Seasonal variation in leaf traits between congeneric savanna and forest trees in Central Brazil: implications for forest expansion into savanna

The ecology of forest and savanna trees species will largely determine the structure and dynamics of the forest–savanna boundaries, but little is known about the constraints to leaf trait variation imposed by selective forces and evolutionary history during the process of savanna invasion by forest species. We compared seasonal patterns in leaf traits related to leaf structure, carbon assimilation, water, and nutrient relations in 10 congeneric species pairs, each containing one savanna species and one forest species. All individuals were growing in dystrophic oxisols in a fire-protected savanna of Central Brazil. We tested the hypothesis that forest species would be more constrained by seasonal drought and nutrient-poor soils than their savanna congeners. We also hypothesized that habitat, rather than phylogeny, would explain more of the interspecific variance in leaf traits of the studied species. We found that throughout the year forest trees had higher specific leaf area (SLA) but lower integrated water use efficiency than savanna trees. Forest and savanna species maintained similar values of predawn and midday leaf water potential along the year. Lower values were measured in the dry season. However, this was achieved by a stronger regulation of stomatal conductance and of CO₂ assimilation on an area basis (A _area) in forest trees, particularly toward the end of the dry season. Relative to savanna trees, forest trees maintained similar (P, K, Ca, and Mg) or slightly higher (N) leaf nutrient concentrations. For the majority of traits, more variance was explained by phylogeny, than by habitat of origin, with the exception of SLA, leaf N concentration, and A _area, which were apparently subjected to different selective pressures in the savanna and forest environments. In conclusion, water shortage during extended droughts would be more limiting for forest trees than nutrient-poor soils.     

Productive leaf functional traits of Chinese savanna species

The river valleys in Southwest China are characterized by a dry?Chot climate and relatively rich soils, and host valley-type savannas that are dominated by deciduous species. However, little is known about the ecological adaptations of Chinese savanna plants to the local environments. We hypothesize that Chinese savanna species mainly possess a drought-avoiding strategy by having a deciduous leaf habit and have productive leaf traits. To test this hypothesis, we measured 26 anatomical, morphological, physiological, and chemical traits for 33 woody species from a valley savanna in Southwest China and compared them with the literature data of other dry and wet tropical tree species and a global dataset. We found that Chinese savanna species showed drought avoidance adaptations and exhibited productive leaf traits, such as thin and dense leaves with high ratio of palisade to spongy mesophyll, leaf nutrient concentrations and photosynthetic capacity. Correlations of photosynthetic capacity with N, P, and stomatal conductance across Chinese savanna species were consistent with global patterns reported for seed plants. However, the Chinese savanna species had consistently greater carbon gain at a given specific leaf area, N, P, and stomatal conductance, suggesting higher nutrient- and intrinsic water use efficiencies. These results suggest that paradoxically, Chinese savanna species are adapted to the stressful dry?Chot valley habitat by having productive leaves.     

Seasonal leaf dynamics across a tree density gradient in a Brazilian savanna

Interactions between trees and grasses that influence leaf area index (LAI) have important consequences for savanna ecosystem processes through their controls on water, carbon, and energy fluxes as well as fire regimes. We measured LAI, of the groundlayer (herbaceous and woody plants <1-m tall) and shrub and tree layer (woody plants >1-m tall), in the Brazilian cerrado over a range of tree densities from open shrub savanna to closed woodland through the annual cycle. During the dry season, soil water potential was strongly and positively correlated with grass LAI, and less strongly with tree and shrub LAI. By the end of the dry season, LAI of grasses, groundlayer dicots and trees declined to 28, 60, and 68% of mean wet-season values, respectively. We compared the data to remotely sensed vegetation indices, finding that field measurements were more strongly correlated to the enhanced vegetation index (EVI, r ²=0.71) than to the normalized difference vegetation index (NDVI, r ²=0.49). Although the latter has been more widely used in quantifying leaf dynamics of tropical savannas, EVI appears better suited for this purpose. Our ground-based measurements demonstrate that groundlayer LAI declines with increasing tree density across sites, with savanna grasses being excluded at a tree LAI of approximately 3.3. LAI averaged 4.2 in nearby gallery (riparian) forest, so savanna grasses were absent, thereby greatly reducing fire risk and permitting survival of fire-sensitive forest tree species. Although edaphic conditions may partly explain the larger tree LAI of forests, relative to savanna, biological differences between savanna and forest tree species play an important role. Overall, forest tree species had 48% greater LAI than congeneric savanna trees under similar growing conditions. Savanna and forest species play distinct roles in the structure and dynamics of savanna–forest boundaries, contributing to the differences in fire regimes, microclimate, and nutrient cycling between savanna and forest ecosystems.     

Allocation to leaf area and sapwood area affects water relations of co-occurring savanna and forest trees

Water availability is a principal factor limiting the distribution of closed-canopy forest in the seasonal tropics, suggesting that forest tree species may not be well adapted to cope with seasonal drought. We studied 11 congeneric species pairs, each containing one forest and one savanna species, to test the hypothesis that forest trees have a lower capacity to maintain seasonal homeostasis in water relations relative to savanna species. To quantify this, we measured sap flow, leaf water potential (Ψ_L), stomatal conductance (g _s), wood density, and Huber value (sapwood area:leaf area) of the 22 study species. We found significant differences in the water relations of these two species types. Leaf area specific hydraulic conductance of the soil/root/leaf pathway (G _t) was greater for savanna species than forest species. The lower G _t of forest trees resulted in significantly lower Ψ_L and g _s in the late dry season relative to savanna trees. The differences in G _t can be explained by differences in biomass allocation of savanna and forest trees. Savanna species had higher Huber values relative to forest species, conferring greater transport capacity on a leaf area basis. Forest trees have a lower capacity to maintain homeostasis in Ψ_L due to greater allocation to leaf area relative to savanna species. Despite significant differences in water relations, relationships between traits such as wood density and minimum Ψ_L were indistinguishable for the two species groups, indicating that forest and savanna share a common axis of water-use strategies involving multiple traits.     

元江干热河谷植物叶片解剖和养分含量特征

研究了元江干热河谷旱田植物(旱季可浇灌,水分较好)和山坡半萨王纳植被中(自然状况,水分较差)共20种的叶片形态解剖特征,以及7种山坡植物叶片养分含量特征.结果表明,山坡植物叶片比叶重大,气孔密度大,气孔长度小,海绵组织/栅栏组织的值小等.元江干热河谷山坡植物叶片养分含量低,1.3%＞Ca＞N＞K＞1%＞Mg＞P＞S.除氮元素外,其它元素种间差别1～3倍.与热带植物群落叶片养元素含量相比,热带雨林＞元江山坡植物＞东南亚沙地旱生林和巴西矮卡廷加群落,表明元江干热河谷植物叶片具有明显的旱生性形态解剖特征,且叶片养分含量也较低.     

Seasonal changes in hydraulic conductance, xylem embolism and leaf area in Eucalyptus tetrodonta and Eucalyptus miniata saplings in a north Australian savanna

Eucalypt saplings in north Australian savannas commonly die back, sometimes to ground level, during the 5 months of the long dry season. Water potentials are lower in saplings than large trees during the dry season, and we hypothesized that low water potentials may lead to high levels of xylem embolism and consequent death of branches and whole shoots. As the dry season progressed, hydraulic conductance of terminal branches decreased by 50% in Eucalyptus tetrodonta but not in Eucalyptus miniata saplings. Hydraulic conductance per leaf area decreased seasonally by 34% in E. tetrodonta branches. These decreases may be associated with the loss of leaves recorded from E. tetrodonta but not E. miniata branches. We modelled the effect of sequential loss of parallel resistors, representing petioles on a branch. This showed there is a non-linear decrease in flow as basal resistors are lost, which can lead to a decrease in mean flow per resistor due to increased mean path-length. Thus the observed loss of basal leaves, together with xylem embolism, probably contributed to the seasonal loss of hydraulic conductance in E. tetrodonta saplings. Loss of hydraulic conductance due to xylem embolism was generally low ( < 15%) in both species, and the seasonal increase in embolism could not fully account for the decline in hydraulic conductance of E. tetrodonta branches. There was little evidence that branch and shoot death was caused by these levels of embolism. Developing an embolism vulnerability curve for species with long vessels is problematic and this issue is discussed.     

Dry season watering alters the significance of climate factors influencing phenology and growth of saplings of savanna woody species in central Zambia,southern Africa

Although tree growth in southern African savannas is correlated with rainfall in the wet season, some studies have shown that tree growth is controlled more by rainfall in the dry season. If more rainfall occurred in the dry season in future climates, it would affect the growth of savanna trees, especially saplings that have shallower roots which limit access to subsoil water during the dry season when leaf flush and shoot extension occur. Recent paleobotanical evidence has revealed that there was relatively more precipitation in the dry season in eastern Africa in the Eocene than under the current climate. Saplings therefore can be expected to respond more to water addition during the dry season than mature trees that have more stored water and deeper roots that access subsoil water. Accordingly, I hypothesized that irrigation in the dry season should (i) advance the onset of the growing season, (ii) increase growth rates and (iii) alter the growth responses of saplings to climate factors. To test these hypotheses saplings of five savanna woody species were irrigated during the hot‐dry season at a site in central Zambia and their monthly and annual growth rates compared to those of conspecifics growing under control conditions. Although the responses among the species were variable, all irrigated saplings had significantly higher monthly and annual growth rates than control plants. In addition, dry season watering significantly altered the climatic determinants of sapling growth by either strengthening the role of the same climatic factors that were important under control conditions or displacing them altogether. In conclusion, more precipitation during the hot‐dry season is likely to have significant positive effects on sapling growth and consequently reduce the sapling‐tree transition periods and promote future tree population recruitment in some southern African savanna tree species.     

Evidence for facultative deciduousness in Colophospermum mopane in semi‐arid African savannas

Leaf phenology dictates the time available for carbon assimilation, transpiration and nutrient uptake in plants. Understanding the environmental cues that control phenology is therefore vital for predicting climate‐related changes to plant and ecosystem function. In contrast to temperate systems, and to a lesser degree, tropical forest systems, the cues initiating leaf drop in tropical savannas are poorly studied. We investigated the cues for leaf fall in a tropical monodominant arid savanna species, Colophospermum mopane, using an irrigation experiment. We tracked soil moisture, solar radiation, air temperature, leaf water status, leaf health and leaf carbon balance through the dry season in both irrigated and control plants. Water was the primary cue driving leaf loss of C. mopane rather than temperature or light. Trees watered throughout the dry season retained their canopies. These leaves remained functional and continued photosynthesis throughout the dry season. Leaf carbon acquisition rates did not decline with leaf age but were affected by soil moisture availability and temperature. Leaf loss did not occur when leaf carbon gain was zero, or when a particular leaf carbon threshold was reached. Colophospermum mopane is facultatively deciduous as water availability determines leaf drop in this widespread arid savanna species. Obligate deciduosity is not the only successful strategy in climates with a long dry season.     

Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G_s) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day^-1 and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. G_s declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G_s and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.     

The distribution and inversion polymorphism of chromosomally recognized taxa of the Anopheles gambiae complex in Mali, West Africa

Data from polytene chromosome studies on the Anopheles gambiae complex in Mali were reviewed. The banding pattern was successfully scored in 17,705 specimens from 76 sampling sites representing the main ecological strata of the country. Two members of the complex, namely An. arabiensis and An. gambiae, were found widespread and frequently sympatric, with the latter prevalent in most localities. Population genetic analysis of the inversion polymorphisms indicated the existence of panmictic conditions for An. arabiensis only, whereas the parallel study of An. gambiae supported its splitting into at least three reproductive units, characterized by different 2R chromosome arrangements, designated Bamako, Mopti and Savanna. The chromosomal evidence was consistent with the hypothesis of complete reproductive isolation between Bamako and Mopti. Partial isolation between these two taxa and Savanna was suggested by the scoring of hypothetical hybrid 2R heterokaryotypes in various samples, but the actual hybrid origin of these specimens was not confirmed. Different patterns of geographical and seasonal distribution were shown as follows. An. arabiensis prevails in arid savannas (Sahel and Northern Sudan savanna) out of the flooded or irrigated zones; it is able to withstand the most arid conditions of Saharan localities and its breeding might extend throughout the dry season. An. gambiae Savanna and Bamako prevail in relatively humid savannas (Southern Sudan savanna) and their breeding generally occurs only during the rainy season. The Savanna taxon was almost absent in flooded or irrigated zones and in riverine localities; the Bamako taxon is distributed along the upper river Niger and its tributaries. An. gambiae Mopti extends its range in all ecological zones present in Mali including the Sahel and predesertic areas, showing high relative frequencies up to absolute dominance in flooded or irrigated areas; its breeding is highly successful also during the dry season. Rainfall at the sampling sites was found to correlate positively with the frequency of Savanna and negatively with the frequency of Mopti. The remarkable ecological flexibility of the latter was found associated with wide seasonal and geographical variations in its 2R inversion polymorphism bc/u. Higher frequencies of the bc arrangement were recorded both in the Southern localities during the dry season and in the Northern more arid localities during the rainy season. The absence or scarcity of An. arabiensis and An. gambiae Savanna in most flooded or irrigated zones suggests their competitive exclusion by An. gambiae Mopti.     

Post‐Establishment Seedling Success in the Brazilian Cerrado: A Comparison of Savanna and Forest Species1

Due to frequent fire, low nutrient availability, and prolonged drought, tropical savanna is a stressful environment for the survival and growth of woody plant seedlings. To understand why forest species do not succeed in this environment while savanna species are able to persist, the effects of fire and woody cover on seedlings of these two functional groups were investigated in the Brazilian Cerrado. Seedlings were established in experimental plots under three densities of woody cover, in sites protected from fire and sites to be subjected to fire. There was a clear difference in the ability of savanna and forest species to survive fire. None of the three forest species were able to survive fire during the first two years of life, whereas eight of the nine savanna species were able to resprout following fire. The small seed size of the ninth savanna species, Miconia albicans, predisposed its seedlings to be sensitive to fire, because there was a strong positive correlation between seed size and survivorship. Savanna species were less dependent on woody cover than were forest species, which exhibited higher growth and survival under tree canopies than in open grassland. The low rates of establishment and survival of forest trees in savanna, combined with high sensitivity to fire, appear sufficient to prevent the expansion of forest into savanna under current fire regimes in the Cerrado.     

Seasonal patterns of leaf production in co‐occurring trees with contrasting leaf phenology: time and quantitative divergences

Neotropical savannas (‘cerrados’) of Central Brazil are characterized by the coexistence of a large diversity of tree species with divergent phenological behaviors, which reflect a great diversity in growth strategies. In the present study time behavior and quantitative aspects of shoot growth, shoot mortality, and leaf longevity and production were analyzed in 12 woody species of contrasting leaf phenology, adopting a functional group approach where 12 species were categorized into three functional groups: evergreen, decidous and brevideciduous, according to their leaf phenology. Shoot growth and leaf production were seasonal for the three functional groups, differing in their time of occurrence, but being concentrated during the last months of the dry season. Shoot growth differed between evergreens and deciduous, as well leaf production. Evergreens had higher rates of shoot growth, produced a higher number of leaves and had longer leaf longevity (around 500 days against 300 days in deciduous and brevideciduous). Leaf longevity was associated with patterns of leaf production when accounting for all phenological groups studied. It was possible to identify different patterns of aerial growth in savanna phenological groups, providing evidence of great functional variability amongst the groups studied.     

季节性干旱对白皮乌口树(Tarenna depauperata Hutchins)水分状况、叶片光谱特征和荧光参数的影响

干热河谷稀树灌丛常绿植物能够忍受长达半年以上的季节性干旱胁迫,但对这些常绿植物响应干旱胁迫的生理生态机制研究很少。本研究以干热河谷稀树灌丛优势常绿植物白皮乌口树(Tarenna depauperata Hutchins)为研究对象,分别在雨季和干季测定其叶片的水势、压力-体积曲线、气体交换参数、叶片光谱特征以及叶绿素荧光和P700的光能分配。结果显示:受严重季节性干旱胁迫的影响,与雨季相比,干季的凌晨叶片水势(Ψpd)下降至-4.5 MPa,水分传导的叶比导率(KL)下降了49.5%,叶绿素反射指数(NDVI)下降了40.6%,花青素反射指数(ARI)上升至0.074(约为雨季的12.3倍),并且雨季和干季的叶片水势、水分传导效率、叶绿素含量和花青素含量均差异显著(P0.05)。与雨季相比,干旱导致光系统Ⅱ(PSⅡ)最大光化学量子效率(Fv/Fm)显著下降至0.72(P0.05),即PSⅡ发生光抑制,而光系统Ⅰ(PSⅠ)的活性(Pm)未发生明显变化;干季叶片的最大非光化学淬灭(NPQ)增加了31%,而激发的最大环式电子传递速率(CEF)下降了66%。表明长期干旱胁迫使CEF的激发受到强烈抑制,即光能捕获效率的降低和NPQ的增强促进了白皮乌口树在长期干旱胁迫下的光保护。     

The water relations of two evergreen tree species in a karst savanna

The ecohydrology of karst has not received much attention, despite the disproportionally large contribution of karst aquifers to freshwater supplies. Karst savannas, like many savannas elsewhere, are encroached by woody plants, with possibly negative consequences on aquifer recharge. However, the role of savanna tree species in hydrological processes remains unclear, not least because the location and water absorption zones of tree roots in the spatially complex subsurface strata are unknown. This study examined the water sources and water relations of two savanna trees, Quercus fusiformis (Small) and Juniperus ashei (Buchholz) in the karst region of the eastern Edwards Plateau, Texas (USA). Stable isotope analysis of stem water revealed that both species took up evaporatively enriched water during the warm season, suggesting a relatively shallow water source in the epikarst, the transition zone between soil and bedrock. Q. fusiformis had consistently higher predawn water potentials than J. ashei during drought, and thus was probably deeper-rooted and less capable of maintaining gas exchange at low water potentials. Although the water potential of both species recovered after drought-breaking spring and summer rain events, associated shifts in stem water isotope ratios did not indicate significant uptake of rainwater from the shallow soil. A hypothesis is developed to explain this phenomenon invoking a piston-flow mechanism that pushes water stored in macropores into the active root zones of the trees. Epikarst structure varied greatly with parent material and topography, and had strong effects on seasonal fluctuations in plant water status. The study suggests that tree species of the Edwards Plateau do not commonly reduce aquifer recharge by tapping directly into perched water tables, but more likely by reducing water storage in the epikarst. A more general conclusion is that models of savanna water relations based on Walter's two-layer model may not apply unequivocally to karst savannas.     

Cascading biodiversity and functional consequences of a global change–induced biome switch

Aim At a regional scale, across southern Africa, woody thickening of savannas is becoming increasingly widespread. Using coupled vegetation and faunal responses (ants), we explore whether major changes in woody cover in savannas represent an increase in the density of savanna trees (C₄ grass layer remains intact) or a ‘regime shift’ in system state from savanna to thicket (=dry forest) where broad‐leaved, forest‐associated trees shade out C₄ grasses. Location Hluhluwe Game Reserve, South Africa. Methods We sampled paired open (low woody cover) and closed (high cover that have undergone an increase in tree density) sites. Vegetation was sampled using belt transects, and a combination of pitfall trapping and Winkler sampling was used for ants. Results Closed habitats did not simply contain a higher density of woody savanna species, but differed significantly in structure, functional composition (high prevalence of broad‐leaved trees, discontinuous C₄ grasses) and system properties (e.g. low flammability). Ant assemblage composition reflected this difference in habitat. The trophic structure of ant assemblages in the two habitats revealed a functional shift with much higher abundances of predatory species in the closed habitat. Main conclusions The predominance of species with forest‐associated traits and concomitant reduction of C₄ grasses in closed sites indicate that vegetation has undergone a shift in fundamental system state (to thicket), rather than simply savanna thickening. This biome shift has cascading functional consequences and implications for biodiversity conservation. The potential loss of many specialist savanna plant species is especially concerning, given the spatial extent and speed of this vegetation switch. Although it is not clear how easily the habitat switch can be reversed and how stable the thicket habitats are, it is likely in the not‐too‐distant future that conservation managers will be forced to make decisions on whether to actively maintain savannas.     

Woody plant phenology in the West Africa savanna

In the savanna of West Africa the seasonality of rainfall, with a drought period of at least four months, strongly influences the vegetation. Rainfall is a very critical abiotic variable and therefore plant species must be well adapted to survive in this habitat.
In our research, phenological patterns of 120 woody plant species have been examined based on the presence of green leaves. According to the patterns found, these species can be classified in phenological groups, which represent different strategies for survival. Two extreme strategies are found to resist drought: (1) by using the waterstorage in the deeper soil layers and river beds and by restricting drought-damage through scleromorphic features, and (2) by avoiding the drought through foliage shedding in the dry period.
The first strategy is represented by the riparian and upland evergreens , and the semi-evergreens . The evergreens bear leaves the whole year, gradually replacing old leaves by new ones. The riparian evergreens are strictly bound to riverbeds and grow in or immediately adjacent to them. The semi-evergreens shed their leaves and start sprouting during a short period (one-two weeks) once a year. Because the evergreens and the semi-evergreens are in leaf in the dry period they have to protect themselves to drought damage by scleromorphic features.
Contrary to these species are the deciduous species which are bare for at least some months per year. When the dry season starts their leaves dry out and are subsequently shed. They start sprouting before or at the beginning of the first rains. Although much less in number, some deciduous trees also have scleromorphic features to resist drought-damage. The strategy of sprouting just before the rainy season begins indicates that certain water resources remain available to these deep-rooting woody plants throughout the year, providing them with a fully operating photosynthetic apparatus when favourable conditions arrive.     

Leaf functional traits of Neotropical savanna trees in relation to seasonal water deficit

The seasonal savannas (cerrados) of Central Brazil are characterized by a large diversity of evergreen and deciduous trees, which do not show a clear differentiation in terms of active rooting depth. Irrespective of the depth of the root system, expansion of new foliage in deciduous species occurs at the end of the dry season. In this study, we examined a suite of leaf traits related to C assimilation, water and nutrients (N, P) in five deciduous and six evergreen trees that were among the dominant families of cerrado vegetation. Maximum CO₂ assimilation on a mass basis (A_mass) was significantly correlated with leaf N and P, and specific leaf area (SLA; leaf area per unit of leaf mass). The highest leaf concentrations of both nutrients were measured in the newly mature leaves of deciduous species at the end of the dry period. The differences in terms of leaf N and P between evergreen and deciduous species decreased during the wet season. Deciduous species also invested less in the production of non-photosynthetic leaf tissues and produced leaves with higher SLA and maintained higher water use efficiency. Thus, deciduous species compensated for their shorter leaf payback period by maintaining higher potential payback capacity (higher values of A_mass) and lower leaf construction costs (higher SLA). Their short leafless period and the capacity to flush by the end of the dry season may also contribute to offset the longer payback period of evergreen species, although it may involve the higher cost of maintaining a deep-root system or a tight control of plant water balance in the shallow-rooted ones.     

Expanding our understanding of leaf functional syndromes in savanna systems: the role of plant growth form

The assessment of leaf strategies has been a common theme in ecology, especially where multiple sources of environmental constraints (fire, seasonal drought, nutrient-poor soils) impose a strong selection pressure towards leaf functional diversity, leading to inevitable tradeoffs among leaf traits, and ultimately to niche segregation among coexisting species. As diversification on leaf functional strategies is dependent on integration at whole plant level, we hypothesized that regardless of phylogenetic relatedness, leaf trait functional syndromes in a multivariate space would be associated with the type of growth form. We measured traits related to leaf gas exchange, structure and nutrient status in 57 coexisting species encompassing all Angiosperms major clades, in a wide array of plant morphologies (trees, shrubs, sub-shrubs, herbs, grasses and palms) in a savanna of Central Brazil. Growth forms differed in mean values for the studied functional leaf traits. We extracted 4 groups of functional typologies: grasses (elevated leaf dark respiration, light-saturated photosynthesis on a leaf mass and area basis, lower values of leaf Ca and Mg), herbs (high values of SLA, leaf N and leaf Fe), palms (high values of stomatal conductance, leaf transpiration and leaf K) and woody eudicots (sub-shrubs, shrubs and trees; low SLA and high leaf Ca and Mg). Despite the large range of variation among species for each individual trait and the independent evolutionary trajectory of individual species, growth forms were strongly associated with particular leaf trait combinations, suggesting clear evolutionary constraints on leaf function for morphologically similar species in savanna ecosystems.     

Micro-site effects of trees and shrubs in dry savannas

Abstract. The physiognomy of dry savannas is described as a combination of discontinuous woody perennials and a continuous grassland matrix. Interactions between these two components are of vital importance for the persistence of a savanna landscape. Earlier savanna models have emphasized competitive interactions for water between the two components. Recent studies have argued that small-scale facilitating interactions between woody perennials and the herbaceous understorey are also important. This phenomenon has been given little theoretical consideration in the savanna literature, but it has been an important topic in agroforestry and arid-grassland ecology. This paper reviews some of the evidence for micro-site effects of trees and shrubs, and attempts to integrate their interactions with the surrounding open grassland. Woody perennials modify the microclimate by interception of solar radiation and rainfall. Their root systems extract nutrients horizontally and vertically, which are concentrated in the sub-canopy soil from litter decomposition and root turnover. Legumes are abundant in dry savannas, and may have symbiotic relationships with Rhizobium bacteria. This symbosis increases the availability of nitrogen in the soil. Isolated trees and shrubs initiate feedback mechanisms in their interactions with other organisms, and contribute to an uneven distribution of water and nutrients in dry savanna. This influences the species composition, and community diversity. Small-scale facilitating interaction between the woody and herbaceous components and competitive interaction on larger scales, are complementary processes which together explain a dynamic coexistence.     

Patterns in leaf traits of leguminous and non-leguminous dominant trees along a rainfall gradient in Ghana

Aims Both dominance distribution of species and the composition of the dominant species determine the distribution of traits within community. Leaf carbon (C) and nitrogen (N) isotopic composition are important leaf traits, and such traits of dominant species are associated with ecosystem C, water and N cycling. Very little is known how dominant species with distinct traits (e.g. N-fixing leguminous and non-leguminous trees) mediate resource utilization of the ecosystems in stressful environment.Methods Leaves of 81 dominant leguminous and non-leguminous trees were collected in forest (moist semi-deciduous and dry semi-deciduous ecosystems) and savanna (costal savanna, Guinean savanna and west Sudanian savanna ecosystems) areas and the transitional zone (between the forest and the savanna) along the transect from the south to the north of Ghana. We measured leaf traits, i.e. leaf δ 13 C, leaf δ 15 N, leaf water content, leaf mass per area (LMA) and C and N concentration. Correlation analyses were used to examine trait–trait relationships, and relationships of leaf traits with temperature and precipitation. We used analysis of covariance to test the differences in slopes of the linear regressions between legumes and non-legumes.Important findings Leaf δ 13 C, δ 15 N, leaf water content and LMA did not differ between leguminous and non-leguminous trees. Leaf N concentration and C:N ratio differed between the two groups. Moreover, leaf traits varied significantly among the six ecosystems. δ 13 C values were negatively correlated with annual precipitation and positively correlated with mean annual temperature. In contrast, leaf δ 15 N of non-leguminous trees were positively correlated with annual precipitation and negatively correlated with mean annual temperature. For leguminous trees, such correlations were not significant. We also found significant coordination between leaf traits. However, the slopes of the linear relationships were significantly different between leguminous and non-leguminous trees. Our results indicate that shifts in dominant trees with distinct water-use efficiency were corresponded to the rainfall gradient. Moreover, leguminous trees, those characterized with relative high water-use efficiency in the low rainfall ecosystems, were also corresponded to the relative high N use efficiency. The high proportion of leguminous trees in the savannas is crucial to mitigate nutrient stress.