Large variation in availability of Maya food plant sources during ancient droughts.

Paleoclimatic evidence indicating a series of droughts in the Yucatan Peninsula during the Terminal Classic period suggests that climate change may have contributed to the disruption or collapse of Classic Maya polities. Although climate change cannot fully account for the multifaceted, political turmoil of the period, it is clear that droughts of strong magnitude could have limited food availability, potentially causing famine, migration, and societal decline. Maize was undoubtedly an important staple food of the ancient Maya, but a complete analysis of other food resources that would have been available during drought remains unresolved. Here, we assess drought resistance of all 497 indigenous food plant species documented in ethnographic, ethnobotanical, and botanical studies as having been used by the lowland Maya and classify the availability of these plant species and their edible components under various drought scenarios. Our analysis indicates availability of 83% of food plant species in short-term drought, but this percentage drops to 22% of food plant species available in moderate drought up to 1 y. During extreme drought, lasting several years, our analysis indicates availability of 11% of food plant species. Our results demonstrate a greater diversity of food sources beyond maize that would have been available to the Maya during climate disruption of the Terminal Classic period than has been previously acknowledged. While drought would have necessitated shifts in dietary patterns, the range of physiological drought responses for the available food plants would have allowed a continuing food supply under all but the most dire conditions.

Drought stress influences foraging preference of a solitary bee on two wildflowers.

BACKGROUND AND AIMS: Pollinators provide critical ecosystem services, maintaining biodiversity and benefiting global food production. However, plants, pollinators, and their mutualistic interactions may be affected by drought, which has increased in severity and frequency under climate change. Using two annual, insect-pollinated wildflowers (Phacelia campanularia and Nemophila menziesii), we asked how drought impacts floral traits and foraging preferences of a solitary bee (Osmia lignaria) and explore potential implications for plant reproduction. METHODS: In greenhouses, we experimentally subjected plants to drought to induce water stress, as verified by leaf water potential. To assess the impact of drought on floral traits, we measured flower size, floral display size, nectar volume, and nectar sugar concentration. To explore how drought-induced effects on floral traits affected bee foraging preferences, we performed choice trials. Individual female bees were placed into foraging arenas with two conspecific plants, one droughted and one non-droughted, and were allowed to forage freely. KEY RESULTS: We determined that P. campanularia is more drought-tolerant than N. menziesii based on measures of turgor loss point, and confirmed that droughted plants were more drought-stressed than non-droughted plants. For droughted plants of both species, floral display size was reduced, and flowers were smaller and produced less, more-concentrated nectar. We found that bees preferred non-droughted flowers of N. menziesii. However, bee preference for non-droughted P. campanularia flowers depended on the time of day and was detected only in the afternoon. CONCLUSIONS: Our findings indicate that bees prefer visiting non-droughted flowers, likely reducing pollination success for drought-stressed plants. Lack of preference for non-droughted P. campanularia flowers in the morning may reflect the higher drought tolerance of this species. This work highlights the potentially intersecting, short-term physiological and pollinator behavioral responses to drought and suggests that such responses may reshape plant-pollinator interactions, ultimately reducing reproductive output for less drought-tolerant wildflowers.

Irrigated urban trees exhibit greater functional trait plasticity compared to natural stands.

Urbanization creates novel ecosystems comprised of species assemblages and environments with no natural analogue. Moreover, irrigation can alter plant function compared to non-irrigated systems. However, the capacity of irrigation to alter functional trait patterns across multiple species is unknown but may be important for the dynamics of urban ecosystems. We evaluated the hypothesis that urban irrigation influences plasticity in functional traits by measuring carbon-gain and water-use traits of 30 tree species planted in Southern California, USA spanning a coastal-to-desert gradient. Tree species respond to irrigation through increasing the carbon-gain trait relationship of leaf nitrogen per specific leaf area compared to their native habitat. Moreover, most species shift to a water-use strategy of greater water loss through stomata when planted in irrigated desert-like environments compared to coastal environments, implying that irrigated species capitalize on increased water availability to cool their leaves in extreme heat and high evaporative demand conditions. Therefore, irrigated urban environments increase the plasticity of trait responses compared to native ecosystems, allowing for novel response to climatic variation. Our results indicate that trees grown in water-resource-rich urban ecosystems can alter their functional traits plasticity beyond those measured in native ecosystems, which can lead to plant trait dynamics with no natural analogue.

Towards a statistically robust determination of minimum water potential and hydraulic risk in plants.

Minimum water potential (Ψmin ) is a key variable for characterizing dehydration tolerance and hydraulic safety margins (HSMs) in plants. Ψmin is usually estimated as the absolute minimum tissue Ψ experienced by a species, but this is problematic because sample extremes are affected by sample size and the underlying probability distribution. We compare alternative approaches to estimate Ψmin and assess the corresponding uncertainties and biases; propose statistically robust estimation methods based on extreme value theory (EVT); and assess the implications of our results for the characterization of hydraulic risk. Our results show that current estimates of Ψmin and HSMs are biased, as they are strongly affected by sample size. Because sampling effort is generally higher for species living in dry environments, the differences in current Ψmin estimates between these species and those living under milder conditions are partly artefactual. When this bias is corrected using EVT methods, resulting HSMs tend to increase substantially with resistance to embolism across species. Although data availability and representativeness remain the main challenges for proper determination of Ψmin , a closer look at Ψ distributions and the use of statistically robust methods to estimate Ψmin opens new ground for characterizing plant hydraulic risks.

Functional traits of leaves and photosynthetic stems of species from a sarcocaulescent scrub in the southern Baja California Peninsula.

PREMISE: Photosynthetic stems represent a source of extra carbon in plants from hot and dry environments, but little is known about how leaves and photosynthetic stems differ in terms of photosynthetic capacity, trait coordination, and responses to seasonal drought in subtropical systems. METHODS: We studied photosynthetic, hydraulic, morphometric (specific leaf area [SLA], wood density [WD]), and biochemical (C and N isotopes) traits in leaves and photosynthetic stems of 12 plant species from a sarcocaulescent scrub in the southern Baja California Peninsula, Mexico, in wet and dry seasons. RESULTS: Leaves and stems had similar mean photosynthetic capacity, as evaluated by chlorophyll fluorescence traits, indicating similar investment in leaf and stem photosynthesis. We did not find a relationship between stem hydraulic conductivity and leaf or stem photosynthetic traits. However, we found resource allocation trade-offs, between WD and both stem hydraulic conductivity and SLA. Leaf and stem photosynthetic traits did not change with season, but specific stem area was one of the few traits that changed the most between seasons-it increased during the dry season by as much as 154% indicating substantial water storage. CONCLUSIONS: Our results indicate the same proportional investment in photosynthetic capacity and dry matter in both leaves and photosynthetic stems across all 12 species. We identified multiple strategies at this seasonal site, with species ranging from high WD, low SLA, low hydraulic conductivity, and high specific bark area on one end of the spectrum and opposite traits on the other end.

Coordination and trade-offs among hydraulic safety, efficiency and drought avoidance traits in Amazonian rainforest canopy tree species.

Predicting responses of tropical forests to climate change-type drought is challenging because of high species diversity. Detailed characterization of tropical tree hydraulic physiology is necessary to evaluate community drought vulnerability and improve model parameterization. Here, we measured xylem hydraulic conductivity (hydraulic efficiency), xylem vulnerability curves (hydraulic safety), sapwood pressure-volume curves (drought avoidance) and wood density on emergent branches of 14 common species of Eastern Amazonian canopy trees in Paracou, French Guiana across species with the densest and lightest wood in the plot. Our objectives were to evaluate relationships among hydraulic traits to identify strategies and test the ability of easy-to-measure traits as proxies for hard-to-measure hydraulic traits. Xylem efficiency was related to capacitance, sapwood water content and turgor loss point, and other drought avoidance traits, but not to xylem safety (P50 ). Wood density was correlated (r = -0.57 to -0.97) with sapwood pressure-volume traits, forming an axis of hydraulic strategy variation. In contrast to drier sites where hydraulic safety plays a greater role, tropical trees in this humid tropical site varied along an axis with low wood density, high xylem efficiency and high capacitance at one end of the spectrum, and high wood density and low turgor loss point at the other.

Stomatal behaviour and stem xylem traits are coordinated for woody plant species under exceptional drought conditions.

Isohydry (maintenance of plant water potential at the cost of carbon gain) and anisohydry (gas exchange maintenance at the cost of declining plant water status) make up two ends of a stomatal drought response strategy continuum. However, few studies have merged measures of stomatal regulation with xylem hydraulic safety strategies based on in situ field measurements. The goal of this study was to characterize the stomatal and xylem hydraulic safety strategies of woody species in the biodiverse Mediterranean-type ecosystem region of California. Measurements were conducted in situ when California was experiencing the most severe drought conditions in the past 1,200 years. We found coordination among stomatal, hydraulic, and standard leaf functional traits. For example, stem xylem vulnerability to cavitation (P50 ) was correlated with the water potential at stomatal closure (Pclose ); more resistant species had a more negative water potential at stomatal closure. The degree of isohydry-anisohydry, defined at Pclose -P50 , was correlated with the hydraulic safety margin across species; more isohydric species had a larger hydraulic safety margin. In addition, we report for the first time Pclose values below -10 MPa. Measuring these traits in a biodiverse region under exceptional drought conditions contributes to our understanding of plant drought responses.

Stem photosynthesis and hydraulics are coordinated in desert plant species.

Coordination between stem photosynthesis and hydraulics in green-stemmed desert plants is important for understanding the physiology of stem photosynthesis and possible drought responses. Plants with photosynthetic stems have extra carbon gain that can help cope with the detrimental effects of drought. We studied photosynthetic, hydraulic and functional traits of 11 plant species with photosynthetic stems from three California desert locations. We compared relationships among traits between wet and dry seasons to test the effect of seasonality on these relationships. Finally, we compared stem trait relationships with analogous relationships in the leaf economics spectrum. We found that photosynthetic and hydraulic traits are coordinated in photosynthetic stems. The slope or intercept of all trait relationships was mediated by seasonality. The relationship between mass-based stem photosynthetic CO2 assimilation rate (Amass ) and specific stem area (SSA; stem surface area to dry mass ratio) was statistically indistinguishable from the leaf economics spectrum. Our results indicate that photosynthetic stems behave like leaves in the coordination of multiple traits related to carbon gain, water movement and water loss. Because of the similarity of the stem Amass -SSA relationship to the leaf Amass -specific leaf area relationship, we suggest the existence of a photosynthetic stem economic spectrum.

Reconciling seasonal hydraulic risk and plant water use through probabilistic soil-plant dynamics.

Current models used for predicting vegetation responses to climate change are often guided by the dichotomous needs to resolve either (i) internal plant water status as a proxy for physiological vulnerability or (ii) external water and carbon fluxes and atmospheric feedbacks. Yet, accurate representation of fluxes does not always equate to accurate predictions of vulnerability. We resolve this discrepancy using a hydrodynamic framework that simultaneously tracks plant water status and water uptake. We couple a minimalist plant hydraulics model with a soil moisture model and, for the first time, translate rainfall variability at multiple timescales - with explicit descriptions at daily, seasonal, and interannual timescales - into a physiologically meaningful metric for the risk of hydraulic failure. The model, parameterized with measured traits from chaparral species native to Southern California, shows that apparently similar transpiration patterns throughout the dry season can emerge from disparate plant water potential trajectories, and vice versa. The parsimonious set of parameters that captures the role of many traits across the soil-plant-atmosphere continuum is then used to establish differences in species sensitivities to shifts in seasonal rainfall statistics, showing that co-occurring species may diverge in their risk of hydraulic failure despite minimal changes to their seasonal water use. The results suggest potential shifts in species composition in this region due to species-specific changes in hydraulic risk. Our process-based approach offers a quantitative framework for understanding species sensitivity across multiple timescales of rainfall variability and provides a promising avenue toward incorporating interactions of temporal variability and physiological mechanisms into drought response models.

Plant hydraulics as a central hub integrating plant and ecosystem function: meeting report for 'Emerging Frontiers in Plant Hydraulics' (Washington, DC, May 2015).

Water plays a central role in plant biology and the efficiency of water transport throughout the plant affects both photosynthetic rate and growth, an influence that scales up deterministically to the productivity of terrestrial ecosystems. Moreover, hydraulic traits mediate the ways in which plants interact with their abiotic and biotic environment. At landscape to global scale, plant hydraulic traits are important in describing the function of ecological communities and ecosystems. Plant hydraulics is increasingly recognized as a central hub within a network by which plant biology is connected to palaeobiology, agronomy, climatology, forestry, community and ecosystem ecology and earth-system science. Such grand challenges as anticipating and mitigating the impacts of climate change, and improving the security and sustainability of our food supply rely on our fundamental knowledge of how water behaves in the cells, tissues, organs, bodies and diverse communities of plants. A workshop, 'Emerging Frontiers in Plant Hydraulics' supported by the National Science Foundation, was held in Washington DC, 2015 to promote open discussion of new ideas, controversies regarding measurements and analyses, and especially, the potential for expansion of up-scaled and down-scaled inter-disciplinary research, and the strengthening of connections between plant hydraulic research, allied fields and global modelling efforts.

Plant hydraulic responses to long-term dry season nitrogen deposition alter drought tolerance in a Mediterranean-type ecosystem.

Anthropogenic nitrogen (N) deposition represents a significant N input for many terrestrial ecosystems. N deposition can affect plants on scales ranging from photosynthesis to community composition, yet few studies have investigated how changes in N availability affect plant water relations. We tested the effects of N addition on plant water relations, hydraulic traits, functional traits, gas exchange, and leaf chemistry in a semi-arid ecosystem in Southern California using long-term experimental plots fertilized with N for over a decade. The dominant species were Artemisia california and Salvia mellifera at Santa Margarita Ecological Reserve and Adenostoma fasciculatum and Ceanothus greggii at Sky Oaks Field Station. All species, except Ceanothus, showed increased leaf N concentration, decreased foliar carbon to N ratio, and increased foliar N isotopic composition with fertilization, indicating that added N was taken up by study species, yet each species had a differing physiological response to long-term N addition. Dry season predawn water potentials were less negative with N addition for all species except Adenostoma, but there were no differences in midday water potentials, or wet season water potentials. Artemisia was particularly responsive, as N addition increased stem hydraulic conductivity, stomatal conductance, and leaf carbon isotopic composition, and decreased wood density. The alteration of water relations and drought resistance parameters with N addition in Artemisia, as well as Adenostoma, Ceanothus, and Salvia, indicate that N deposition can affect the ability of native Southern California shrubs to respond to drought.

Lianas always outperform tree seedlings regardless of soil nutrients: results from a long-term fertilization experiment.

Lianas are a prominent growth form in tropical forests, and there is compelling evidence that they are increasing in abundance throughout the Neotropics. While recent evidence shows that soil resources limit tree growth even in deep shade, the degree to which soil resources limit lianas in forest understories, where they coexist with trees for decades, remains unknown. Regardless, the physiological underpinnings of soil resource limitation in deeply shaded tropical habitats remain largely unexplored for either trees or lianas. Theory predicts that lianas should be more limited by soil resources than trees because they occupy the quick-return end of the "leaf economic spectrum," characterized by high rates of photosynthesis, high specific leaf area, short leaf life span, affinity to high-nutrient sites, and greater foliar nutrient concentrations. To address these issues, we asked whether soil resources (nitrogen, phosphorus, and potassium), alone or in combination, applied experimentally for more than a decade would cause significant changes in the morphology or physiology of tree and liana seedlings in a lowland tropical forest. We found evidence for the first time that phosphorus limits the photosynthetic performance of both trees and lianas in deeply shaded understory habitats. More importantly, lianas always showed significantly greater photosynthetic capacity, quenching, and saturating light levels compared to trees across all treatments. We found little evidence for nutrient x growth form interactions, indicating that lianas were not disproportionately favored in nutrient-rich habitats. Tree and liana seedlings differed markedly for six key morphological traits, demonstrating that architectural differences occurred very early in ontogeny prior to lianas finding a trellis (all seedlings were self-supporting). Overall, our results do not support nutrient loading as a mechanism of increasing liana abundance in the Neotropics. Rather, our finding that lianas always outperform trees, in terms of photosynthetic processes and under contrasting rates of resource supply of macronutrients, will allow lianas to increase in abundance if disturbance and tree turnover rates are increasing in Neotropical forests as has been suggested.

Global-scale environmental control of plant photosynthetic capacity.

Photosynthetic capacity, determined by light harvesting and carboxylation reactions, is a key plant trait that determines the rate of photosynthesis; however, in Earth System Models (ESMs) at a reference temperature, it is either a fixed value for a given plant functional type or derived from a linear function of leaf nitrogen content. In this study, we conducted a comprehensive analysis that considered correlations of environmental factors with photosynthetic capacity as determined by maximum carboxylation (V(cm)) rate scaled to 25 degrees C (i.e., V(c),25; µmol CO2 x m(-2)x s(-1)) and maximum electron transport rate (J(max)) scaled to 25 degrees C (i.e., J25; µmol electron x m(-2) x s(-1)) at the global scale. Our results showed that the percentage of variation in observed V(c),25 and J25 explained jointly by the environmental factors (i.e., day length, radiation, temperature, and humidity) were 2-2.5 times and 6-9 times of that explained by area-based leaf nitrogen content, respectively. Environmental factors influenced photosynthetic capacity mainly through photosynthetic nitrogen use efficiency, rather than through leaf nitrogen content. The combination of leaf nitrogen content and environmental factors was able to explain -56% and -66% of the variation in V(c),25 and J25 at the global scale, respectively. Our analyses suggest that model projections of plant photosynthetic capacity and hence land-atmosphere exchange under changing climatic conditions could be substantially improved if environmental factors are incorporated into algorithms used to parameterize photosynthetic capacity in ESMs.

Coordination of stem and leaf hydraulic conductance in southern California shrubs: a test of the hydraulic segmentation hypothesis.

Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, 'expendable' organs such as leaves and longer lived, more 'expensive' organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or 'safety valve'. We tested the HSH in woody species from a Mediterranean-type ecosystem by measuring leaf hydraulic conductance (Kleaf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kleaf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψstem-Ψ50)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole-plant water use strategies. For hydrated shoots, Kleaf was negatively correlated with KS , supporting the HSH. Additionally, Kleaf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio. Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS , or a low safety margin. This critical role of leaves appears to contribute importantly to plant ecological specialization in a drought-prone environment.

Light use efficiency of California redwood forest understory plants along a moisture gradient.

We investigated photosynthesis of five plant species growing in the understory at three sites (1,170-, 1,600- and 2,100-mm annual moisture inputs), along the geographical range of coastal California redwood forest, to determine whether greater inputs of rain and fog at northern sites enhance photosynthetic utilization of fluctuating light. Measurements of understory light environment and gas exchange were carried out to determine steady state and dynamic photosynthetic responses to light. Leaf area index ranged from 4.84 at the 2,100-mm site to 5.98 at the 1,170-mm site. Maximum rates of net photosynthesis and stomatal conductance (g) did not vary appreciably within species across sites. Photosynthetic induction after a change from low to high light was significantly greater in plants growing in lower light conditions regardless of site. Photosynthetic induction also increased with the rate of g in diffuse light, prior to the increase to saturating light levels. Post-illumination CO2 assimilation was the largest factor contributing to variation in C gain during simulated lightflecks. The duration of post-illumination photosynthetic activity, total CO2 assimilation per light received, and light use efficiency during simulated lightflecks increased significantly with moisture inputs in four out of five species. Increasing leaf N concentration with increasing moisture inputs in three out of five species, coupled with changes in leaf N isotopic composition with the onset of the summer fog season suggest that natural N deposition increases with rain and fog inputs and contributes to greater utilization of fluctuating light availability in coastal California redwood forests.

Determinants of change in subtropical tree diameter growth with ontogenetic stage.

We evaluated the degree to which relative growth rate (RGR) of saplings and large trees is related to seven functional traits that describe physiological behavior and soil environmental factors related to topography and fertility for 57 subtropical tree species in Dinghushan, China. The mean values of functional traits and soil environmental factors for each species that were related to RGR varied with ontogenetic stage. Sapling RGR showed greater relationships with functional traits than large-tree RGR, whereas large-tree RGR was more associated with soil environment than was sapling RGR. The strongest single predictors of RGR were wood density for saplings and slope aspect for large trees. The stepwise regression model for large trees accounted for a larger proportion of variability (R(2) = 0.95) in RGR than the model for saplings (R(2) = 0.55). Functional diversity analysis revealed that the process of habitat filtering likely contributes to the substantial changes in regulation of RGR as communities transition from saplings to large trees.

The benefits of woody plant stem photosynthesis extend to hydraulic function and drought survival in Parkinsonia florida.

As climate change exacerbates drought stress in many parts of the world, understanding plant physiological mechanisms for drought survival is critical to predicting ecosystem responses. Stem net photosynthesis, which is common in arid environments, may be a drought survival trait, but whether the additional carbon fixed by stems contributes to plant hydraulic function and drought survival in arid land plants is untested. We conducted a stem light-exclusion experiment on saplings of a widespread North American desert tree species, Parkinsonia florida L., and after shading acclimation, we then subjected half of the plants to a drought treatment to test the interaction between light exclusion and water limitation on growth, leaf and stem photosynthetic gas exchange, xylem embolism assessed with micro-computed tomography and gravimetric techniques, and survival. Growth, stem photosynthetic gas exchange, hydraulic function and survival all showed expected reductions in response to light exclusion. However, stem photosynthesis mitigated the drought-induced reductions in gas exchange, xylem embolism (percent loss of conductivity, PLC) and mortality. The highest mortality was in the combined light exclusion and drought treatment, and was related to stem PLC and native sapwood-specific hydraulic conductivity. This research highlights the integration of carbon economy and water transport. Our results show that additional carbon income by photosynthetic stems has an important role in the growth and survival of a widespread desert tree species during drought. This shift in function under conditions of increasing stress underscores the importance of considering stem photosynthesis for predicting drought-induced mortality not only for the additional supply of carbon, but also for its extended benefits for hydraulic function.

Biocrust carbon exchange varies with crust type and time on Chihuahuan Desert gypsum soils.

Introduction: In dryland systems, biological soil crusts (biocrusts) can occupy large areas of plant interspaces, where they fix carbon following rain. Although distinct biocrust types contain different dominant photoautotrophs, few studies to date have documented carbon exchange over time from various biocrust types. This is especially true for gypsum soils. Our objective was to assess the carbon exchange of biocrust types established at the world's largest gypsum dune field at White Sands National Park. Methods: We sampled five different biocrust types from a sand sheet location in three different years and seasons (summer 2020, fall 2021, and winter 2022) for carbon exchange measurements in controlled lab conditions. Biocrusts were rehydrated to full saturation and light incubated for 30 min, 2, 6, 12, 24, and 36 h. Samples were then subject to a 12-point light regime with a LI-6400XT photosynthesis system to determine carbon exchange. Results: Biocrust carbon exchange values differed by biocrust type, by incubation time since wetting, and by date of field sampling. Lichens and mosses had higher gross and net carbon fixation rates than dark and light cyanobacterial crusts. High respiration rates were found after 0.5 h and 2 h incubation times as communities recovered from desiccation, leveling off after 6 h incubation. Net carbon fixation of all types increased with longer incubation time, primarily as a result of decreasing respiration, which suggests rapid recovery of biocrust photosynthesis across types. However, net carbon fixation rates varied from year to year, likely as a product of time since the last rain event and environmental conditions preceding collection, with moss crusts being most sensitive to environmental stress at our study sites. Discussion: Given the complexity of patterns discovered in our study, it is especially important to consider a multitude of factors when comparing biocrust carbon exchange rates across studies. Understanding the dynamics of biocrust carbon fixation in distinct crust types will enable greater precision of carbon cycling models and improved forecasting of impacts of global climate change on dryland carbon cycling and ecosystem functioning.

Plant physiological indicators for optimizing conservation outcomes.

Plant species of concern often occupy narrow habitat ranges, making climate change an outsized potential threat to their conservation and restoration. Understanding the physiological status of a species during stress has the potential to elucidate current risk and provide an outlook on population maintenance. However, the physiological status of a plant can be difficult to interpret without a reference point, such as the capacity to tolerate stress before loss of function, or mortality. We address the application of plant physiology to conservation biology by distinguishing between two physiological approaches that together determine plant status in relation to environmental conditions and evaluate the capacity to avoid stress-induced loss of function. Plant physiological status indices, such as instantaneous rates of photosynthetic gas exchange, describe the level of physiological activity in the plant and are indicative of physiological health. When such measurements are combined with a reference point that reflects the maximum value or environmental limits of a parameter, such as the temperature at which photosynthesis begins to decline due to high temperature stress, we can better diagnose the proximity to potentially damaging thresholds. Here, we review a collection of useful plant status and reference point measurements related to photosynthesis, water relations and mineral nutrition, which can contribute to plant conservation physiology. We propose that these measurements can serve as important additional information to more commonly used phenological and morphological parameters, as the proposed parameters will reveal early warning signals before they are visible. We discuss their implications in the context of changing temperature, water and nutrient supply.