Maximize Carbon Gain Research Articles

A moving target: trade-offs between maximizing carbon and minimizing hydraulic stress for plants in a changing climate.

Observational evidence indicates that tree leaf area may acclimate in response to changes in water availability to alleviate hydraulic stress. However, the underlying mechanisms driving leaf area changes and consequences of different leaf area allocation strategies remain unknown. Here, we use a trait-based hydraulically enabled tree model with two endmember leaf area allocation strategies, aimed at either maximizing carbon gain or moderating hydraulic stress. We examined the impacts of these strategies on future plant stress and productivity. Allocating leaf area to maximize carbon gain increased productivity with high CO2, but systematically increased hydraulic stress. Following an allocation strategy to avoid increased future hydraulic stress missed out on 26% of the potential future net primary productivity in some geographies. Both endmember leaf area allocation strategies resulted in leaf area decreases under future climate scenarios, contrary to Earth system model (ESM) predictions. Leaf area acclimation to avoid increased hydraulic stress (and potentially the risk of accelerated mortality) was possible, but led to reduced carbon gain. Accounting for plant hydraulic effects on canopy acclimation in ESMs could limit or reverse current projections of future increases in leaf area, with consequences for the carbon and water cycles, and surface energy budgets.

Ecosystem Water‐Saving Timescale Varies Spatially With Typical Drydown Length

AbstractStomatal optimization theory is a commonly used framework for modeling how plants regulate transpiration in response to the environment. Most stomatal optimization models assume that plants instantaneously optimize a reward function such as carbon gain. However, plants are expected to optimize over longer timescales given the rapid environmental variability they encounter. There are currently no observational constraints on these timescales. Here, a new stomatal model is developed and is used to analyze the timescales over which stomatal closure is optimized. The proposed model assumes plants maximize carbon gain subject to the constraint that they cannot draw down soil moisture below a critical value. The reward is integrated over time, after being weighted by a discount factor that represents the timescale (τ) that a plant considers when optimizing stomatal conductance to save water. The model is simple enough to be analytically solvable, which allows the value of τ to be inferred from observations of stomatal behavior under known environmental conditions. The model is fitted to eddy covariance data in a range of ecosystems, finding the value of τ that best predicts the dynamics of evapotranspiration at each site. Across 82 sites, the climate metrics with the strongest correlation to τ are measures of the average number of dry days between rainfall events. Values of τ are similar in magnitude to the longest such dry period encountered in an average year. The results here shed light on which climate characteristics shape spatial variations in ecosystem‐level water use strategy.

Mechanical advantage makes stomatal opening speed a function of evaporative demand.

Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals, particularly evaporative demand driven by vapor pressure deficit (VPD) influences the kinetics of this response. In angiosperms, and some fern species from the family Marsileaceae, a mechanical interaction between the guard cells and the epidermal cells determines the aperture of the pore. Here, we examine whether this mechanical interaction influences the speed of stomatal opening in the light. To test this, we investigated the speed of stomatal opening in response to light across a range of VPDs in seven plant species spanning the evolutionary diversity of guard cell and epidermal cell mechanical interactions. We found that stomatal opening speed is a function of evaporative demand in angiosperm species and Marsilea, which have guard cell and epidermal cell mechanical interactions. Stomatal opening speeds did not change across a range of VPD in species of gymnosperm and fern, which do not have guard cell mechanical interactions with the epidermis. We find that guard cell and epidermal cell mechanical interactions may play a key role in regulating stomatal responsiveness to light. These results provide valuable insight into the adaptive relevance of mechanical advantage.

An Inverse Scaling Relationship between Stomatal Density and Mean Nearest Neighbor Distance: Evidence from a Photinia Hybrid and One of Its Parents.

Stomata are involved in transpiration and CO2 uptake by mediating gas exchange between internal plant tissues and the atmosphere. The capacity for gas exchange depends on stomatal density (SD), stomatal size, and pore dimensions. Most published work on stomatal quantification has assumed that stomatal distribution and stomatal density are spatially homogeneous across the leaf, but this assumption has been seldom tested. We selected 32 leaves from a Photinia hybrid, Photinia × fraseri 'Red Robin', and one of its parents, P. serratifolia. For each leaf, the leaf surface was divided into three or four equidistant layers along the apical-basal axis, and, in each layer, two positions, one closer to the midrib and the other closer to the leaf margin, were further selected. We calculated SD and mean nearest neighbor distance (MNND) for each lamina section and tested the scaling relationship between SD and MNND of the sampled stomatal centers using reduced major axis protocols. In addition, we calculated the stomatal aggregation index (SAI) for each lamina section to examine the spatial arrangement of stomata at the given size of field of view of 1.2 mm × 0.9 mm. We observed that SD decreased from the lamina apex towards the base for central lamina areas but varied little at leaf margins. An inverse scaling relationship between SD and MNND was observed for both species. This relationship could be used for SD estimation using the rapidly estimated trait, MNND. SAI did not vary significantly throughout leaf lamina, and the numerical values of SAI for all fields of view were greater than one, which indicates significant spatial repulsion between stomata. The study suggests that SD varies across leaf lamina to fine-tune plant water use and maximize carbon gain. However, spatial structures of stomata from different lamina sections exhibit similar patterns (i.e., spatial inhibition between stomata at small scales), probably due to hierarchical leaf vein patterns.

Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access.

High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (Anet ) and stomatal conductance (gs ). However, evidence for decoupling of gs from Anet under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum Tair > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced Anet and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced-gs was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.

Photosynthetic response dynamics in the invasive species Tithonia diversifolia and two co-occurring native shrub species under fluctuating light conditions

To determine the invasiveness of invasive plants, many studies have compared photosynthetic traits or strategies between invasive and native species. However, few studies have compared the photosynthetic dynamics between invasive and native species during light fluctuations. We compared photosynthetic induction, relaxation dynamics and leaf traits between the invasive species, Tithonia diversifolia and two native species, Clerodendrum bungei and Blumea balsamifera, in full-sun and shady habitats. The photosynthetic dynamics and leaf traits differed among species. T. diversifolia showed a slower induction speed and stomatal opening response but had higher average intrinsic water-use efficiency than the two native species in full-sun habitats. Thus, the slow induction response may be attributed to the longer stomatal length in T. diversifolia. Habitat had a significant effect on photosynthetic dynamics in T. diversifolia and B. balsamifera but not in C. bungei. In shady habitat, T. diversifolia had a faster photosynthetic induction response than in full-sun habitat, leading to a higher average stomatal conductance during photosynthetic induction in T. diversifolia than in the two native species. In contrast, B. balsamifera had a larger stomatal length and slower photosynthetic induction and relaxation response in shady habitat than in full-sun habitat, resulting in higher carbon gain during photosynthetic relaxation. Nevertheless, in both habitats, T. diversifolia had an overall higher carbon gain during light fluctuations than the two native species. Our results indicated that T. diversifolia can adopt more effective response strategies under fluctuating light environments to maximize carbon gain, which may contribute to its successful invasion.

The Plant Leaf: A Biomimetic Resource for Multifunctional and Economic Design

As organs of photosynthesis, leaves are of vital importance for plants and a source of inspiration for biomimetic developments. Leaves are composed of interconnected functional elements that evolved in concert under high selective pressure, directed toward strategies for improving productivity with limited resources. In this paper, selected basic components of the leaf are described together with biomimetic examples derived from them. The epidermis (the "skin" of leaves) protects the leaf from uncontrolled desiccation and carries functional surface structures such as wax crystals and hairs. The epidermis is pierced by micropore apparatuses, stomata, which allow for regulated gas exchange. Photosynthesis takes place in the internal leaf tissue, while the venation system supplies the leaf with water and nutrients and exports the products of photosynthesis. Identifying the selective forces as well as functional limitations of the single components requires understanding the leaf as an integrated system that was shaped by evolution to maximize carbon gain from limited resource availability. These economic aspects of leaf function manifest themselves as trade-off solutions. Biomimetics is expected to benefit from a more holistic perspective on adaptive strategies and functional contexts of leaf structures.

Allometric relationships between leaf and petiole traits across 31 floating-leaved plants reveal a different adaptation pattern from terrestrial plants.

Allometric scaling between stomata and xylem for terrestrial woody plants is a widely observed pattern that may be constrained by water transport. Floating-leaved plants, a particular life form of aquatic plants, have leaves in direct contact with both air and water and a poorly developed xylem that may not be limited by water supply as for terrestrial plants. However, whether such an allometric scaling relationship still exists in floating-leaved plants has not been explored. We analysed 31 floating-leaved species/varieties with a range in leaf area covering six orders of magnitude. For all 31 floating-leaved plants, we studied the allometric relationships between leaf area and petiole transverse area, and between total stomatal area and petiole vascular area. The slopes of both relationships were similar to the slope of the allometric relationship (1.23) between total stomatal area and xylem area of 53 terrestrial plants. However, for ten of them with xylem that can be clearly defined, the strong positive relationship between total stomatal area and petiole xylem area had a significantly smaller slope than that of terrestrial plants (0.64 vs. 1.23). Furthermore, after considering phylogeny, the scaling relationships between total stomatal area and petiole traits in floating-leaved plants remained significant. We speculated that for floating-leaved plants, the hyperallometric relationship (slope >1) between the construction of leaf/stoma and petiole was promoted by the high demand for photosynthesis and thus more leaves/stomata. While the hypoallometric relationship (slope <1) between stomatal and xylem area was related more to hydraulic processes, the selection pressure on stomata was lower than xylem of floating-leaved plants. Allometric relationships among the hydraulic traits on water transport of aquatic plants are the result of natural selection to achieve maximum carbon gain, which is similar to terrestrial plants.

Vapor pressure deficit constrains transpiration and photosynthesis in holm oak: A comparison of three methods during summer drought.

High rates of vapor pressure deficit (VPD) can severely decrease plant productivity by reducing stomatal conductance, which might be exacerbated during Mediterranean summers due to soil water deficit. In this study, we monitored the response of holm oak, the archetype of Mediterranean trees, to changes in VPD during a summer drought period to evaluate the effects and consequences on gas exchange of the two water stresses (atmospheric and soil). Measurements were performed on trees growing in an experimental plantation over two summers with moderate drought stress by using three different methods: at the leaf level with an infrared gas analyzer, using a whole-plant chamber for short-term monitoring at the tree level, and measuring the canopy temperature for long-term monitoring. The three methods provided negative relationships between leaf conductance and VPD but with discrepancies probably associated with the measurement scale. Overall, the results showed that atmospheric and soil water stress had an additive effect. Under well-watered conditions, an increase in VPD was partially compensated by a reduction in stomatal conductance, resulting in a slight increase in the transpiration rates. With soil water deficit, the response to VPD resulted in a further decrease in stomatal conductance, reducing transpiration as a water saving strategy. The decrease in conductance in response to VPD was transitory, recovering to initial values as soon as the VPD decreased, both under well-watered and drought conditions. Due to this high sensitivity to atmospheric drought, the maximum carbon gain rates of holm oak were restricted to a short environmental window, which might modulate its physiological performance and natural distribution.

Radiation Interception, Conversion and Partitioning Efficiency in Potato Landraces: How Far Are We from the Optimum?

Crop efficiencies associated with intercepted radiation, conversion into biomass and allocation to edible organs are essential for yield improvement strategies that would enhance genetic properties to maximize carbon gain without increasing crop inputs. The production of 20 potato landraces—never studied before—was analyzed for radiation interception (), conversion () and partitioning () efficiencies. Additionally, other physiological traits related to senescence delay (normalized difference vegetation index (NDVI)), tuberization precocity (), photosynthetic performance and dry tuber yield per plant (TY) were also assessed. Vegetation reflectance was remotely acquired and the efficiencies estimated through a process-based model parameterized by a time-series of airborne imageries. The combination of and , closely associated with an early tuber maturity and a NDVI explained 39% of the variability grouping the most productive genotypes. TY was closely correlated to senescence delay (r = 0.74), indicating the usefulness of remote sensing methods for potato yield diversity characterization. About 89% of TY was explained by the first three principal components, associated mainly to , and , respectively. When comparing potato with other major crops, its is very close to the theoretical maximum. These findings suggest that there is room for improving and to enhance potato production.

Stomatal regulation of plants in response to drought stress

The regulation on carbon acquisition and water loss plays a critical role in plant growth and survival. Stomata are important portals for plants to control the exchanges of carbon and water between leaves and the atmosphere. Therefore, understanding stomatal control mechanisms and modelling stomatal conductivity are indispensable to accurately simulate carbon and water cycling in terrestrial ecosystems. As global climate change is accelerating in recent years, drought events have become more and more frequent and thus profoundly affect the survival, growth and distribution of plants. In order to deeply understand the underlying mechanism of carbon-water coupling of plants and predict the dynamics of plants and communities under global changes, it is crucial to explore responses of stomatal regulation of plants to drought stress. In this review, we synthesized recent research progress on mechanisms and modeling of plant stomatal regulation under drought stress. First, this review described the active and passive regulation of plant stomatal control in response to drought stress, and discussed the evolution of plant stomatal regulation, including the passive hydraulic regulation of ferns and lycophytes, the active regulation of angiosperms, and the dual-control mechanism of gymnosperms that was proposed as an important transitional type during evolution from ferns to angiosperms. Then, we analyzed the relationship between stomatal and hydraulic regulations, and discussed the debates on the decoupling of plant water potential from stomatal conductivity. The application of stomatal-conductivity optimization models was introduced based on the water use efficiency hypothesis and the maximum carbon gain hypothesis. The model based on the latter had a greater potential of prediction and practical application. Finally, we proposed two issues that should be urgently addressed: 1) to scale up the research of plant stomatal regulation from leaf or individual to ecosystem or even larger scales so as to improve the mechanistic models of carbon and water cycling in terrestrial ecosystems; and 2) to quantify the hydroactive feedback processes of plant stomatal regulation so as to modify current hydraulics models of plant stomatal function.

Functional traits of expanding, thicket‐forming shrubs: contrasting strategies between exotic and native species

AbstractWoody expansion has been documented for decades in many different systems globally, often yielding vast changes in ecosystem functioning. While causes and consequences of woody expansion have been well documented, few studies have addressed plant functional traits that promote dramatic and rapid expansion in range. Our objectives were to investigate plant functional traits that contribute to the colonization, rapid expansion, and thicket formation of an invasive, N‐fixing shrub, Elaeagnus umbellata Thunb. (Elaeagnaceae), and a native, N‐fixing shrub Morella cerifera (L.) Small (Myricaceae) and compare to native, sympatric, non‐expanding shrub species. Quantified functional traits included morphological (e.g., specific leaf area, leaf area) and physiological characteristics (e.g., electron transport rate, hydraulic conductivity) and were linked to two primary resources: light and water, which directly influence plant growth. Elaeagnus umbellata and M. cerifera rely on different strategies to maximize carbon gain, yet resulting physiological efficiency is similar. Elaeagnus umbellata invests a substantial amount of energy into growth during a short amount of time (i.e., deciduous growing season), using an acquisitive trait strategy to outcompete co‐occurring woody species, while M. cerifera is productive year‐round and uses a combination of conservative and acquisitive traits to outcompete co‐occurring woody species. The majority of quantified functional traits of E. umbellata and several of M. cerifera are indicative of efficient light capture, utilization, and internal water movement. These factors contribute to rapid range expansion and thicket formation by promoting enhanced productivity while simultaneously inhibiting colonization and expansion of co‐occurring species. Suites of functional traits are important for expansive success and thicket formation, yet differences in functional traits represent alternative strategies for colonization, rapid expansion, and thicketization.

Optimal stomatal behavior with competition for water and risk of hydraulic impairment

For over 40 y the dominant theory of stomatal behavior has been that plants should open stomates until the carbon gained by an infinitesimal additional opening balances the additional water lost times a water price that is constant at least over short periods. This theory has persisted because of its remarkable success in explaining strongly supported simple empirical models of stomatal conductance, even though we have also known for over 40 y that the theory is not consistent with competition among plants for water. We develop an alternative theory in which plants maximize carbon gain without pricing water loss and also add two features to both this and the classical theory, which are strongly supported by empirical evidence: (i) water flow through xylem that is progressively impaired as xylem water potential drops and (ii) fitness or carbon costs associated with low water potentials caused by a variety of mechanisms, including xylem damage repair. We show that our alternative carbon-maximization optimization is consistent with plant competition because it yields an evolutionary stable strategy (ESS)-species with the ESS stomatal behavior that will outcompete all others. We further show that, like the classical theory, the alternative theory also explains the functional forms of empirical stomatal models. We derive ways to test between the alternative optimization criteria by introducing a metric-the marginal xylem tension efficiency, which quantifies the amount of photosynthesis a plant will forego from opening stomatal an infinitesimal amount more to avoid a drop in water potential.

Supplement understanding of the relative importance of biophysical factors in determination of photosynthetic capacity and photosynthetic productivity in rice ecosystems

Extensive variation in gross primary production (GPP) among growing seasons in rice ecosystems, which has previously been reported, may be caused by a number of biophysical factors. Two field-based studies, one on Oryza sativa L. cv. Odae in the Haean Basin and the other on O. sativa L. cv. Unkwang in Gwangju, South Korea, were conducted to evaluate physiological regulators of the ecosystem. A literature survey across climatic zones was additionally undertaken to unravel the roles of relevant climatic factors. The results from the two field studies showed that the correlation between the fraction of canopy light interception (fPAR) and leaf area index (LAI) across all data sets exhibited a common trajectory. Analogous values of canopy light use efficiency (LUEc) at similar growth stages of the two rice crops, but, clearly varietal and nutritional differences in LAI development were present, which implies that temporal differences in carbon gain capacity are related to modifications in LAI, thereby influencing fPAR. The literature survey revealed that most variations in growing season GPP can be explained by changes in daily solar radiation (SR) which is negatively correlated to the proportion of rainy days; this is supported by the two field studies. In the context of temperature warming, a reduction in SR noticeably exacerbated impacts on photosynthetic productivity. Moreover, global values of maximum carbon gain capacity over the growing season (GPPday-max) in field-cultivated rice were significantly affected by SR. We suggest that: 1) temporal differences in carbon gain capacity and variations in GPPday-max are jointly driven by LAI development, which varies according to nutrient availability and rice genotypes, and SR, especially at the reproductive stage; 2) although photosynthetic productivity of rice ecosystems is vulnerable to temperature increasing and prolonged growth duration contributes to larger photosynthetic productivity, its fluctuations across growing seasons are profoundly mediated by SR, which statistically correlates with the amount of precipitation; and 3) the important regulating implication of SR changes is attributed to the structure rather than physiology dependent light sensitivity of canopy photosynthesis at the reproductive stage. Further researches regarding interactions among climate change, phenology and canopy carbon dynamics are discussed.

Optimal plant water economy

It was shown over 40 years ago that plants maximize carbon gain for a given rate of water loss if stomatal conductance, gs , varies in response to external and internal conditions such that the marginal carbon revenue of water, ∂A/∂E, remains constant over time. This theory has long held promise for understanding the physiological ecology of water use and for informing models of plant-atmosphere interactions. Full realization of this potential hinges on three questions: (i) Are analytical approximations adequate for applying the theory at diurnal time scales? (ii) At what time scale is it realistic and appropriate to apply the theory? (iii) How should gs vary to maximize growth over long time scales? We review the current state of understanding for each of these questions and describe future research frontiers. In particular, we show that analytical solutions represent the theory quite poorly, especially when boundary layer or mesophyll resistances are significant; that diurnal variations in hydraulic conductance may help or hinder maintenance of ∂A/∂E, and the matter requires further study; and that optimal diurnal responses are distinct from optimal long-term variations in gs , which emerge from optimal shifts in carbon partitioning at the whole-plant scale.

Trait Acclimation Mitigates Mortality Risks of Tropical Canopy Trees under Global Warming.

There is a heated debate about the effect of global change on tropical forests. Many scientists predict large-scale tree mortality while others point to mitigating roles of CO2 fertilization and – the notoriously unknown – physiological trait acclimation of trees. In this opinion article we provided a first quantification of the potential of trait acclimation to mitigate the negative effects of warming on tropical canopy tree growth and survival. We applied a physiological tree growth model that incorporates trait acclimation through an optimization approach. Our model estimated the maximum effect of acclimation when trees optimize traits that are strongly plastic on a week to annual time scale (leaf photosynthetic capacity, total leaf area, stem sapwood area) to maximize carbon gain. We simulated tree carbon gain for temperatures (25–35°C) and ambient CO2 concentrations (390–800 ppm) predicted for the 21st century. Full trait acclimation increased simulated carbon gain by up to 10–20% and the maximum tolerated temperature by up to 2°C, thus reducing risks of tree death under predicted warming. Functional trait acclimation may thus increase the resilience of tropical trees to warming, but cannot prevent tree death during extremely hot and dry years at current CO2 levels. We call for incorporating trait acclimation in field and experimental studies of plant functional traits, and in models that predict responses of tropical forests to climate change.

Interdependence of chronic hydraulic dysfunction and canopy processes can improve integrated models of tree response to drought

AbstractHydraulic systems of plants have evolved in the context of carbon allocation and fitness trade‐offs of maximizing carbon gain and water transport in the face of short and long‐term fluctuations in environmental conditions. The resulting diversity of traits include a continuum of isohydry‐anisohydry or high to low relative stomatal closure during drought, shedding of canopy foliage or disconnecting roots from soil to survive drought, and adjusting root areas to efficiently manage canopy water costs associated with photosynthesis. These traits are examined within TREES, an integrated model that explicitly couples photosynthesis and carbon allocation to soil‐plant hydraulics and canopy processes. Key advances of the model are its ability to account for differences in soil and xylem cavitation, transience of hydraulic impairment associated with delayed or no refilling of xylem, and carbon allocation to plant structures based on photosynthetic uptake of carbon and hydraulic limitations to water transport. The model was used to examine hydraulic traits of cooccurring isohydric (piñon pine) and anisohydric (one‐seed juniper) trees from a field‐based experimental drought. Model predictions of both transpiration and leaf water potential were improved when there was no refilling of xylem over simulations where xylem was able refill in response to soil water recharge. Model experiments with alternative root‐to‐leaf area ratios (RR/L) showed the RR/L that supports maximum cumulative water use is not beneficial for supporting maximum carbon gain during extended drought, illustrating how a process model reveals trade‐offs in plant traits.

Is the dry season an important driver of phenology and growth for two Brazilian savanna tree species with contrasting leaf habits?

Brazilian savanna (known as cerrado) has highly seasonal variation in rainfall yet trees have widely different phenological strategies ranging from evergreen to fully deciduous. While qualitative patterns of canopy phenology are well-known, few studies have quantitatively measured schedules of leaf and branch phenology. We measured the leaf and vegetative phenology of two widely distributed cerrado tree species Vochysia divergens Pohl., an evergreen species, and Curatella americana L., a semi-deciduous species, over a 1-year period and hypothesized that the dry season would represent a trigger for leaf abscission and leaf and branch growth. Leaf and branch emergence and leaf abscission for the semi-deciduous species were coincident with the end and beginning of the dry season, respectively, and were significantly correlated with dry season meteorology, but with time lags that varied depending on the meteorological variable. Leaf and branch emergence and leaf abscission for the evergreen species were also coincident with the dry season, but correlations with meteorological variables were weaker and seasonal patterns were more subtle. V. divergens leaves also suffered more from herbivory than C. americana, and there is evidence that herbivory may have altered patterns of leaf emergence for V. divergens. V. divergens leaves survived longer than C. americana leaves, and relative branch growth rates were significantly higher for C. americana. While our study was limited to only two tree species and 1 year, we demonstrated quantitatively that patterns of leaf and branch phenology were highly correlated with climatic variations. A strategy of leaf emergence and branch growth initiation during the dry season likely maximizes carbon gain by increasing rates of C assimilation by plants at the onset of the rainy season.

Sapling performance along resource gradients drives tree species distributions within and across tropical forests

Niche differentiation is a major hypothesized determinant of species distributions, but its practical importance is heavily debated and its underlying mechanisms are poorly understood. Trait‐based approaches have been used to infer niche differentiation and predict species distributions. For understanding underlying mechanisms, individual traits should be scaled up to whole‐plant performance, which has rarely been done.We measured seven key traits that are important for carbon and water balance for 37 tropical tree species. We used a process‐based plant physiological model to simulate the carbon budget of saplings along gradients of light and water availability, and quantified the performance of the species in terms of their light compensation points (a proxy for shade tolerance), water compensation points (proxy for drought tolerance), and maximum carbon gain rates (proxy for potential growth rate). We linked species performances to their observed distributions (the realized niches) at two spatial scales in Bolivian lowland forests: along a canopy openness gradient at local scale (∼1 km2) and along a rainfall gradient (1100–2200 mm/yr) at regional (∼1000 km) scale.We show that the water compensation point was the best predictor of species distributions along water and light resource gradients within and across tropical forests. A sensitivity analysis suggests that the stomatal regulation of minimum leaf water potentials, rather than stem hydraulic traits (sapwood area and specific conductivity), contributed to the species differences in the water compensation point of saplings. The light compensation point and maximum carbon gain, both driven by leaf area index and leaf nitrogen concentration, also contributed to differential species distributions at the local scale, but not or only marginally at the regional scale. Trait‐and‐physiology‐based simulations of whole‐plant performance thus help to evaluate the possible roles of individual traits in physiological processes underlying species performance along environmental gradients. The development of such whole‐plant concepts will improve our ability to understand responses of plant communities to shifts in resource availability and stress under global change.

Effects of vegetation structure on biomass accumulation in a Balanced Optimality Structure Vegetation Model (BOSVM v1.0)

Abstract. A myriad of interactions exist between vegetation and local climate for arid and semi-arid regions. Vegetation function, structure and individual behavior have large impacts on carbon–water–energy balances, which consequently influence local climate variability that, in turn, feeds back to the vegetation. In this study, a conceptual vegetation structure scheme is formulated and tested in the new Balanced Optimality Structure Vegetation Model (BOSVM) to explore the importance of vegetation structure and vegetation adaptation to water stress on equilibrium biomass states. Surface energy, water and carbon fluxes are simulated for a range of vegetation structures across a precipitation gradient in West Africa and optimal vegetation structures that maximize biomass for each precipitation regime are determined. Two different strategies of vegetation adaptation to water stress are included. Under dry conditions vegetation tries to maximize the water use efficiency and leaf area index as it tries to maximize carbon gain. However, a negative feedback mechanism in the vegetation–soil water system is found as the vegetation also tries to minimize its cover to optimize the surrounding bare ground area from which water can be extracted, thereby forming patches of vertical vegetation. Under larger precipitation, a positive feedback mechanism is found in which vegetation tries to maximize its cover as it then can reduce water loss from bare soil while having maximum carbon gain due to a large leaf area index. The competition between vegetation and bare soil determines a transition between a "survival" state to a "growing" state.