCsite = 8.27%, SE = 1.38% for Dataset 2). The proportion of variance explained by site decreased by 24%–44% and the proportion of variance explained by species changed only slightly, when height is included in the allometric biomass models (i.e., models based on diameter at breast height alone, compared to models based on diameter at breast height and tree height). Conclusions: Allometric biomass models were more species-specific than they were site-specific. Therefore, the species (i.e., differences between species) seems to be a more important driver of variability in allometric models compared to site (i.e., differences between sites). Including height in allometric biomass models helped reduce the dependency of these models, on sites only.

The Effect of Tree Crown Allometry on Community Dynamics in Mixed-Species Stands versus Monocultures. A Review and Perspectives for Modeling and Silvicultural Regulation

Many recent studies have shown that the structure, density, and productivity of mixed-species stands can differ from the weighted mean of monospecific stands of the respective species. The tree and stand properties emerging by inter-specific neighborhood should be considered in models for understanding and practical management. A promising approach for this is a more realistic representation of the individual tree allometry in models and management concepts, as tree allometry determines many structural and functional aspects at the tree and stand level. Therefore, this paper is focused on the crown allometry in mixed and mono-specific stands. Firstly, we review species-specific differences in the crown allometry in monospecific stands. Secondly, we show how species-specific differences and complementarities in crown allometry can emerge in mixed-species stands. Thirdly, the consequences of allometric complementarity for the canopy packing density will be analyzed. Fourthly, we trace the crown allometry from the tree level to the stand density and show the relevance for the self-thinning in mixed versus monospecific stands. Fifth, the consequence of the findings for modeling and regulating tree and stand growth will be discussed. The review deals mainly with widespread even-aged, mono-layered stands, but the main results apply for more heterogeneous stands analogously.

How a measure of tree structural complexity relates to architectural benefit-to-cost ratio, light availability, and growth of trees

Aboveground tree architecture is neither fully deterministic nor random. It is likely the result of mechanisms that balance static requirements and light-capturing efficiency. Here, we used terrestrial laser scanning data to investigate the relationship between tree architecture, here addressed using the box-dimension (D b), and the architectural benefit-to-cost ratio, the light availability, and the growth of trees. We detected a clear relationship between D b and the benefit-to-cost ratio for the tested three temperate forest tree species (Fagus sylvatica L., Fraxinus excelsior L., and Acer pseudoplatanus L.). In addition, we could also show that D b is positively related to the growth performance of several tropical tree species. Finally, we observed a negative relationship between the strength of competition enforced on red oak (Quercus rubra L.) trees and their D b. We therefore argue that D b is a meaningful and integrative measure that describes the structural complexity of the aboveground compartments of a plant as well as its relation to structural efficiency (benefit-to-cost ratio), productivity, and growing conditions (competition or availability of light).

Biomechanical control of beech pole verticality (Fagus sylvatica) before and after thinning: theoretical modelling and ground-truth data using terrestrial LiDAR

PREMISE OF THE STUDY

Thinning is a frequent disturbance in managed forests, especially to increase radial growth. Due to buckling and bending risk associated with height and mass growth, tree verticality is strongly constrained in slender trees growing in dense forests and poor light conditions. Tree verticality is controlled by uprighting movements implemented from local curvatures induced by wood maturation stresses and/or eccentric radial growth. This study presents the first attempt to compare the real uprighting movements in mature trees using a theoretical model of posture control.

METHODS

Stem lean and curvature were measured by Terrestrial LiDAR Scanner (TLS) technology before and 6 years after thinning and compared to unthinned control poles. Measures for several tree and wood traits were pooled together to implement a widely used biomechanical model of tree posture control. Changes in observed stem lean were then compared with the model predictions, and discrepancies were reviewed.

KEY RESULTS

Even under a highly constrained environment, most control poles were able to counterbalance gravitational curvature and avoid sagging. Thinning stimulated uprighting movements. The theoretical uprighting curvature rate increased just after thinning, then slowed after 2 years, likely due to the stem diameter increase. The biomechanical model overestimated the magnitude of uprighting.

CONCLUSIONS

Most suppressed beech poles maintain a constant lean angle, and uprighting movements occur after thinning, indicating that stem lean is plastic in response to light conditions. Acclimation of posture control to other changes in growth condition should be investigated, and lean angles should be measured in forest inventories as an indicator of future wood quality.

Phylogeny of Schinus L. (Anacardiaceae) with a new infrageneric classification and insights into evolution of spinescence and floral traits

Schinus, best known by its few cultivated and invasive species, is the largest genus of Anacardiaceae in southern South America. It is remarkably diverse compared to closely related genera, with approximately 42 species, most of which occur in several arid vegetation types and extend into Andean and Atlantic moist forests. The most comprehensive taxonomic revision of the genus dates to 1957, recognizing S. subg. Schinus and S. subg. Duvaua, the latter of which were further divided into two sections. Subsequent studies have highlighted morphological inconsistencies in this infrageneric classification, and species delimitation remains a challenge. Schinus has been poorly sampled in previous phylogenetic studies of Anacardiaceae, and thus any assumptions about its monophyly and relationships remain untested. We investigated the phylogenetic relationships of 44 Schinus taxa and sampled 122 specimens, including the outgroup, using nine nuclear and two plastid DNA sequence regions, most of them developed recently for Commiphora (Burseraceae, sister to Anacardiaceae). We used maximum parsimony, maximum likelihood, and Bayesian inference to infer relationships among species. We also constructed a morphological dataset, including vegetative anatomical features, and compared these characters to hypotheses based on molecular evidence in order to achieve a better understanding of the relationships among the species of Schinus and to related genera, aiming also to identify morphological characters and putative synapomorphies for major clades, and to discuss hypotheses regarding the evolution of structural traits in the genus. Our analyses strongly support the monophyly of Schinus, but also indicate that S. subg. Schinus and the sections of S. subg. Duvaua are polyphyletic. The phylogenetic relationships that emerged from our analyses include eight relatively well-supported lineages, but relationships among closely related species remain unclear in some clades. Ancestral state reconstructions demonstrate that several morphological and leaf-anatomical characters are valuable in characterizing some lineages. By contrast, most of the traits that have traditionally been used to circumscribe groups in Schinus show high levels of homoplasy. In light of these results, we present a novel sectional classification of Schinus based on a combination of character states associated with geographic distribution, corresponding to lineages that are mostly allopatric or at least ecologically distinct.

Human impacts in African savannas are mediated by plant functional traits

Tropical savannas have a ground cover dominated by C₄ grasses, with fire and herbivory constraining woody cover below a rainfall-based potential. The savanna biome covers 50% of the African continent, encompassing diverse ecosystems that include densely wooded Miombo woodlands and Serengeti grasslands with scattered trees. African savannas provide water, grazing and browsing, food and fuel for tens of millions of people, and have a unique biodiversity that supports wildlife tourism. However, human impacts are causing widespread and accelerating degradation of savannas. The primary threats are land cover-change and transformation, landscape fragmentation that disrupts herbivore communities and fire regimes, climate change and rising atmospheric CO₂ . The interactions among these threats are poorly understood, with unknown consequences for ecosystem health and human livelihoods. We argue that the unique combinations of plant functional traits characterizing the major floristic assemblages of African savannas make them differentially susceptible and resilient to anthropogenic drivers of ecosystem change. Research must address how this functional diversity among African savannas differentially influences their vulnerability to global change and elucidate the mechanisms responsible. This knowledge will permit appropriate management strategies to be developed to maintain ecosystem integrity, biodiversity and livelihoods.

Competing consumers: contrasting the patterns and impacts of fire and mammalian herbivory in Africa

Fire and herbivory are the two consumers of above-ground biomass globally. They have contrasting impacts as they differ in terms of selectivity and temporal occurrence. Here, we integrate continental-scale data on fire and herbivory in Africa to explore (i) how environmental drivers constrain these two consumers and (ii) the degree to which each consumer affects the other. Environments conducive to mammalian herbivory are not necessarily the same as those conducive to fire, although their spheres of influence do overlap-especially in grassy ecosystems which are known for their frequent fires and abundance of large mammalian herbivores. Interactions between fire and herbivory can be competitive, facultative or antagonistic, and we explore this with reference to the potential for alternative ecosystem states. Although fire removes orders of magnitude more biomass than herbivory their methane emissions are very similar, and in the past, herbivores probably emitted more methane than fire. We contrast the type of herbivory and fire in different ecosystems to define 'consumer-realms'.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

A holistic approach to determine tree structural complexity based on laser scanning data and fractal analysis

The three‐dimensional forest structure affects many ecosystem functions and services provided by forests. As forests are made of trees it seems reasonable to approach their structure by investigating individual tree structure. Based on three‐dimensional point clouds from laser scanning, a newly developed holistic approach is presented that enables to calculate the box dimension as a measure of structural complexity of individual trees using fractal analysis. It was found that the box dimension of trees was significantly different among the tested species, among trees belonging to the same species but exposed to different growing conditions (at gap vs. forest interior) or to different kinds of competition (intraspecific vs. interspecific). Furthermore, it was shown that the box dimension is positively related to the trees’ growth rate. The box dimension was identified as an easy to calculate measure that integrates the effect of several external drivers of tree structure, such as competition strength and type, while simultaneously providing information on structure‐related properties, like tree growth.

Ecological and evolutionary legacy of megafauna extinctions

For hundreds of millions of years, large vertebrates (megafauna) have inhabited most of the ecosystems on our planet. During the late Quaternary, notably during the Late Pleistocene and the early Holocene, Earth experienced a rapid extinction of large, terrestrial vertebrates. While much attention has been paid to understanding the causes of this massive megafauna extinction, less attention has been given to understanding the impacts of loss of megafauna on other organisms with whom they interacted. In this review, we discuss how the loss of megafauna disrupted and reshaped ecological interactions, and explore the ecological consequences of the ongoing decline of large vertebrates. Numerous late Quaternary extinct species of predators, parasites, commensals and mutualistic partners were associated with megafauna and were probably lost due to their strict dependence upon them (co-extinctions). Moreover, many extant species have megafauna-adapted traits that provided evolutionary benefits under past megafauna-rich conditions, but are now of no or limited use (anachronisms). Morphological evolution and behavioural changes allowed some of these species partially to overcome the absence of megafauna. Although the extinction of megafauna led to a number of co-extinction events, several species that likely co-evolved with megafauna established new interactions with humans and their domestic animals. Species that were highly specialized in interactions with megafauna, such as large predators, specialized parasites, and large commensalists (e.g. scavengers, dung beetles), and could not adapt to new hosts or prey were more likely to die out. Partners that were less megafauna dependent persisted because of behavioural plasticity or by shifting their dependency to humans via domestication, facilitation or pathogen spill-over, or through interactions with domestic megafauna. We argue that the ongoing extinction of the extant megafauna in the Anthropocene will catalyse another wave of co-extinctions due to the enormous diversity of key ecological interactions and functional roles provided by the megafauna.

Long-term livestock exclusion facilitates native woody plant encroachment in a sandy semiarid rangeland

The role of livestock grazing in regulating woody cover and biomass in grass-dominant systems is well recognized. However, the way in which woody plant populations in respond when livestock are removed from grazing in the absence of other disturbances, such as fire, remains unclear.

We conducted a 10-year, replicated fencing experiment in a sandy semiarid rangeland in northern China (which has a mean annual rainfall of 365 mm), where fires have been actively suppressed for decades.

Fencing dramatically influenced the growth and age structure of the native tree species, Ulmus pumila, which is the sole dominant tree in the area. After a decade, the density of the U. pumila tree population in the fencing plots increased doubly and canopy cover increased triply. The proportion of both saplings (U₂) and young trees (U₃) increased in fencing plots but decreased in grazing plots after the 10-year treatment period. The effects of fencing on U. pumila trees varied by age class, with potential implications for the future structure of the U. pumila tree community. Decadal fencing led to approximately 80-fold increase in recruitment and a nearly 2.5-fold decrease in the mortality of both U₂ and U₃. Further, livestock grazing generated a “browsing trap” to the recruitment of both U₂ and U₃, and had a small impact on the mortality of old trees. A long-term, fencing-driven shift in woody species composition was mediated via its effects on both recruitment and mortality rates.

Synthesis and applications. Our results demonstrate that in the long-term absence of both fire and livestock, native woody plant encroachment tends to occur in sandy rangelands, transforming the woody plant demography in the process. The feasibility of full livestock exclusion in sandy rangelands requires further discussion. A balanced amount of livestock grazing may provide critical ecosystem services by regulating woody cover and mediating woody plant encroachment.

Do wide crowns in arid woodland trees reflect hydraulic limitation and reduction of self-shading?

In arid regions many tree species develop broad crowns. A number of hypotheses involve trade-offs between growth in height and horizontal spreading, but there is no explanation for the switch from vertical to horizontal growth during development. Using Acacia papyrocarpa Benth as a model, we measured tree height and crown shape across different sites and topographic positions. We also measured δ13C of phyllodes from crown tops and lateral spreading branches. Trees were significantly taller at the base of a hill, where water availability is typically greater, than on the adjacent steep hillslope. In contrast, δ13C from the treetops was not significantly different across this topographic gradient, despite variation in tree height. In addition, δ13C was higher at treetops than in lower, lateral branches. These observations are consistent with hydraulic limitation to tree height. The shape of mature and young crowns in open environments was not symmetrical. At all sites, branches were shortest, but tree crowns tallest, on south-facing (i.e. shadiest) aspect of crowns. This suggests that light limitation may also affect crown development. If upper branches become water-limited and lower branches light-limited, then middle lateral branches become the less-stressed part of the crown and may grow more, producing a broad crown.

Have plants evolved to self-immolate?

By definition fire prone ecosystems have highly combustible plants, leading to the hypothesis, first formally stated by Mutch in 1970, that community flammability is the product of natural selection of flammable traits. However, proving the "Mutch hypothesis" has presented an enormous challenge for fire ecologists given the difficulty in establishing cause and effect between landscape fire and flammable plant traits. Individual plant traits (such as leaf moisture content, retention of dead branches and foliage, oil rich foliage) are known to affect the flammability of plants but there is no evidence these characters evolved specifically to self-immolate, although some of these traits may have been secondarily modified to increase the propensity to burn. Demonstrating individual benefits from self-immolation is extraordinarily difficult, given the intersection of the physical environmental factors that control landscape fire (fuel production, dryness and ignitions) with community flammability properties that emerge from numerous traits of multiple species (canopy cover and litter bed bulk density). It is more parsimonious to conclude plants have evolved mechanisms to tolerate, but not promote, landscape fire.

Heavy browsing affects the hydraulic capacity of Ceanothus rigidus (Rhamnaceae)

Defoliation by herbivores can reduce carbon assimilation, change plant water relations, and even shift the biotic structure of plant communities. In this study, we took advantage of a long-term deer exclosure experiment to examine the consequences of persistent deer herbivory on plant water relations and the xylem structure-function relationships in Ceanothus rigidus, a maritime chaparral shrub in coastal California. Browsed plants had thicker stems with many intertwined short distal twigs, and significantly higher sapwood-to-leaf area ratios than their non-browsed counterparts. Leaf area-specific hydraulic conductivity was similar in both browsed and non-browsed plants, but xylem area-specific conductivity was significantly lower in the browsed plants. Vessel diameters were equivalent in both plant groups, but the number of vessels on a transverse area basis was nearly 40% lower in the browsed plants, accounting for their lower transport efficiency. Mid-day in situ water potentials and losses of hydraulic conductivity due to embolism were similar in both groups of plants but stomatal conductance was higher in the browsed shrubs in the early part of the growing season. We discuss our findings in the context of whole-plant ecophysiology, and explore the consequences of herbivory on hormonal signals, wood anatomy, and xylem function.

Tree species from different functional groups respond differently to environmental changes during establishment

Savanna plant communities change considerably across time and space. The processes driving savanna plant species diversity, coexistence and turnover along environmental gradients are still unclear. Understanding how species respond differently to varying environmental conditions during the seedling stage, a critical stage for plant population dynamics, is needed to explain the current composition of plant communities and to enable us to predict their responses to future environmental changes. Here we investigate whether seedling response to changes in resource availability, and to competition with grass, varied between two functional groups of African savanna trees: species with small leaves, spines and N-fixing associations (fine-leaved species), and species with broad leaves, no spines, and lacking N-fixing associations (broad-leaved species). We show that while tree species were strongly suppressed by grass, the effect of resource availability on seedling performance varied considerably between the two functional groups. Nutrient inputs increased stem length only of broad-leaved species and only under an even watering treatment. Low light conditions benefited mostly broad-leaved species' growth. Savannas are susceptible to ongoing global environment changes. Our results suggest that an increase in woody cover is only likely to occur in savannas if grass cover is strongly suppressed (e.g. by fire or overgrazing). However, if woody cover does increase, broad-leaved species will benefit most from the resulting shaded environments, potentially leading to an expansion of the distribution of these species. Eutrophication and changes in rainfall patterns may also affect the balance between fine- and broad-leaved species.

Allometric convergence in savanna trees and implications for the use of plant scaling models in variable ecosystems

Theoretical models of allometric scaling provide frameworks for understanding and predicting how and why the morphology and function of organisms vary with scale. It remains unclear, however, if the predictions of ‘universal’ scaling models for vascular plants hold across diverse species in variable environments. Phenomena such as competition and disturbance may drive allometric scaling relationships away from theoretical predictions based on an optimized tree. Here, we use a hierarchical Bayesian approach to calculate tree-specific, species-specific, and ‘global’ (i.e. interspecific) scaling exponents for several allometric relationships using tree- and branch-level data harvested from three savanna sites across a rainfall gradient in Mali, West Africa. We use these exponents to provide a rigorous test of three plant scaling models (Metabolic Scaling Theory (MST), Geometric Similarity, and Stress Similarity) in savanna systems. For the allometric relationships we evaluated (diameter vs. length, aboveground mass, stem mass, and leaf mass) the empirically calculated exponents broadly overlapped among species from diverse environments, except for the scaling exponents for length, which increased with tree cover and density. When we compare empirical scaling exponents to the theoretical predictions from the three models we find MST predictions are most consistent with our observed allometries. In those situations where observations are inconsistent with MST we find that departure from theory corresponds with expected tradeoffs related to disturbance and competitive interactions. We hypothesize savanna trees have greater length-scaling exponents than predicted by MST due to an evolutionary tradeoff between fire escape and optimization of mechanical stability and internal resource transport. Future research on the drivers of systematic allometric variation could reconcile the differences between observed scaling relationships in variable ecosystems and those predicted by ideal models such as MST.