Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance and the LL of several tree species in a seasonally dry tropical ecosystem. We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ ) for the most common species of the Brazilian semiarid, locally known as Caatinga. Leaf life span was negatively related to Ψ and positively related to S : G, which was negatively related to Ψ . This means that greater S : G increases LL by reducing Ψ . Lignin content was not correlated with any variable. We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S : G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.

Deciduous and evergreen trees are usually considered the main coexisting functional groups in seasonally dry tropical forests (SDTF). We compared leaf and stem traits of 22 woody species in the Brazilian Caatinga to investigate whether deciduous (DC) and evergreen (EV) species have divergent water-use strategies. Our hypothesis was that DC trees compensate for their short leaf longevity by being less conservative in water use and showing higher variation in the seasonal water potential after leaf shedding. Evergreen species should exhibit a highly conservative water use strategy, which reduces variations in seasonal water potential and the negative effects of desiccation. Our leaf dynamics results indicate that the crown area of DC trees is more sensitive to air and soil drought, whereas EV trees are only sensitive to soil drought. Deciduous species exhibit differences in a set of leaf traits confirming their acquisitive strategy, which contrasts with evergreen species. However, when stomatal traits are considered, we found that DC and EV have similar stomatal regulation strategies (partially isohydric). We also found divergent physiological strategies within DC. For high wood density DC, the xylem water potential (Ψ xylem ) continued to drop during the dry season. We also found a negative linear relationship between leaf life span (LL) and the transpiration rate per unit of hydraulic conductivity (Λ), indicating that species with high LL are less vulnerable to hydraulic conductivity loss than early-deciduous species. Collectively, our results indicate divergence in the physiology of deciduous species, which suggests that categorizing species based solely on their leaf phenology may be an oversimplification.

Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga.• We performed scanning electron, bright-field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan.• All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species.• We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

Lianas are abundant in seasonal tropical forests, where they avoid seasonal water stress presumably by accessing deep-soil water reserves. Although lianas are favoured in seasonal environments, their occurrence and abundance are low in semiarid environments. We hypothesized that lianas do not tolerate the great water shortage in the soil and air characteristic of semiarid environments, which would increase the risk of embolism. We compared the rooting depth of coarse roots, leaf dynamics, leaf water potential (ψ leaf ), embolism resistance (P 50 ) and lethal levels of embolism (P 88 ) between congeneric lianas that occur with different abundances in two semiarid sites differing in soil characteristics and vapour pressure deficit in the air (VPD air ). Regardless of soil texture and depth, water availability was restricted to the rainy season. All liana species were drought deciduous and had superficial coarse roots (not deeper than 35 cm). P 50 varied from À1.8 to À2.49 MPa, and all species operated under narrow safety margins against catastrophic (P 50 ) and irreversible hydraulic failure (P 88 ), even during the rainy season. In short, lianas that occur in semiarid environments have lower resistance to cavitation and limit carbon fixation to the rainy season because of leaf fall in the early dry season. We suggest that leaf shedding and shallow roots impairing carbon gain and growth in the dry season may explain why liana abundance is lower in semiarid than in other seasonally dry environments.

Xylem anatomical traits can provide insights regarding the mechanisms affecting the distribution of vascular plants across environmental gradients. In this study, we aimed to investigate the hypothesis that lianas occurring in semiarid environments have characteristics that maximize the xylem resistance to tension-induced cavitation along the root--stem-branch continuum. To gather information regarding the hydraulic architecture of the lianas, we analyzed several anatomical traits of wood: cross-sectional area occupied by the parenchyma (A P ), fibers (A F ), and vessels (A V ); average vessel diameter (d); wood density (WD); pit diameter in the vessel wall (d pit ); pit density (N pit ), and potential hydraulic conductivity (Kp) in branches, stems, and roots of three congeneric species of lianas that occur in two vegetation types of the semiarid regions of Brazil. We found that lianas in these semiarid environments possess a number of the following xylem traits that may allow resistance to tension-induced cavitation:1) lower vessel diameter, lower Kp, and higher hydraulic safety in roots and branches; 2) Dimorphic vessels, ensuring both hydraulic efficiency and safety; and 3) small diameter of pits (potentially associated with a decrease in the membrane area of the vessel pits). This suite of traits may provide insight into mechanisms allowing lianas to occur in semiarid environments.

Although the conservation of tropical biodiversity depends on protected areas, there is still a very large 'gap' of knowledge on the flora of Brazilian reserves, especially in the Northeast region of Brazil. Field and herbarium surveys of the phanerogamic flora of the Ubajara National Park, located on the Brazilian Northeast, were made and analyses on phenology and dispersal syndromes were performed. 418 taxa (213 trees and shrubs, 100 terrestrial herbs, 68 climbing plants, 33 sub-shrubs, two epiphytes, one hemiparasite and one aquatic herb) were recorded. The most representative families were: Fabaceae, Malvaceae, Asteraceae, Rubiaceae and Euphorbiaceae. The annual flowering / fruiting peak hypothesis was not fully confirmed, therefore, the forest may be an important food resource for the fauna all year long (especially in the moister region). Zoochory was the predominant dispersal syndrome in the moister area, whereas, autochory and anemochory together, predominated in the drier area.

The relative importance of different community assembly mechanisms varies depending on the environment. According to the stress-dominance hypothesis (SDH), assembly mechanisms range from strong abiotic filtering to competition as the environment becomes more favourable. Most evidence for the SDH comes from studies in gradients of conditions (i.e. abiotic environmental factors that influence the functioning of organisms but are not consumed by them). However, we hypothesized that in resource gradients, competition increases as abiotic filtering becomes stronger. To test our hypothesis, we set up eight plots at different sites along an abiotic severity gradient in the Brazilian semi-arid region (BSAR). In each plot, we identified and measured each woody plant species found, and we recorded 11 functional traits of the main species, dividing the traits into alpha (competition effects) and beta (abiotic filtering effects). We investigated the presence of phylogenetic signal in the traits, the community phylogenetic and phenotypic patterns, and associated the variation in these patterns with the availability of water and soil nutrients. We found phylogenetic signal for most (91%) of the traits analysed. The phylogenetic patterns varied from clustered in stressful sites to random or overdispersed in favourable sites, and we concluded that these phylogenetic patterns were the result of historical processes influencing community assembly in different environments in the BSAR. In general, the phenotypic patterns varied from clustered at the most stressful end to random at less stressful sites. Our results show that in resource gradients, any restriction of the resource (hydric or edaphic) intensifies abiotic filtering and, at the same time, increases the competitive hierarchy among species. On the other hand, stochastic processes seem to have a stronger influence under more favourable abiotic conditions, where abiotic filtering and competition are weaker. Thus, we conclude that the SDH is not supported in resource gradients.

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