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Background and Aims The process of domestication has driven dramatic shifts in plant functional traits, including leaf mass per area (LMA). It remains unclear whether domestication has produced concerted shifts in the lower-level anatomical traits that underpin LMA and how these traits in turn affect photosynthesis. Methods In this study we investigated controls of LMA and leaf gas exchange by leaf anatomical properties at the cellular, tissue and whole-leaf levels, comparing 26 wild and 31 domesticated genotypes of cotton (Gossypium). Key Results As expected, domesticated plants expressed lower LMA, higher photosynthesis and higher stomatal conductance, suggesting a shift towards the ‘faster’ end of the leaf economics spectrum. At whole-leaf level, variation in LMA was predominantly determined by leaf density (LD) both in wild and domesticated genotypes. At tissue level, higher leaf volume per area (Vleaf) in domesticated genotypes was driven by a simultaneous increase in the volume of epidermal, mesophyll and vascular bundle tissue and airspace, while lower LD resulted from a lower volume of palisade tissue and vascular bundles (which are of high density), paired with a greater volume of epidermis and airspace, which are of low density. The volume of spongy mesophyll exerted direct control on photosynthesis in domesticated genotypes but only indirect control in wild genotypes. At cellular level, a shift to larger but less numerous cells with thinner cell walls underpinned a lower proportion of cell wall mass, and thus a reduction in LD. Conclusions Taken together, cotton domestication has triggered synergistic shifts in the underlying determinants of LMA but also photosynthesis, at cell, tissue and whole-leaf levels, resulting in a marked shift in plant ecological strategy.

Background and Aims Displacement of native plant species by non-native invaders may result from differences in their carbon economy, yet little is known regarding how variation in leaf traits influences native–invader dynamics across climate gradients. In Hawaii, one of the most heavily invaded biodiversity hotspots in the world, strong spatial variation in climate results from the complex topography, which underlies variation in traits that probably drives shifts in species interactions. Methods Using one of the most comprehensive trait data sets for Hawaii to date (91 species and four islands), we determined the extent and sources of variation (climate, species and species origin) in leaf traits, and used mixed models to examine differences between natives and non-native invasives. Key Results We detected significant differences in trait means, such that invasives were more resource acquisitive than natives over most of the climate gradients. However, we also detected trait convergence and a rank reversal (natives more resource acquisitive than invasives) in a sub-set of conditions. There was significant intraspecific trait variation (ITV) in leaf traits of natives and invasives, although invasives expressed significantly greater ITV than natives in water loss and photosynthesis. Species accounted for more trait variation than did climate for invasives, while the reverse was true for natives. Incorporating this climate-driven trait variation significantly improved the fit of models that compared natives and invasives. Lastly, in invasives, ITV was most strongly explained by spatial heterogeneity in moisture, whereas solar energy explains more ITV in natives. Conclusions Our results indicate that trait expression and ITV vary significantly between natives and invasives, and that this is mediated by climate. These findings suggest that although natives and invasives are functionally similar at the regional scale, invader success at local scales is contingent on climate.

"Least-cost theory" posits that C3 plants should balance rates of photosynthetic water loss and carboxylation in relation to the relative acquisition and maintenance costs of resources required for these activities. Here we investigated the dependency of photosynthetic traits on climate and soil properties using a new Australia-wide trait dataset spanning 528 species from 67 sites. We tested the hypotheses that plants on relatively cold or dry sites, or on relatively more fertile sites, would typically operate at greater CO2 drawdown (lower ratio of leaf internal to ambient CO2 , Ci :Ca ) during light-saturated photosynthesis, and at higher leaf N per area (Narea ) and higher carboxylation capacity (Vcmax 25 ) for a given rate of stomatal conductance to water vapour, gsw . These results would be indicative of plants having relatively higher water costs than nutrient costs. In general, our hypotheses were supported. Soil total phosphorus (P) concentration and (more weakly) soil pH exerted positive effects on the Narea -gsw and Vcmax 25 -gsw slopes, and negative effects on Ci :Ca . The P effect strengthened when the effect of climate was removed via partial regression. We observed similar trends with increasing soil cation exchange capacity and clay content, which affect soil nutrient availability, and found that soil properties explained similar amounts of variation in the focal traits as climate did. Although climate typically explained more trait variation than soil did, together they explained up to 52% of variation in the slope relationships and soil properties explained up to 30% of the variation in individual traits. Soils influenced photosynthetic traits as well as their coordination. In particular, the influence of soil P likely reflects the Australia's geologically ancient low-relief landscapes with highly leached soils. Least-cost theory provides a valuable framework for understanding trade-offs between resource costs and use in plants, including limiting soil nutrients.

Background: Climates are changing at a rate that exceeds the adaptive capacity of species, especially endangered species. Genetic variation and phenotypic plasticity are important for population persistence, yet few studies have linked traits to seedling performance under drought in endangered species. Aims: We assessed intraspecific variation and trait plasticity under drought, to understand how an endangered species would cope with increasingly severe droughts. Methods: Using greenhouse experiments, we assessed drought tolerance of a critically endangered Hawaiian shrub, Schiedea obovata. Seedlings from five maternal families (three selfed and two interpopulation crosses) were subjected to daily watering or a simulated drought, and we measured growth, traits linked to drought tolerance, and days until death under terminal drought. Results: Drought reduced growth but not days until death. We detected genetic variation in growth, but no trait plasticity except in carbon:nitrogen, which decreased under drought. We did not detect traits that enhanced performance under drought but identified four physiological traits whose effects on growth varied under control and drought. Conclusions: Our results indicate moderate drought tolerance of an endangered shrub, and low trait plasticity. Conservation of endangered species under shifting climates will benefit from studies of stress tolerance, particularly at the vulnerable seedling stage.

Climate change models project an increase in the frequency and duration of drought globally. Changes in rainfall are expected to have particularly detrimental effects on seedlings due to their inability to reach deep water sources. We conducted a greenhouse experiment to test how the timing of watering affects seedling performance, physiology, and morphology in a keystone island endemic tree, Metrosideros polymorpha. Seedlings were grown for 60 days in one of five conditions: control treatment with daily watering to 100% field capacity (FC), a 28-day pulse drought with no water, and three groups maintained at 80% FC and watered every 2, 7, or 14 days. Seedling drought tolerance depended on the duration of drought; growth for plants watered to 80% FC every 2 or 14 days was comparable to 100% FC plants, but was reduced in the 28-day pulse drought and 7-day 80% FC. Survival was high in all treatments except the pulse drought. Stomatal conductance, chlorophyll content, and C:N ratio were positively related to height and biomass under drought. Traits associated with resource investment, fluorescence, and water use were plastic in their expression, shifting under drought compared to control conditions. M. polymorpha seedlings exhibit trait plasticity that allows them to tolerate drought, but its duration and intensity influence their survival and growth. Our results highlight the need for experimental studies manipulating temporal variation in water with explicit consideration of the role of trait plasticity in drought tolerance.

As the most remote archipelago in the world, the Hawaiian Islands are home to a highly endemic and disharmonic biota that has fascinated biologists for centuries. Forests are the dominant terrestrial biome in Hawai‘i, spanning complex, heterogeneous climates across substrates that vary tremendously in age, soil structure, and nutrient availability. Species richness is low in Hawaiian forests compared to other tropical forests, as a consequence of dispersal limitation from continents and adaptive radiations in only some lineages, and forests are dominated by the widespread Metrosideros species complex. Low species richness provides a relatively tractable model system for studies of community assembly, local adaptation, and species interactions. Moreover, Hawaiian forests provide insights into predicted patterns of evolution on islands, revealing that while some evidence supports “island syndromes,” there are exceptions to them all. For example, Hawaiian plants are not as a whole less defended against herbivores, less dispersible, more conservative in resource use, or more slow-growing than their continental relatives. Clearly, more work is needed to understand the drivers, sources, and constraints on phenotypic variation among Hawaiian species, including both widespread and rare species, and to understand the role of this variation for ecological and evolutionary processes, which will further contribute to conservation of this unique biota. Today, Hawaiian forests are among the most threatened globally. Resource management failures – the proliferation of non-native species in particular – have led to devastating declines in native taxa and resulted in dominance by novel species assemblages. Conservation and restoration of Hawaiian forests now rely on managing threats including climate change, ongoing species introductions, novel pathogens, lost mutualists, and altered ecosystem dynamics through the use of diverse tools and strategies grounded in basic ecological, evolutionary, and biocultural principles. The future of Hawaiian forests thus depends on the synthesis of ecological and evolutionary research, which will continue to inform future conservation and restoration practices.

Summary The understorey of tropical forests is heterogeneous across time, and plants that inhabit this layer may exhibit adaptations (e.g. trait plasticity) that enable them to exploit this variability to their advantage. We tested the hypothesis that two widespread understorey herbs would perform equally well in a variable as in a constant environment, using a 2-year shade-house experiment. We measured demographic traits (growth and survival), a physiological trait (maximum photosynthetic capacity), and life-history traits (leaf life span and biomass allocation) of Heliconia tortuosa and Calathea crotalifera. We investigated how these traits were affected by light availability at the seedling stage, precipitation, and whether individuals experienced a constant or variable light environment. Whether or not a variable environment was favourable for plants depended upon precipitation and the environment in which individuals started life. At low precipitation, plants in a variable light environment grew more than those in a constant environment, but only when individuals had lived as seedlings in low light. At high precipitation, plants in a constant environment grew more than those in a variable environment, regardless of early conditions. Survival was lower in a variable environment at low precipitation, and more so at high precipitation. Photosynthetic capacity was lower for individuals in a variable environment than in a constant environment when they had lived in high light as seedlings. Calathea grew faster and survived more poorly than Heliconia, independently of the treatments. Calathea grew more at high than low precipitation while Heliconia grew more at low than high precipitation. Leaf life span and biomass allocation did not differ among treatments, although Calathea had a significantly greater proportion of its biomass above-ground vs. that of Heliconia. Synthesis. Environmental variability had a neutral or positive effect on biomass allocation, photosynthetic capacity, and leaf life span for these species. Survival was the only trait that was always lower in a variable environment. The effect of environmental variability was dependent on early life conditions as well as precipitation, suggesting that generalist species may experience high fitness as forest environments become more variable by maintaining high growth at the expense of survival.

Photosynthetic 'least-cost' theory posits that the optimal trait combination for a given environment is that where the summed costs of photosynthetic water and nutrient acquisition/use are minimised. The effects of soil water and nutrient availability on photosynthesis should be stronger as climate-related costs for both resources increase. Two independent datasets of photosynthetic traits, Globamax (1509 species, 288 sites) and Glob13C (3645 species, 594 sites), were used to quantify biophysical and biochemical limitations of photosynthesis and the key variable Ci /Ca (CO2 drawdown during photosynthesis). Climate and soil variables were associated with both datasets. The biochemical photosynthetic capacity was higher on alkaline soils. This effect was strongest at more arid sites, where water unit-costs are presumably higher. Higher values of soil silt and depth increased Ci /Ca , likely by providing greater H2 O supply, alleviating biophysical photosynthetic limitation when soil water is scarce. Climate is important in controlling the optimal balance of H2 O and N costs for photosynthesis, but soil properties change these costs, both directly and indirectly. In total, soil properties modify the climate-demand driven predictions of Ci /Ca by up to 30% at a global scale.

Temporal variability in light from gaps in the tree canopy strongly influences the vital rates of understory plants. From 2012-2015, we estimated the size-specific vital rates of two herbs, Calathea crotalifera and Heliconia tortuosa, over a range of light environments. We estimated maximum photosynthetic capacity (Amax) for a subset of individuals each year during three annual censuses, and modelled future size as a linear function of current size (a plant trait that changes ontogenetically), canopy openness (an environmental variable), and Amax (a potentially plastic physiological trait). We estimated what the demographic success would be of a population comprised of individuals with a particular fixed Amax for each of several levels of canopy openness if the environment remained constant, by evaluating corresponding Integral Projection Models and their deterministic growth rates (λ). We then estimated their demographic success in the stochastic light environment (λS) and its elasticities. As light increased, deterministic λ increased for Calathea by 33% but decreased for Heliconia by 52%, and increasing Amax had no effect on λ for Calathea but increased λ for Heliconia in low light. As Amax increased, λS increased for Heliconia, but not Calathea. We also investigated whether photosynthetic rates would influence the elasticities of λS, including its response to perturbation of: vital rates in each environment (ESβ), vital rates over all environments (ES), and variability of vital rates among environments (ESσ). ES, ESσ, and ESβ were influenced by Amax for Heliconia but not Calathea. Events that affect some vital rates in high light have a greater impact on overall fitness than events that affect the same vital rates in shady environments, and there is greater potential for selection on traits of large individuals in high light than in low light for Heliconia, while the reverse was true for Calathea. Photosynthetic rates, through their effects on growth, can strongly influence the population dynamics of plants in random light environments, but the magnitude of this effect varies between species. In the species for which fitness was independent of Amax, Calathea, there would be little opportunity for selection on photosynthetic rates. This article is protected by copyright. All rights reserved.

UNLABELLED • PREMISE OF THE STUDY For tropical forest understory plants, the ability to grow, survive, and reproduce is limited by the availability of light. The extent to which reproduction incurs a survival or growth cost may change with light availability, plant size, and adaptation to shade, and may vary among similar species.• METHODS We estimated size-specific rates of growth, survival, and reproduction (vital rates), for two neotropical understory herbs (order Zingiberales) in a premontane tropical rainforest in Costa Rica. During three annual censuses we monitored 1278 plants, measuring leaf area, number of inflorescences, and canopy openness. We fit regression models of all vital rates and evaluated them over a range of light levels. The best fitting models were selected using Akaike's Information Criterion.• KEY RESULTS All vital rates were significantly influenced by size in both species, but not always by light. Increasing light resulted in higher growth and a higher probability of reproduction in both species, but lower survival in one species. Both species grew at small sizes but shrank at larger sizes. The size at which shrinkage began differed among species and light environments. Vital rates of large individuals were more sensitive to changes in light than small individuals.• CONCLUSIONS Increasing light does not always positively influence vital rates; the extent to which light affects vital rates depends on plant size. Differences among species in their abilities to thrive under different light conditions and thus occupy distinct niches may contribute to the maintenance of species diversity.

Global climate change is expected to increase the aridity of semiarid landscapes by increasing heat stress and decreasing soil moisture, negatively impacting plant survival, growth, and reproduction and forcing major shifts in plant community composition. To better understand potential vegetation shifts, we investigated how aspect-mediated differences in soil moisture and nitrogen influence tree-grass and tree species coexistence in pinyon-juniper woodlands in the southwestern United States (at the Sevilleta Long Term Ecological Research Station in New Mexico). We measured soil moisture, total mineralized soil nitrogen, grass cover, and woody cover and demography of the two dominant trees—two-needle pinyon pine (Pinus edulis) and one-seeded juniper (Juniperus monosperma)—along 32 transects on north-facing and south-facing slopes. Tree cover was greater on north-facing than on south-facing slopes, reflecting significantly higher soil moisture on northern aspects. Aspect did not affect soil nitrogen or grass cover. Population structure for juniper was influenced by aspect while reproductive output was influenced by aspect in both species, suggesting that differences in drought tolerance between these codominant tree species may be responsible for differences in the utilization of resources in our study area. Our results suggest that soil moisture can affect tree cover and tree-grass coexistence more than does nitrogen. Increasing aridity due to global climate change is likely to decrease woody cover in semiarid landscapes, resulting in systems codominated by grasses and woody species.