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The exploration of unexploited geothermal resources is required to encourage the use of renewable energy. This study focuses on La Jolla beach, Ensenada, Mexico. The beach hosts a thermal anomaly with temperatures of up to 52 °C at the surface and up to 93 °C at 20 cm depth. The objectives were to: map the thermal anomaly, understand the impact of tides, quantify the thermal water discharge rate and heat output, and discuss a direct use of the energy. The mapping was performed with Unmanned Aerial Vehicles equipped with optical and thermal cameras at two different dates. Additional temperature measurements were performed with a thermocouple, while the total fluid discharge was estimated from flow measurements. A comparison between the campaigns indicated that the highest surface temperature area was more than three times larger in 2019 than in 2018 (259 m2 vs. 69 m2). Such change was due to the tidal range and associated hydrostatic pressure variations. The total thermal water discharge is 330 ± 44 L s−1, which corresponds to an advective heat output of 40.5 ± 5.2 MWt. The use of this energy in a Multi-Effect Distillation desalinization plant can contribute to cover the shortage of freshwater in Ensenada.

The drought susceptibility of woody saplings may explain their low survival in arid environments. Therefore, it is critical to determine which morphological and physiological traits are more responsive to drought among young plants. This study tested whether plant responses to experimental drought differ between two plant functional groups: the deciduous and evergreen species. We predicted that deciduous species would present a tighter stomatal control under drought, coupled with fast carbon fixation under no stress, tending toward isohydry and faster growth rates than the evergreen species. Using 1-year-old saplings from three evergreen and four deciduous Sonoran Desert tree species, we evaluated their hydraulic and gas exchange traits under three experimental irrigation conditions: high, intermediate and low water availability. We measured CO2 assimilation rates (A), stomatal conductance (gs), the level of iso-anisohydry (as the plant’s ability to maintain constant their water potential) and seven morphological and growth-related traits throughout 2 months. Under high water availability, saplings reached their maximum values of A and gs, which were significantly higher for deciduous than evergreen species. Correlations among hydroscape area (HA) and leaf traits positioned species along the iso/anisohydric continuum. Deciduous species presented isohydric characteristics, including low HA, high gs, A and Huber values (HVs), and traits indicative of a faster use of resources, such as low stem-specific density (SSD) and low leaf mass per area (LMA). By contrast, evergreen species showed traits that indicate slow resource use and anisohydric behavior, such as high HA, SSD and LMA, and low gs, A and HVs. Deciduous species drastically reduced gas exchange rates in response to drought, while evergreen maintained low rates independently of drought intensity. Overall, desert saplings showed strategies concordant with the iso-anisohydric continuum and the fast–slow use of resources.

In urban Mediterranean ecosystems, invasive alien plants should have characteristics that enable faster resource acquisition and utilization than native species. We tested this hypothesis by comparing stem and leaf functional traits from five abundant native woody shrubs and five of the most abundant alien species within an urban coastal scrub community in Ensenada, Baja California, Mexico. Twelve anatomical and ecophysiological traits were studied: light-saturated CO2 assimilation rate, stomatal conductance, stem and leaf water potentials during transpiration, % stem allocation to bark, xylem and pith, stem xylem vessel diameter and density, leaf size, and % allocation to spongy and palisade mesophyll. Alien species varied more in these traits, and their trait combinations reflected less investment in long-lived tissues; they had higher CO2 assimilation and stomatal conductance rates, higher values of leaf and stem water potential, and higher allocation to pith, larger stem xylem vessels, and larger leaf areas. In contrast, native woody shrub species were relatively convergent in leaf and stem traits reflecting more conservative resource use. Within disturbed urban environments, alien species may outcompete the native stress-tolerant species through rapid resource acquisition, enhanced by a broader set of functional trait combinations across species.

Owing to periodic inundation, wetland plants may have adapted to minimize leaf gas exchange impairments. Increasing the leaf cuticle surface roughness might contribute to buffer flooding stress, since it might increase both leaf water repellency (i.e. leaf hydrophobicity) and the presence of gas films. Thus, a higher leaf hydrophobicity, as well as an increased cuticle roughness, can be expected in species subject to continuous water-logging. We tested this hypothesis by: 1) measuring the contact angle in the liquid-solid interface on leaves of 52 plant species from three coastal wetlands with different diversity and flooding regime in Baja California, Mexico, and 2) by performing scanning electron microscopic analyses in the most hydrophilic and hydrophobic species. Contrary to our expectations, the highest values of leaf water repellency were found in species from non-flooded areas adjacent to the wetlands, whereas neutral leaf water repellency values were found in species subject to flooding, as indicated by the contact angle measurements (77 to 100°). The most water repellent species presented a striking layer of wax on top of the leaf cuticles, while the lest water repellent species were mostly glabrous, which suggest that the augmentation of wax covering in the cuticle might be responding to other environmental factors, such as irradiation.

Leaf density links the resistance to mechanical and hydraulic stress in Mediterranean shrubs as it is associated with the water potential at turgor loss and the moduli of elasticity and strength. Understanding the patterns of hydraulic and mechanical trait variation in vascular plants is critical to predicting species’ stress tolerance. Although previous work has shown that hydraulic and mechanical traits are decoupled in stems, there is little information available for leaves, which are organs more diversified in structure, function, and possibly drought tolerance strategies across habitats. We tested for coordination between leaf hydraulic traits related to drought tolerance and the mechanical resistance of leaves, for 17 shrub species from the arid and semiarid vegetation of the California Floristic Province. Bayesian and phylogenetic correlations showed that across species, hydraulic and mechanical traits both had strong associations with the water potential at turgor loss, and with leaf tissue density. However, leaf maximum hydraulic conductance and the water potential at 50% and 80% loss of hydraulic conductance were statistically independent of two key mechanical traits, the leaf modulus of elasticity and leaf structural strength. Our results suggest that leaf biomechanical traits, which reflect construction costs and contribute to leaf longevity, are decoupled from hydraulic capacity and safety. The independence of hydraulic and mechanical protection in leaves enables a wide range of trait combinations in leaves, which would increase their adaptive potential across ecosystems.

BackgroundIn arid and semiarid shrublands, water availability directly influences ecosystem properties. However, few empirical tests have determined the association between particular soil and hydrology traits with biodiversity and ecosystem biomass at the local scale.MethodsWe tested if plant species richness (S) and aboveground biomass (AGB) were associated with soil and topographic properties on 36 plots (ca. 12.5 m2) in 17 hectares of chaparral in the Mediterranean-climate of Valle de Guadalupe, Baja California, México. We used close-to-the-ground aerial photography to quantify sky-view cover per species, including all growth forms. We derived an elevation model (5 cm) from other aerial imagery. We estimated six soil properties (soil water potential, organic matter content, water content, pH, total dissolved solids concentration, and texture) and four landscape metrics (slope, aspect, elevation, and topographic index) for the 36 plots. We quantified the biomass of stems, leaves, and reproductive structures, per species.Results86% of AGB was in stems, while non-woody species represented 0.7% of AGB but comprised 38% ofS(29 species). Aboveground biomass and species richness were unrelated across the landscape.Swas correlated with aspect and elevation (R= 0.53, aspectP= 0.035, elevationP= 0.05), while AGB (0.006–9.17 Kg m-2) increased with soil water potential and clay content (R= 0.51,P= 0.02, andP= 0.04). Only three species (11% of totalS) occupied 65% of the total plant cover, and the remaining 26 represented only 35%. Cover was negatively correlated withS(R= -0.38,P= 0.02). 75% of AGB was concentrated in 30% of the 36 plots, and 96% of AGB corresponded to only 20% of 29 species.DiscussionAt the scale of small plots in our studied Mediterranean-climate shrubland in Baja California, AGB was most affected by soil water storage. AGB and cover were dominated by a few species, and only cover was negatively related toS.Swas comprised mostly by uncommon species and tended to increase as plant cover decreased.

Plants from arid environments display covarying traits to survive or resist drought. Plant drought resistance and ability to survive long periods of low soil water availability should involve leaf phenology coordination with leaf and stem functional traits related to water status. This study tested correlations between phenology and functional traits involved in plant water status regulation in 10 Sonoran Desert tree species with contrasting phenology. Species seasonal variation in plant water status was defined by calculating their relative positions along the iso/anisohydric regulation continuum based on their hydroscape areas (HA)—a metric derived from the relationship between predawn and midday water potentials—and stomatal and hydraulic traits. Additionally, functional traits associated with plant water status regulation, including lamina vessel hydraulic diameter (DHL), stem-specific density (SSD) and leaf mass per area (LMA) were quantified per species. To characterize leaf phenology, leaf longevity (LL) and canopy foliage duration (FD) were determined. Hydroscape area was strongly correlated with FD but not with leaf longevity (LL); HA was significantly associated with SSD and leaf hydraulic traits (DHL, LMA) but not with stem hydraulic traits (vulnerability index, relative conductivity); and FD was strongly correlated with LMA and SSD. Leaf physiological characteristics affected leaf phenology when it was described as canopy FD better than when described as LL. Stem and leaf structure and hydraulic functions were not only relevant for categorizing species along the iso/anisohydric continuum but also allowed identifying different strategies of desert trees within the ‘fast–slow’ plant economics spectrum. The results in this study pinpoint the set of evolutionary pressures that shape the Sonoran Desert Scrub physiognomy.

Salinity tolerance in plant species varies widely due to adaptation and acclimation processes at the cellular and whole-plant scales. In mangroves, extreme substrate salinity induces hydraulic failure and ion excess toxicity and reduces growth and survival, thus suggesting a potentially critical role for physiological acclimation to salinity. We tested the hypothesis that osmotic adjustment, a key type of plasticity that mitigates salinity shock, would take place in coordination with declines in whole-plant hydraulic conductance in a common garden experiment using saplings of three mangrove species with different salinity tolerances (Avicennia germinans L., Rhizophora mangle L. and Laguncularia racemosa (L.) C.F. Gaertn., ordered from higher to lower salinity tolerance). For each mangrove species, four salinity treatments (1, 10, 30 and 50 practical salinity units) were established and the time trajectories were determined for leaf osmotic potential (Ψs), stomatal conductance (gs), whole-plant hydraulic conductance (Kplant) and predawn disequilibrium between xylem and substrate water potentials (Ψpdd). We expected that, for all three species, salinity increments would result in coordinated declines in Ψs, gs and Kplant, and that the Ψpdd would increase with substrate salinity and time of exposure. In concordance with our predictions, reductions in substrate water potential promoted a coordinated decline in Ψs, gs and Kplant, whereas the Ψpdd increased substantially during the first 4 days but dissipated after 7 days, indicating a time lag for equilibration after a change in substratum salinity. Our results show that mangroves confront and partially ameliorate acute salinity stress via simultaneous reductions in Ψs, gs and Kplant, thus developing synergistic physiological responses at the cell and whole-plant scales.

International audience; As trees worldwide experience mortality or dieback with increasing drought and low tundras grow taller with warming, understanding the link between plant height and climate is increasingly important. We show that taller plants have predictably wider water-conducting conduits, and that wider conduits within species are more vulnerable to conduction-blocking embolisms. These two observations suggest that tall plants in formerly moist areas die because their wide conduits are excessively vulnerable under novel drought conditions. Also, the cold that limits conduit diameter, and therefore height, in tundra plants is relaxed under warming, permitting wider conduits and taller plants. That plant height appears linked to climate via plant hydraulics helps explain why vegetation height differs across biomes and is altering with climate change.Understanding how plants survive drought and cold is increasingly important as plants worldwide experience dieback with drought in moist places and grow taller with warming in cold ones. Crucial in plant climate adaptation are the diameters of water-transporting conduits. Sampling 537 species across climate zones dominated by angiosperms, we find that plant size is unambiguously the main driver of conduit diameter variation. And because taller plants have wider conduits, and wider conduits within species are more vulnerable to conduction-blocking embolisms, taller conspecifics should be more vulnerable than shorter ones, a prediction we confirm with a plantation experiment. As a result, maximum plant size should be short under drought and cold, which cause embolism, or increase if these pressures relax. That conduit diameter and embolism vulnerability are inseparably related to plant size helps explain why factors that interact with conduit diameter, such as drought or warming, are altering plant heights worldwide.

Background and Aims Rhizophora species of mangroves have a conspicuous system of stilt-like roots (rhizo-phores) that grow from the main stem and resemble flying buttresses. As such, the development of rhizophores canbe predicted to be important for the effective transmission of dynamic loads from the top of the tree to the ground,especially where the substrate is unstable, as is often the case in the habitats where Rhizophora species typicallygrow. This study tests the hypothesis that rhizophore architecture in R. mangle co-varies with their proximity to themain stem, and with stem size and crown position. Methods The allometry and wood mechanical properties of R. mangle (red mangrove) trees growing in amangrove basin forest within a coastal lagoon in Mexico were compared with those of coexisting, non-buttressedmangrove trees of Avicennia germinans. The anatomy of rhizophores was related to mechanical stress due to crownorientation (static load) and to prevailing winds (dynamic load) at the study site. Key Results Rhizophores buttressed between 10 and 33 % of tree height. There were significant and directscaling relationships between the number, height and length of rhizophores vs. basal area, tree height and crownarea. Wood mechanical resistance was significantly higher in the buttressed R. mangle (modulus of elasticity,MOE¼18 162GPa)thaninA. germinans (MOE ¼12 160 5GPa). Slenderness ratios (total height/stem diame-ter) were higher in R. mangle, but there were no interspecies differences in critical buckling height. When in prox-imity to the main stem, rhizophores had a lower length/height ratio, higher eccentricity and higher xylem/bark andpith proportions. However, there were no directional trends with regard to prevailing winds or tree leaning. Conclusions In comparison with A. germinans, a tree species with wide girth and flare at the base, R. mangle sup-ports a thinner stem of higher mechanical resistance that is stabilized by rhizophores resembling flying buttresses.This provides a unique strategy to increase tree slenderness and height in the typically unstable substrate on whichthe trees grow, at a site that is subject to frequent storms.Key words: Allometry, Avicennia germinans, biomechanics, flying buttresses, mangroves, Mexico, modulus ofelasticity, Rhizophora mangle, root anatomy, rhizophores, thigmomorphogenesis

Understanding the response of species to land-use change is necessary for the improvement of management and conservation policies. We assessed the impact of vegetation cover and land-use change on Morelet's crocodile populations in nine wetlands located in central Veracruz, Mexico, to test the hypothesis that higher forest cover and lower anthropogenic impact increases crocodile density. We correlated the relative density of crocodiles with wetland attributes, including depth, pH, salinity, and richness and structure of bordering vegetation; and with anthropogenic disturbance factors, including the proportion of modified land surrounding the water body, road density, and road type. The relative density of crocodiles was correlated positively with vegetation cover and correlated negatively with plant density and with the presence of nonpaved roads, particularly in the zone that bordered the water bodies (core zone). Our findings suggest that as long as the water bodies are bordered with arboreal v...

• Premise of the study: Xylem sap osmolality and salinity is a critical unresolved issue in plant function with impacts on transport efficiency, pressure gradients, and living cell turgor pressure, especially for halophytes such as mangrove trees.• Methods: We collected successive xylem vessel sap samples from stems and shoots of Avicennia germinans and Laguncularia racemosa using vacuum and pressure extraction and measured their osmolality. Following a series of extractions with the pressure chamber, we depressurized the shoot and pressurized again after various equilibration periods (minutes to hours) to test for dynamic control of osmolality. Transpiration and final sap osmolality were measured in shoots perfused with deionized water or different seawater dilutions.• Key results: For both species, the sap osmolality values of consecutive samples collected by vacuum extraction were stable and matched those of the initial samples extracted with the pressure chamber. Further extraction of samples with the pressure chamber decreased sap osmolality, suggesting reverse osmosis occurred. However, sap osmolalities increased when longer equilibration periods after sap extraction were allowed. Analysis of expressed sap with HPLC indicated a 1:1 relation between measured osmolality and the osmolality of the inorganic ions in the sap (mainly Na+, K+, and Cl-), suggesting no contamination by organic compounds. In stems perfused with deionized water, the sap osmolality increased to mimic the native sap osmolality.• Conclusions: Xylem sap osmolality and ionic contents are dynamically adjusted by mangroves and may help modulate turgor pressure, hydraulic conductivity, and water potential, thus being important for mangrove physiology, survival, and distribution.

The role of plant polyphenols as defenses against insect herbivores is controversial. We combined correlative field studies across three geographic regions (Northern Mexico, Southern Mexico, and Costa Rica) with induction experiments under controlled conditions to search for candidate compounds that might play a defensive role in the foliage of the tropical oak, Quercus oleoides. We quantified leaf damage caused by four herbivore guilds (chewers, skeletonizers, leaf miners, and gall forming insects) and analyzed the content of 18 polyphenols (including hydrolyzable tannins, flavan-3-ols, and flavonol glycosides) in the same set of leaves using high performance liquid chromatography and mass spectrometry. Foliar damage ranged from two to eight percent per region, and nearly 90% of all the damage was caused by chewing herbivores. Damage due to chewing herbivores was positively correlated with acutissimin B, catechin, and catechin dimer, and damage by mining herbivores was positively correlated with mongolinin A. By contrast, gall presence was negatively correlated with vescalagin and acutissimin B. By using redundancy analysis, we searched for the combinations of polyphenols that were associated to natural herbivory: the combination of mongolinin A and acutissimin B had the highest association to herbivory. In a common garden experiment with oak saplings, artificial damage increased the content of acutissimin B, mongolinin A, and vescalagin, whereas the content of catechin decreased. Specific polyphenols, either individually or in combination, rather than total polyphenols, were associated with standing leaf damage in this tropical oak. Future studies aimed at understanding the ecological role of polyphenols can use similar correlative studies to identify candidate compounds that could be used individually and in biologically meaningful combinations in tests with herbivores and pathogens.

Question Along an altitudinal gradient of 2000 m in a semi-tropical mountain, we explored the relation between tree specific diversity and community functional composition by studying variations in tree allometry, stem and leaf functional traits, and their relationship with temperature and precipitation. Location Tequila Volcano, Jalisco, Mexico (20°48′ N, 103°51′ W). Methods We surveyed tree specific diversity, five forest structural parameters and six functional traits in ten horizontal transects (50–75 m in length) located every 200 m along a 2000-m gradient (from 800 to 2800 m a.s.l.). We calculated alpha and beta diversity, and quantified the community-weighted means for wood and bark density, Huber value (sapwood to leaf area ratio), leaf area, leaf dry mass content and leaf mass per unit area. The patterns of association were explored using Pearson correlations, and summarized using PCA. Results Alpha diversity was independent of altitude, and species turnover was almost complete between consecutive transects. Altitude (and its associated abiotic factors, temperature and precipitation) were highly correlated with functional traits. Maximum tree height, total basal area and the community-weighted mean values for leaf mass per unit area, leaf dry mass content, and the Huber values were positively correlated with altitude, and the opposite was found for the number of basal stems. Stem and leaf trait values were correlated along the altitudinal gradient. Conclusions Altitude imposes environmental filters at the community scale that determine a high species replacement. Stem and leaf traits were correlated along the gradient; trees at higher altitudes were taller with a single stem, higher density of wood and bark, and leaves with higher leaf mass per area and dry mass content than in the low-altitude sites. These results suggest the consistency of a fast–slow acquisitive trade-off across environments, tending to promote slow acquisition and high longevity at higher altitudes.

The causes underlying bark diversity are unclear. Variation has been frequently attributed to environmental differences across sites. However, variation may also result from tradeoffs and coordination between bark's multiple functions. Bark traits may also covary with wood and leaf traits as part of major dimensions of plant variation. To assess hypotheses regarding tradeoffs and functional coordination, we measured bark traits reflecting protection, storage, mechanics, and photosynthesis in branches of 90 species spanning a wide phylogenetic and environmental range. We also tested associations between bark, wood, and leaf traits. We partitioned trait variation within species, and within and across communities to quantify variation associated with across-site differences. We observed associations between bark mechanics and storage, density and thickness, and thickness and photosynthetic activity. Increasing bark thickness contributed significantly to stiffer stems and greater water storage. Bark density, water content, and mechanics covaried strongly with the equivalent wood traits, and to a lesser degree with leaf size, xylem conductivity, and vessel diameter. Most variation was observed within sites and had low phylogenetic signal. Compared with relatively minor across-site differences, tradeoffs and coordination among functions of bark, leaves, and wood are likely to be major and overlooked factors shaping bark ecology and evolution.

Leaf vein traits are implicated in the determination of gas exchange rates and plant performance. These traits are increasingly considered as causal factors affecting the 'leaf economic spectrum' (LES), which includes the light-saturated rate of photosynthesis, dark respiration, foliar nitrogen concentration, leaf dry mass per area (LMA) and leaf longevity. This article reviews the support for two contrasting hypotheses regarding a key vein trait, vein length per unit leaf area (VLA). Recently, Blonder et al. (2011, 2013) proposed that vein traits, including VLA, can be described as the 'origin' of the LES by structurally determining LMA and leaf thickness, and thereby vein traits would predict LES traits according to specific equations. Careful re-examination of leaf anatomy, published datasets, and a newly compiled global database for diverse species did not support the 'vein origin' hypothesis, and moreover showed that the apparent power of those equations to predict LES traits arose from circularity. This review provides a 'flux trait network' hypothesis for the effects of vein traits on the LES and on plant performance, based on a synthesis of the previous literature. According to this hypothesis, VLA, while virtually independent of LMA, strongly influences hydraulic conductance, and thus stomatal conductance and photosynthetic rate. We also review (i) the specific physiological roles of VLA; (ii) the role of leaf major veins in influencing LES traits; and (iii) the role of VLA in determining photosynthetic rate per leaf dry mass and plant relative growth rate. A clear understanding of leaf vein traits provides a new perspective on plant function independently of the LES and can enhance the ability to explain and predict whole plant performance under dynamic conditions, with applications towards breeding improved crop varieties.

Summary The mechanical resistance of leaves has key ecological implications but its basis has not been well understood, particularly at the tissue scale. We tested the hypotheses that leaf mechanical resistance should be a function of tissue density, increasing from the lamina to the midrib, and higher in drought-tolerant than drought-avoiding species. In a common garden study, we quantified nine leaf biomechanical traits, including measurements of material and structural resistance, and in addition 17 morphological traits, at the tissue and whole-leaf scales, for 21 species from three semi-arid communities of California, USA. The mechanical properties of leaves depended strongly on tissue density. Material resistance was significantly greater in the midrib than in the leaf lamina, and tissue resistances were significantly correlated among tissues, lower in deciduous coastal sage species and higher in evergreen drought-tolerant chaparral species. The proportion of the biomass invested in the midrib was lower in species bearing midribs and laminas of high material resistance. Our results support the hypothesis of a hierarchical partitioning of leaf mechanical resistance among leaf tissues reflecting the investment of dry mass. Also, our data indicated a mechanical compensation in leaf design, where leaves with high material resistance and density deploy a relatively minor proportion of support tissue in the midrib. Finally, our results establish a quantitative basis for differences among communities in leaf biomechanics. Our results supported the classical characterization of the mediterranean-climate flora of California according to the dramatic increase in the mean leaf mechanical resistance from species of coastal sage to chaparral, with diverse leaf types in the Mojave Desert species.

In tropical latitudes, the analysis of leaf phenology in tree species of lineages with temperate origin can help better understanding the potential effects of climate change on these forests. Over three years (2008-2010), we recorded the timing of bud burst (BB), leaf unfolding (LU), and leaf spreading (LS) and their relation to temperature, precipitation, and soil water potential in two deciduous oak species (Quercus magnoliifolia Nee and Quercus resinosa Liebm.) along an altitudinal gradient at the Tequila Volcano, central Mexico. Quercus magnoliifolia was monitored at three altitudes, 1450, 1667, and 1787 m, and Q. resinosa was monitored at 1787, 2055, and 2110 m. The onset of BB, LU, and LS occurred earlier at lower elevations with higher temperature in Q. magnoliifolia, but in Q. resinosa only the onset of BB occurred later at lower elevations with higher temperature. BB, LU, and LS were not correlated with rainfall and soil water potential in the two species. The total duration time of leaf development was not significantly correlated with rainfall in Q. magnoliifolia, but a significant negative correlation with rainfall was found in Q. resinosa. Results indicated that leaf phenology of the two examined oak species exhibited contrasting responses to temperature and precipitation. Resume : Sous les tropiques, l'etude de la phenologie des feuilles chez les especes arborescentes qui font partie d'une lignee d'origine temperee peut aider amieux comprendre les effets des changements climatiques sur ces forets. Pendant trois ans (2008-2010), nous avons note les dates du debourrement (BB), du deploiement des feuilles (LU) et de l'expansion des feuilles (LS) en relation avec la temperature, la precipitation et le potentiel hydrique dans le sol chez deux especes decidues de chene (Quercus magnoliifolia Nee et Q. resinosa Liebm.) le long d'un gradient altitudinal au volcan de Tequila, dans le centre du Mexique. Le suivi a ete effectue atrois altitudes : 1450, 1667 et 1787 m dans le cas de Q. magnoliifolia ainsi que 1787, 2055 et 2110 m dans le cas de Q. resinosa. Le debut de BB, LU et LS est survenu plus tot aux plus basses altitudes ou la temperature etait plus elevee dans le cas de Q. magnoliifolia, mais chez Q. resinosa seul le debut de BB est survenu plus tard aux plus basses altitudes ou la temperature etait plus elevee. BB, LU et LS n'etaient pas correles avec la precipitation ni avec le potentiel hydrique dans le sol chez les deux especes. La duree totale du developpement des feuilles n'etait pas significativement correlee avec la precipitation chez Q. magnoliifolia mais une correlation negative significative avec la precipitation a ete trouvee chez Q. resinosa. Les resultats indiquent que la phenologie des feuilles des deux especes de chene qui ont ete etudiees montre que ces especes reagissent differemment ala temperature et ala precipitation. (Traduit par la Redaction)

In tropical dry forests, spatial heterogeneity in soil water availability is thought to determine interspecific differences in key components of resource use strategies, such as leaf phenology and xylem function. To understand the environmental drivers of variation in leaf phenology and xylem function, we explored the relation of soil water potential to topographic metrics derived from a digital elevation model. Subsequently, we compared nine xylem hydraulic, mechanical and storage traits in 18 species in three phenological classes (readily deciduous, tardily deciduous, and evergreen) in the dry tropical forest of Chamela, Mexico. Soil water potential was negatively correlated with elevation, insolation and water flow accumulation. Evergreen species characterized low-elevation moist sites, whereas deciduous species dominated hills and dry sites. Overall, evergreen species had lower xylem specific conductivity than deciduous species, and tardily deciduous species were different from readily deciduous and evergreen species in five of eight xylem traits. In dry tropical forests, water availability promotes divergence in leaf phenology and xylem traits, ranging from low wood density, evergreen species in moist sites to a combination of low wood density, readily deciduous species plus high wood density, tardily deciduous species in dry sites.

With data from 15 species in eight families of tropical dry forest trees, we provide evidence of coordination between the stem and leaf economic spectra. Species with low-density, flexible, breakable, hydraulically efficient but cavitationally vulnerable wood shed their leaves rapidly in response to drought and had low leaf mass per area and dry mass content. In contrast, species with the opposite xylem syndrome shed their costlier but more drought-resistant leaves late in the dry season. Our results explain variation in the timing of leaf shedding in tropical dry forests: selection eliminates combinations such as low-productivity leaves atop highly vulnerable xylem or water-greedy leaves supplied by xylem of low conductive efficiency. Across biomes, rather than a fundamental trade-off underlying a single axis of trait covariation, the relationship between leaf and stem economics is likely to occupy a wide space in which multiple combinations are possible.

The competition for resources increases size inequality in trees, particularly under low abiotic stress. Because mangrove communities are subject to site-specific salinity (and therefore abiotic stress) gradients, these habitats should differ in height–diameter allometry and size inequality. The size inequality (by the Gini Coefficient, G) and maximum potential height (Hmax from a height–diameter asymptotic model) were determined within the mangrove forest of a coastal lagoon in Veracruz, Mexico in 20 0.25-ha plots, 10 in interdistributary basins (IBs, lower salinity) having Avicennia germinans, Laguncularia racemosa and Rhizophora mangle and 10 in mudflats (MFs, higher salinity) dominated by A. germinans. Size inequality was significantly higher in IBs (G = 0.59 ± 0.02 vs. 0.39 ± 0.03). Due to their significant intercorrelation G, total basal area and density were synthesized in one PCA axis accounting for 67% of total variance and inversely correlated with salinity (R = −0.65, P = 0.003). The height–diameter scaling model reached a stable asymptote (Hmax range: 16–21 m; coefficient of variation CV: 7.7) in IBs, suggesting that trees can still increase their diameter after achieving maximum height. In MFs, no stable asymptote was reached (Hmax range: 11–26 m; CV: 32.5), suggesting a lower growth rate of diameter in the MF trees when compared with IB trees.

The variation in leaf mass per area, leaf nutrients (% carbon, nitrogen and phosphorus), and the allometric relation between tree height and diameter of the black mangrove, Avicennia germinans, were explored in nine mangrove forests in similar environments along a 5° latitudinal gradient in the central region of the Gulf of Mexico, as indicated by a southward increase in temperature and precipitation. There was no correlation between leaf nitrogen or phosphorus content and latitude. Leaf mass per area and leaf carbon content were positively correlated with latitude and negatively correlated with temperature and annual rainfall, whereas asymptotic tree height and maximum diameter showed the opposite trend. Such patterns suggest a trade-off between leaf traits and tree size which may be constrained by the same environmental factors along a dry-cold to humid-warm latitudinal gradient.