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Seedling dynamics are crucial for understanding spatial plant distribution patterns, yet little is known about seedling establishment in Neotropical savannas because empirical studies at the community level are scarce., Over 2 years, we studied the recruitment and survival of an initial seedling assemblage and three cohorts of recruits of woody plants within 216 plots of 1 m2 located along a tree density gradient in the savannas of central Brazil. These savannas differ in tree density and canopy cover, from closed (high canopy cover) to open savannas (low canopy cover), and are located along shallow topographic gradients., We measured community-wide seedling limitation (i.e. proportion of 1-m2 plots without seedlings of any woody species), photosynthetic photon flux density, litter cover, soil moisture and soil nutrients in each savanna type. Because closed savannas had lower PPFD and higher leaf litter cover than open savannas, we evaluated the effects of light level and litter cover on seedling emergence of nine dominant savanna woody species under controlled conditions in a glasshouse., Density, recruitment and survival of seedlings decreased over time because of mortality in all savanna types, but they were consistently higher in closed than in open savannas. Community-wide seedling limitation was significantly lower in closed (0.16 ± 0.03) than in open (0.30 ± 0.05) savannas., In the glasshouse, high litter cover and very low light levels reduced seedling emergence of most species, suggesting an adaptation to delay seed germination until the wet season when soil water availability is high and leaf litter rapidly decomposes., Synthesis: In Neotropical savannas, tree canopy cover facilitates seedling establishment of woody species by reducing stressful environmental conditions. In particular, low irradiance and high litter cover in closed savannas enhance the recruitment and survival of woody seedlings relative to open savannas by reducing soil water deficits and increasing nutrient availability in the upper soil layers. The higher seedling limitation of tree species in open than in closed savannas contributes to maintain relatively different balances between trees and herbaceous plants along topographic gradients in Neotropical savannas and helps to explain spatial distribution patterns of woody species in these ecosystems. [ABSTRACT FROM AUTHOR]

Relationships between diel changes in stem expansion and contraction and discharge and refilling of stemwater storage tissues were studied in six dominant Neotropical savanna (cerrado) tree species from central Brazil. Two stem tissues were studied, the active xylem or sapwood and the living tissues located between the cambium and the cork, made up predominantly of parenchyma cells (outer parenchyma). Outer parenchyma and sapwood density ranged from 320 to 410 kg m-3 and from 420 to 620 kg m-3, respectively, depending on the species. The denser sapwood tissues exhibited smaller relative changes in cross-sectional area per unit change in water potential compared with the outer parenchyma. Despite undergoing smaller relative changes in cross-sectional area, the sapwood released about 3.5 times as much stored water for a given change in area as the outer parenchyma. Cross-sectional area decreased earlier in the morning in the outer parenchyma than in the sapwood with lag times up to 30 min for most species. The relatively small lag time between dimensional changes of the two tissues suggested that they were hydraulically well connected. The initial morning increase in basal sap flow lagged about 10 to 130 min behind that of branch sap flow. Species- specific lag times between morning declines in branch and main stem cross-sectional area were a function of relative stem water storage capacity, which ranged from 16 to 31% of total diurnal water loss. Reliance on stored water to temporarily replace transpirational losses is one of the homeostatic mechanisms that constrain the magnitude of leaf water deficits in cerrado trees. [ABSTRACT FROM AUTHOR]

Under certain environmental conditions, nocturnal transpiration can be relatively high in temperate and tropical woody species. We have previously shown that nocturnal sap flow accounts for up to 28% of total daily transpiration in woody species growing in a nutrient-poor Brazilian Cerrado ecosystem. In the present study, we assessed the effect of increased nutrient supply on nocturnal transpiration in three dominant Cerrado tree species to explore the hypothesis that, in nutrient-poor systems, continued transpiration at night may enhance delivery of nutrients to root-absorbing surfaces. We compared nocturnal transpiration of trees growing in unfertilized plots and plots to which nitrogen (N) and phosphorus (P) had been added twice yearly from 1998 to 2005. Three independent indicators of nocturnal transpiration were evaluated: sap flow in terminal branches, stomatal conductance (gs), and disequilibrium in water potential between covered and exposed leaves (ΔΨL). In the unfertilized trees, about 25% of the total daily sap flow occurred at night. Nocturnal sap flow was consistently lower in the N- and P-fertilized trees, significantly so in trees in the N treatment. Similarly, nocturnal gs was consistently lower in fertilized trees than in unfertilized trees where it sometimes reached values of 150 mmol m-2 s-1 by the end of the dark period. Predawn gs and the percentage of nocturnal sap flow were linearly related. Nocturnal ΔΨL was significantly greater in the unfertilized trees than in N- and P-fertilized trees. The absolute magnitude of ΔΨL increased linearly with the percentage of nocturnal sap flow. These results are consistent with the idea that enhancing nutrient uptake by allowing additional transpiration to occur at night when evaporative demand is lower may avoid excessive dehydration associated with increased stomatal opening during the day when evaporative demand is high. [ABSTRACT FROM AUTHOR]

Biophysical characteristics of sapwood and outer parenchyma water storage compartments were studied in stems of eight dominant Brazilian Cerrado tree species to assess the impact of differences in tissue capacitance on whole-plant water relations. The rate of decline in tissue water potential with relative water content ( RWC) was greater in the outer parenchyma than in the sapwood for most of the species, resulting in tissue-and species-specific differences in capacitance. Sapwood capacitance on a tissue volume basis ranged from 40 to 160 kg m−3 MPa−1, whereas outer parenchyma capacitance ranged from 25 to only 60 kg m−3 MPa−1. In addition, osmotic potentials at full turgor and at the turgor loss point were more negative for the outer parenchyma compared with the sapwood, and the maximum bulk elastic modulus was higher for the outer parenchyma than for the sapwood. Sapwood capacitance decreased linearly with increasing sapwood density across species, but there was no significant correlation between outer parenchyma capacitance and tissue density. Midday leaf water potential, the total hydraulic conductance of the soil/leaf pathway and stomatal conductance to water vapour ( gs) all increased with stem volumetric capacitance, or with the relative contribution of stored water to total daily transpiration. However, the difference between the pre-dawn water potential of non-transpiring leaves and the weighted average soil water potential, a measure of the water potential disequilibrium between the plant and soil, increased asymptotically with total stem capacitance across species, implying that overnight recharge of water storage compartments was incomplete in species with greater capacitance. Overall, stem capacitance contributes to homeostasis in the diurnal and seasonal water balance of Cerrado trees. [ABSTRACT FROM AUTHOR]

Abstract: Environmental controls of stand-level tree transpiration (E) and seasonal patterns of soil water utilization were studied in five central Brazilian savanna (Cerrado) sites differing in tree density. Tree density of Cerrado vegetation in the study area consistently changes along topographic gradients from ∼1000treesha−1 in open savannas (campo sujo) at the lower end of the topographic gradient to >3000treesha−1 in woodlands (cerradão) at the upper end of the gradient. Tree canopy resistance (r C) increased linearly with increasing daily mean air saturation deficit (D) at all sites, but cerradão and cerrado denso sites with higher tree density and higher tree leaf area index (LAI) had lower r C values at all values of D compared to physiognomies with lower tree density, suggesting that r C was less sensitive to changes in D in physiognomies with high tree density and LAI. During the peak of the dry season, mean soil water potential at 0.20m depth was most negative in the sites with the lowest tree basal area and increased linearly with basal area across sites. In contrast, soil water storage in the 0.10–2.50m layer decreased exponentially with increasing basal area, consistent with trees in higher density sites utilizing a larger proportion of available soil water at depth during the dry season. Maximum tree transpiration was highest in the cerradão and cerrado denso (∼0.81mmday−1). Despite higher evaporative demand during the dry season, E was similar between the dry and wet seasons within each study site, which was associated with lower LAI and canopy conductance (g C) during the dry season compared to the wet season. Leaf area index was a good predictor of E and g C. For both dry and wet season data combined, E increased asymptotically with increasing LAI across all physiognomic types, allowing LAI to be used as a predictor of spatial variation of E. The lack of seasonality in E across the Cerrado physiognomies studied could not be explained by individual constraining variables such as D or soil water potential near the surface, but was consistent with the influence of multiple regulatory effects of D and soil water potential on seasonal changes in leaf area and g C. [Copyright &y& Elsevier]