Gas exchange and water-use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water-use efficiency and carbon isotope discrimination in C(3)plant canopies. I then present a case study of water-use efficiency in northern Australian tree species. In general, C(3)plants face a trade-off whereby increasing stomatal conductance for a given set of conditions will result in a higherCO(2)assimilation rate, but a lower photosynthetic water-use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water-use efficiency. This may explain why community-level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water-use efficiency that take place during leaf expansion in C(3)plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water-use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respiredCO(2)can increase whole-plant water-use efficiency. Finally, I discuss the role of water-use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmosphericCO(2). In coming decades, increases in plant water-use efficiency caused by risingCO(2)are likely to partially mitigate impacts on plants of drought stress caused by global warming

Carbon Isotope Effects in Relation to CO2 Assimilation by Tree Canopies

The carbon atoms deposited in tree rings originate from the CO2 in the atmosphere to which the tree’s canopy is exposed. Thus, the first control on the stable carbon-isotope composition of tree rings is by δ13C of atmospheric CO2. There has been an inter-annual trend of decreasing δ13C of atmospheric CO2 over the past two centuries as a result of combustion of fossil fuels and land-use change. Atmospheric CO2 is, for the most part, well mixed, but the sub-canopy air space can become depleted in 13C due to inputs from soil and plant respiration when turbulent exchange with the troposphere is hindered, for example by a high leaf area index at night. This is less likely to occur during daytime when turbulence is higher and photosynthesis takes place. Discrimination against 13C (∆13C) occurs upon assimilation of atmospheric CO2 by C3 photosynthesis. Trees using the C3 photosynthetic pathway comprise the overwhelming majority of all trees. The primary control on the extent of discrimination during C3 photosynthesis is the drawdown in CO2 concentration from the air outside the leaf to the site of carboxylation in the chloroplast. Part of this drawdown is captured by ci/ca, that is, the ratio of intercellular to ambient CO2 concentrations. The ci/ca represents the balance between the CO2 supply by stomata and its demand by photosynthesis. It can be related to water-use efficiency, the amount of CO2 taken up by photosynthesis for a given amount of water loss to the atmosphere, assuming a given evaporative demand. To predict time-averaged ci/ca from wood ∆13C, a simplified, linear model can be employed. In this linear model, the slope is determined by b¯¯, the effective enzymatic discrimination. The value of b¯¯ can be estimated by comparing wood ∆13C to representative measurements of ci/ca. The b¯¯ was originally estimated from observations of leaf tissue to have a value of 27‰. We compiled data for woody stem tissue here, and our analysis suggests that a lower b¯¯ should be used in the simplified model for wood (b¯¯ = 25.5‰) than for leaves (b¯¯ = 27‰). This is also consistent with widespread observations that woody tissues are enriched in 13C compared to leaves

Nitrogen to phosphorus ratio of plant biomass versus soil solution in a tropical pioneer tree, Ficus insipida

It is commonly assumed that the nitrogen to phosphorus (N:P) ratio of a terrestrial plant reflects the relative availability of N and P in the soil in which the plant grows. Here, this was assessed for a tropical pioneer tree, Ficus insipida. Seedlings were grown in sand and irrigated with nutrient solutions containing N:P ratios ranging from <1 to >100. The experimental design further allowed investigation of physiological responses to N and P availability. Homeostatic control over N:P ratios was stronger in leaves than in stems or roots, suggesting that N:P ratios of stems and roots are more sensitive indicators of the relative availability of N and P at a site than N:P ratios of leaves. The leaf N:P ratio at which the largest plant dry mass and highest photosynthetic rates were achieved was ∼11, whereas the corresponding whole-plant N:P ratio was ∼6. Plant P concentration varied as a function of transpiration rate at constant nutrient solution P concentration, possibly due to transpiration-induced variation in the mass flow of P to root surfaces. The transpiration rate varied in response to nutrient solution N concentration, but not to nutrient solution P concentration, demonstrating nutritional control over transpiration by N but not P. Water-use efficiency varied as a function of N availability, but not as a function of P availability

Epiphytic ant-plant obtains nitrogen from both native and invasive ant inhabitants

Ant-plants have been extensively used as model systems in the study of the evolution and ecology of mutualisms. Using a 15N isotope labeling experiment, we found that both a native ant mutualist (Philidris cordata) and an invasive ant (Pheidole megacephala) provide nitrogen to the Australian ant-plant Myrmecodia beccarii

Epiphytic ant-plant obtains nitrogen from both native and invasive ant inhabitants

Ant-plants have been extensively used as model systems in the study of the evolution and ecology of mutualisms. Using a 15N isotope labeling experiment, we found that both a native ant mutualist (Philidris cordata) and an invasive ant (Pheidole megacephala) provide nitrogen to the Australian ant-plant Myrmecodia beccarii

Epiphytic ant-plant obtains nitrogen from both native and invasive ant inhabitants

Ant-plants have been extensively used as model systems in the study of the evolution and ecology of mutualisms. Using a 15N isotope labeling experiment, we found that both a native ant mutualist (Philidris cordata) and an invasive ant (Pheidole megacephala) provide nitrogen to the Australian ant-plant Myrmecodia beccarii

Assessing the CO2 concentration at the surface of photosynthetic mesophyll cells

We present a robust estimation of the CO2 concentration at the surface of photosynthetic mesophyll cells (cw), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments. We introduce calculations to estimate cw using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances. The cw was lower than adaxial and abaxial estimated intercellular CO2 concentrations (ci). Differences between cw and the ci of each surface were usually larger than 10 μmol mol−1. Differences between adaxial and abaxial ci ranged from a few μmol mol−1 to almost 50 μmol mol−1, where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial ci and the ci estimated by mixing both fluxes ranged from −30 to +20 μmol mol−1, where the largest differences were found under high ASD or high ambient CO2 concentrations. Accounting for cw improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO2 and water vapor gradients within the leaf

Transpiration efficiency of a tropical pioneer tree (Ficus insipida) in relation to soil fertility

The response of whole-plant water-use efficiency, termed transpiration efficiency (TE), to variation in soil fertility was assessed in a tropical pioneer tree, Ficus insipida Willd. Measurements of stable isotope ratios (d13C, d18O, d15N), elemental concentrations (C, N, P),plant growth, instantaneous leaf gas exchange, and whole-plant water use were used to analyse the mechanisms controlling TE. Plants were grown individually in 19 l pots with non-limiting soil moisture. Soil fertility was altered by mixing soil with varying proportions of rice husks, and applying a slow release fertilizer. A large variation was observed in leaf photosynthetic rate, mean relative growth rate (RGR), and TE in response to experimental treatments; these traits were well correlated with variation in leaf N concentration. Variation in TE showed a strong dependence on the ratio of intercellular to ambient CO2 mole fractions (ci/ca); both for instantaneous measurements of ci/ca (R2=0.69, P <0.0001, n=30), and integrated estimates based on C isotope discrimination (R2 =0.88, P<0.0001, n=30). On the other hand, variations in the leaf-to-air humidity gradient, unproductive water loss, and respiratory C use probably played only minor roles in modulating TE in the face of variable soil fertility. The pronounced variation in TE resulted from a combination of the strong response of ci/ca to leaf N, and inherently high values of ci/ca for this tropical tree species; these two factors conspired to cause a 4-fold variation among treatments in (1–ci/ca ), the term that actually modifies TE. Results suggest that variation in plant N status could have important implications for the coupling between C and water exchange in tropical forest trees

Predicting species abundance by implementing the ecological niche theory

Species are not uniformly distributed across the landscape. For every species, there should be few favoured sites where abundance is high and many other sites of lower suitability where abundance is low. Consequently, local abundance could be thought of as a natural expression of species response to local conditions. The correlation between abundance and environmental suitability has been well documented, and a recent meta-analysis has suggested that this relationship could be a generality. Despite the importance and potential implication of the abundance–suitability relationship, its predictive power for meaningful extrapolations has been surprisingly poorly explored. In this study, we showed how a highly predictable trend can be extracted from the abundance–suitability relationship, accurately predicting the variation in species abundance at a high spatial resolution. We produced high-quality environmental suitability estimations for 50 endemic species in the Australian Wet Tropics. Environmental suitability derived from species distribution models was related to observed abundance estimated using data from 29 years of uninterrupted monitoring effort. We used the fitted relationship to accurately predict abundance at a fine scale across the species range. Our results showed that the abundance–suitability relationship was strong for endemic species in the Australian Wet Tropics. The predictive power of our models was high, explaining, on average, 55% of the deviance across taxa. Despite interspecific variation in the strength of the abundance–suitability relationship associated with potential intrinsic estimation biases, our approach provides a powerful tool for predicting abundance across the species range at a fine scale. The potential for robust abundance predictions from occurrence-based species distribution models shown in this study are numerous, and it could have a significant impact in enhancing species conservation and management decisions