Morphological and moisture availability controls of the leaf area‐to‐sapwood area ratio: analysis of measurements on Australian trees

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The influence of predawn leaf water potential on stomatal responses to atmospheric water content at constant Ci and on stem hydraulic conductance and foliar ABA concentrations

Stomata and photosynthetic responses to increasing leaf-to-air vapour pressure difference (V) were investigated in watered and droughted Eucalyptus tetrodonta when either ambient CO2 (Ca) or internal CO2 concentration (Ci) were constant. Stem hydraulic conductance and xylem and foliar abscisic acid levels were measured periodically during the drought period.As V increased, stomatal conductance (gs) declined. Maintaining Ci did not affect the response of Gs to V or predawn leaf water potential (pd). In fully watered plants the decline in gs was insufficient to prevent increased transpiration rates (Et) with increasing V. In contrast, in droughted plants, stomatal closure was sufficient to prevent increasing Et. Stomatal sensitivity to increasing V was increased by drought. As drought developed, the three phases of stomatal responses to V progressively collapsed to one where feedforward mechanisms result in decreasing Et with increasing V. Thus as drought developed the feedforward response of stomata to V entirely dominated.Net photosynthesis (Pn) and gs responded in unison to changes in V and pd. The decline in Pn as V increased, despite Ci being maintained constant, was observed in fully watered plants and in severely droughted plants. Pn was most dependent on gs at large values of V and in droughted plants.As drought progressed and pd declined, stem hydraulic conductance decreased and foliar ABA concentrations increased. The decline in maximum gs was correlated with foliar ABA levels. It is concluded here that hydraulic signals, be they atmospheric water or soil water in origin, and possibly also chemical signals regulate gs, which in turn can limit assimilation rates in seasonally dry savannas.Key words: Eucalyptus tetrodonta, stomatal sensitivity, transpiration, hydraulic conductance, ABA.      

Ecophysiological traits of deciduous and evergreen woody species in the seasonally dry tropics

Seasonally dry tropical ecosystems occur in the Americas, Africa, India and Australia. They sustain large human populations, determine regional climate, are sites of biological and cultural conservation, and have significant economic value. Evergreen, deciduous and semi- and brevideciduous trees frequently co-occur. Recent research reveals how these various phenological groups respond to changes in soil and atmospheric water content. Cost-benefit analyses of evergreen and deciduous species show how leaves of deciduous species live fast and die young, whereas leaves of evergreen species live slowly but for longer.     

Tree allometry and improved estimation of carbon stocks and balance in tropical forests

Tropical forests hold large stores of carbon, yet uncertainty remains regarding their quantitative contribution to the global carbon cycle. One approach to quantifying carbon biomass stores consists in inferring changes from long-term forest inventory plots. Regression models are used to convert inventory data into an estimate of aboveground biomass (AGB). We provide a critical reassessment of the quality and the robustness of these models across tropical forest types, using a large dataset of 2,410 trees ≥ 5 cm diameter, directly harvested in 27 study sites across the tropics. Proportional relationships between aboveground biomass and the product of wood density, trunk cross-sectional area, and total height are constructed. We also develop a regression model involving wood density and stem diameter only. Our models were tested for secondary and old-growth forests, for dry, moist and wet forests, for lowland and montane forests, and for mangrove forests. The most important predictors of AGB of a tree were, in decreasing order of importance, its trunk diameter, wood specific gravity, total height, and forest type (dry, moist, or wet). Overestimates prevailed, giving a bias of 0.5–6.5% when errors were averaged across all stands. Our regression models can be used reliably to predict aboveground tree biomass across a broad range of tropical forests. Because they are based on an unprecedented dataset, these models should improve the quality of tropical biomass estimates, and bring consensus about the contribution of the tropical forest biome and tropical deforestation to the global carbon cycle. Electronic Supplementary Material Supplementary material is available for this article at     

Convergence of tree water use within an arid-zone woodland

We examined spatial and temporal patterns of tree water use and aspects of hydraulic architecture in four common tree species of central Australia—Corymbia opaca, Eucalyptus victrix, E. camaldulensis and Acacia aneura—to better understand processes that constrain water use in these environments. These four widely distributed species occupy contrasting niches within arid environments including woodlands, floodplains and riparian environments. Measurements of tree water use and leaf water potential were made at two sites with contrasting water table depths during a period of high soil water availability following summer rainfall and during a period of low soil water availability following 7 months of very little rainfall during 2007. There were significant differences in specific leaf area (SLA), sapwood area to leaf area ratios and sapwood density between species. Sapwood to leaf area ratio increased in all species from April to November indicating a decline in leaf area per unit sapwood area. Despite very little rainfall in the intervening period three species, C. opaca, E. victrix and E. camaldulensis maintained high leaf water potentials and tree water use during both periods. In contrast, leaf water potential and water use in the A. aneura were significantly reduced in November compared to April. Despite contrasting morphology and water use strategies, we observed considerable convergence in water use among the four species. Wood density in particular was strongly related to SLA, sapwood area to leaf area ratios and soil to leaf conductance, with all four species converging on a common relationship. Identifying convergence in hydraulic traits can potentially provide powerful tools for scaling physiological processes in natural ecosystems.     

Reproductive Phenology of Woody Species in a North Australian Tropical Savanna1

Interspecific and interannual variation in reproductive phenology was quantified for 50 common species of trees and shrubs from a mesic savanna near Darwin, northern Australia. The presence of buds, flowers, and fruit was noted over a 30-month period, from September 1992 to February 1995. Surveys were undertaken at monthly intervals for the less common species, and at bimonthly intervals for ten of the common trees and tall shrubs. The majority of species flowered each year at about the same time. There was no evidence of sub-annual or continuous regimes of reproductive phenology. There was no supra-annual carryover of seed-bearing fruit in the canopy of any species. The peak flowering periods were the mid to late dry season (July–August) and the transition between the dry season and the wet season (October–November). The two dominant trees–Eucalyptus miniata and E, tetrodonta– flowered during the dry season, thereby providing resources for some elements of the vertebrate fauna. Flowering and fruiting were uncommon at the end of the wet season (February/March), although two species that flower and fruit at this time (E. porrecta and Terminalia ferdinandianas may provide resources to consumers at a time when floral or fruit resources are otherwise scarce. Because the peak of reproductive activity takes place during the late dry season, fruit maturity and seed dispersal have occurred prior to the onset of the rainy season for most species, and germination and seedling establishment potentially may take effect in response to the first rains. Late dry season fires, which tend to be extensive and intense, are a potential threat to the floral and fruit reserves within these savannas.