Arctic rodents influence tundra plant communities by altering species diversity, structure, and nutrient dynamics. These dynamics are intensified during rodent population peaks. Plants are known to induce defenses in response to rodent herbivory. However, changes in plant tissue digestibility may also play a role in deterring rodents or impacting their survival. This study presents a first look at the impacts of rodent herbivory on crude protein (CP) and acid detergent fiber (ADF) of two of the most common graminoid species ( Carex nigra and Deschampsia cespitosa ) and graminoid genus ( Calamagrostis spp . ) in the tundra meadows of the Varanger Peninsula, Norway. We selected 32 experimental plots representing both rodent-disturbed and adjacent, undisturbed control graminoid patches. In the summer of a rodent population peak, the disturbed plots had higher ADF (28.5%) values than less disturbed ones (26.6%), controlling for plant species. We also found differences between species, with Carex nigra having the lowest fiber content (24.3%, ADF) and highest protein content (18.2% CP)—making it the most palatable species. These results show that rodent activity can potentially alter plant food quality, suggesting that increasing fiber content may be a defensive response to herbivory.

Certain graminoids can be successful in grasslands to the extent it is a phenomenon called "the Viking Syndrome". Nevertheless, forbs also make up a substantial part of vascular plant diversity in grasslands and are important resources of mammalian herbivores. Here we assess the hypothesis that forb recruitment is constrained by dominant graminoids, limiting access to safe sites for germination. We report on a disturbance experiment of plots with four different graminoid species in tundra grasslands of the Varanger Peninsula, Norway. Plots were selected to sample both rodent-disturbed and undisturbed areas. The dominant graminoids in each plot were removed, reducing their shading capabilities and belowground rhizome and root systems. Results show that forb recruitment one year following disturbance was significantly enhanced by manual graminoid removal. Dominant graminoid type, small rodent disturbance, initial forb abundance, and abiotic conditions had no effect on forb recruitment, whereas initial species richness had a positive relationship. Furthermore we found that manual disturbance had low impact on the species exchange ratio based on richness (SERr), suggesting that disturbance did not reduce the capacity of species to reside and move within the grassland. Our findings support the hypothesis that forb recruitment is limited by dominant graminoids. ### Competing Interest Statement The authors have declared no competing interest.

Science, engineering, and society increasingly require integrative thinking about emerging problems in complex systems, a notion referred to as convergence science. Due to the concurrent pressures of two main stressors-rapid climate change and industrialization, Arctic research demands such a paradigm of scientific inquiry. This perspective represents a synthesis of a vision for its application in Arctic system studies, developed by a group of disciplinary experts consisting of social and earth system scientists, ecologists, and engineers. Our objective is to demonstrate how convergence research questions can be developed via a holistic view of system interactions that are then parsed into material links and concrete inquiries of disciplinary and interdisciplinary nature. We illustrate the application of the convergence science paradigm to several forms of Arctic stressors using the Yamal Peninsula of the Russian Arctic as a representative natural laboratory with a biogeographic gradient from the forest-tundra ecotone to the high Arctic.

Terrestrial ecosystems release ~106–130 PgC yr–1 into the atmosphere through respiration, counterbalancing photosynthetic carbon uptake and determining the strength of the land carbon sink. The effect of anthropogenic warming on the land carbon sink will depend on the temperature response of respiration. In this Review, we explore the relationships between temperature and ecosystem respiration from experimental and observational data at leaf, microbial, ecosystem and global scales. Contrary to the assumed monotonic increase in respiration with increasing temperature derived from Earth system models, empirical findings indicate a unimodal temperature response with a peak in respiration at an optimal temperature (Topt). This unimodality is observed across a range of organization levels with Topt values of 40–60 °C at the leaf and plant level, 11–46 °C at a microbial level and 6.5–33.3 °C at the global scale. Various mechanisms contribute to this unimodal pattern including enzyme deactivation, the thermodynamics of enzyme-catalysed reactions and changes in temperature-dependent factors such as soil moisture, nutrient availability and vegetation physiology. Incorporating the unimodality of these observed temperature responses of ecosystem respiration into Earth system models could facilitate attribution studies to identify the mechanisms responsible for the peaked response and increase the accuracy of carbon sequestration predictions. The future of the land carbon sink depends on the temperature response of ecosystem respiration. This Review explores observational and experimental evidence for a unimodal temperature response of respiration and the implications for carbon sequestration predictions.

Societal Impact StatementOver half of Earth's human population lives in urban areas where pollution from fossil fuel combustion threatens urban air quality and public health. The isotopic composition of plant tissues can be used as a tool to estimate local variation in the amount of fossil fuel–derived carbon dioxide. We present a case study that examines this variation against the backdrop of historic discriminatory policies in Saint Paul, Minnesota, USA. Data suggest higher loads of atmospheric fossil carbon in neighborhood greenspaces that are situated closer to large roads, which often correspond with historical policies. Evidence of unequal fossil fuel combustion, and potentially pollution load, may precipitate changes in urban management or pollution mitigation strategies.

Respiration plays a key role in the terrestrial carbon cycle and is a fundamental metabolic process in all plant tissues and cells. We review respiration from the perspective of plants that grow in their natural habitat and how it is influenced by wide-ranging elements at different scales, from metabolic substrate availability to shifts in climate. Decades of field-based measurements have honed our understanding of the biological and environmental controls on leaf, root, stem, and whole-organism respiration. Despite this effort, there remain gaps in our knowledge within and across species and ecosystems, especially in more challenging-to-measure tissues like roots. Recent databases of respiration rates and associated leaf traits from species representing diverse biomes, plant functional types, and regional climates have allowed for a wider-lens view at modeling this important CO2 flux. We also re-analyze published data sets to show that maximum leaf respiration rates (R max) in species from around the globe are related both to leaf economic traits and environmental variables (precipitation and air temperature), but that root respiration does not follow the same latitudinal trends previously published for leaf data. We encourage the ecophysiological community to continue to expand their study of plant respiration in tissues that are difficult to measure and at the whole plant and ecosystem levels to address outstanding questions in the field.

AbstractIn this chapter, we introduce our perspectives and guidance for authentically integrating principles of EcoJustice – transparency of and access to information, diversity of cultural perspectives, and equity in decision-making - into undergraduate ecology and environmental science classrooms. As a professor of ecology and a recently graduated undergraduate student of biology and history, we found that EcoJustice can act as an inspiration and source of content and a framework to guide authentic, growth-focused pedagogy. Inclusion of diverse cultural perspectives on how science is performed may deepen connection to content in students representing marginalized communities and broaden discussions and understanding of what science is. We detail the benefits of broadening science pedagogy and pivoting from a practice where single actors and sources of knowledge are emphasized to one that emphasizes a more authentic, diverse, and democratic representation of science. We focus on the strengths of including multiple perspectives on pedagogy and sources of knowledge in the classroom. To promote inclusion, we suggest multiple assessments that emphasize individuality, creativity, application, and fun – often sorely missing from undergraduate classrooms. Finally, we offer guiding questions for faculty and student leaders to reflect on while developing courses and structuring learning environments that value and promote authenticity.

Redlining was a practice of financial discrimination in the mid-20th century in which banks refused loans or increased interest rates based on the grade of an applicant’s neighborhood as designated by the federally sponsored Home Owner’s Lending Commission (HOLC). The HOLC primarily graded neighborhoods from “A” (best) to “D” (hazardous) based on characteristics including the racial demographics and economic status of the residents, with neighborhoods with higher percentages of non-white and/or recent immigrant residents given lower grades; this and similar discriminatory practices can be traced to modern-day economic and environmental inequalities between neighborhoods. The legacy of redlining and related housing discrimination on modern-day urban air quality, which presents a significant threat to public health, remains an important issue in addressing environmental injustice in U.S. cities. In our study, we used remotely sensed estimates of the air pollutant nitrogen dioxide (NO2) collected with the TROPOMI satellite sensor, and shapefiles of redlined neighborhoods, to determine whether air quality varies among historic HOLC grades in 11 U.S. Midwestern metropolitan areas. This approach allowed us to test these tools for within-city analysis of NO2 for which high spatial and temporal resolution measurements are not often available, despite their importance for monitoring impacts on human health. We found that NO2 levels were as much as 16% higher in neighborhoods that were graded “D” compared to those graded “A” (as in Chicago), with the mean difference across all cities an increase of 7.3% ± 5.9%. These results present evidence of persistent modern-day inequality in urban air quality associated with historic discriminatory policies and should be used as an argument for government action improving air quality in neighborhoods that were poorly graded by the HOLC.

Dataset contains leaf physiological variables and leaf traits from Rhamnus cathartica (buckthorn) and Prunus serotina (Black cherry) measured in Summer 2019 at Macalester College's Ordway Field Station. All data were collected on understory individuals from 4 sites within the forest in late May and early July, 2019. We sampled from 'tree' individuals and seedlings of both species. All methods and measurement protocols are published in Heskel et al. 2022 in AoB-PLANTS, currently in revision. Data includes:Vcmax (umol m2-s-1) Jmax (umol m2-s-1) Fv/Fm (no units) Leaf Stomatal Density (stomata mm-2) Dark respiration (umol m2-s-1) Asat (umol m2-s-1) A400 (umol m2-s-1) Carbon Gain Efficiency (CGE, no units) CGE_400 (CGE at 400 PAR, no units) Leaf Mass per Are (LMA, g m-2)

1. Photosynthetic traits suggest that shade tolerance may explain the contrasting success of two conifer taxa, Podocarpaceae and Pinaceae, in tropical forests. Needle-leaved species from Pinus (Pinaceae) are generally absent from tropical forests, whereas Pinus krempfii, a flat-leaved pine, and numerous flat-leaved Podocarpaceae are abundant. Respiration (R) traits may provide additional insight into the drivers of the contrasting success of needle- and flat-leaved conifers in tropical forests. 2. We measured the short-term respiratory temperature (RT) response between 10 and 50 degrees C and foliar morphological traits of three needle- and seven flat-leaved conifer species coexisting in a tropical montane forest in the Central Highlands of Vietnam containing notable conifer diversity. We fit a lognormal polynomial model to each RT curve and extracted the following three parameters: a (basal R), and b and c (together describing the shape of the response). 3. Needle-leaved species (Pinus kesiya, Pinus dalatensis and Dacrydium elatum) had higher rates of area-based R at 25 degrees C (R25-area) as well as higher area-based modelled basal respiration (a) than flat-leaved species (P. krempfii, Podocarpus neriifolius, Dacrycarpus imbricatus, Nageia nana, Taxus wallichiana, Keteeleria evelyniana and Fokienia hodginsii). No significant differences were found between needle- and flat-leaved species in mass-based R-25 (R25-mass) or in the shape of the RT response (b and c); however, interspecific differences in R25-mass, R at nighttime temperature extremes (R-4.1 and R-20.6) and leaf traits were apparent. 4. Differences in R25-area and a suggest that needle-leaved foliage may be more energetically costly to maintain than flat-leaved foliage, providing new insight and additional support for the hypothesis that shade tolerance is an important driver of Podocarpaceae success and Pinaceae absence in the majority of tropical forests. 5. Interspecific differences in R25-mass and leaf traits highlight that varying ecological strategies are employed by conifers to coexist and survive in the Central Highlands of Vietnam. Ultimately, these data further our understanding of current conifer biogeographical distributions and underscore the need for additional studies to elucidate the effects of extreme temperature events on the continued survival of conifers in this unique forest.

Leaf-level gas exchange data support the mechanistic understanding of plant fluxes of carbon and water. These fluxes inform our understanding of ecosystem function, are an important constraint on parameterization of terrestrial biosphere models, are necessary to understand the response of plants to global environmental change, and are integral to efforts to improve crop production. Collection of these data using gas analyzers can be both technically challenging and time consuming, and individual studies generally focus on a small range of species, restricted time periods, or limited geographic regions. The high value of these data is exemplified by the many publications that reuse and synthesize gas exchange data, however the lack of metadata and data reporting conventions make full and efficient use of these data difficult. Here we propose a reporting format for leaf-level gas exchange data and metadata to provide guidance to data contributors on how to store data in repositories to maximize their discoverability, facilitate their efficient reuse, and add value to individual datasets. For data users, the reporting format will better allow data repositories to optimize data search and extraction, and more readily integrate similar data into harmonized synthesis products. The reporting format specifies data table variable naming and unit conventions, as well as metadata characterizing experimental conditions and protocols. For common data types that were the focus of this initial version of the reporting format, i.e., survey measurements, dark respiration, carbon dioxide and light response curves, and parameters derived from those measurements, we took a further step of defining required additional data and metadata that would maximize the potential reuse of those data types. To aid data contributors and the development of data ingest tools by data repositories we provided a translation table comparing the outputs of common gas exchange instruments. Extensive consultation with data collectors, data users, instrument manufacturers, and data scientists was undertaken in order to ensure that the reporting format met community needs. The reporting format presented here is intended to form a foundation for future development that will incorporate additional data types and variables as gas exchange systems and measurement approaches advance in the future. The reporting format is published in the U.S. Department of Energy's ESS-DIVE data repository, with documentation and future development efforts being maintained in a version control system.

Warming-induced nutrient enrichment in the Arctic may lead to shifts in leaf-level physiological properties and processes with potential consequences for plant community dynamics and ecosystem function. To explore the physiological responses of Arctic tundra vegetation to increasing nutrient availability, we examined how a set of leaf nutrient and physiological characteristics of eight plant species (representing four plant functional groups) respond to a gradient of experimental nitrogen (N) and phosphorus (P) enrichment. Specifically, we examined a set of chlorophyll fluorescence measures related to photosynthetic efficiency, performance and stress, and two leaf nutrient traits (leaf %C and %N), across an experimental nutrient gradient at the Arctic Long Term Ecological Research site, located in the northern foothills of the Brooks Range, Alaska. In addition, we explicitly assessed the direct relationships between chlorophyll fluorescence and leaf %N. We found significant differences in physiological and nutrient traits between species and plant functional groups, and we found that species within one functional group (deciduous shrubs) have significantly greater leaf %N at high levels of nutrient addition. In addition, we found positive, saturating relationships between leaf %N and chlorophyll fluorescence measures across all species. Our results highlight species-specific differences in leaf nutrient traits and physiology in this ecosystem. In particular, the effects of a gradient of nutrient enrichment were most prominent in deciduous plant species, the plant functional group known to be increasing in relative abundance with warming in this ecosystem.

Two simplifying hypotheses have been proposed for whole-plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first-principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carryover of fixed carbon between years, while the second implies far too great an increase in respiration during stand development – leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass.

When solar-induced chlorophyll fluorescence (SIF) is developed as a novel approach to quantify gross primary production (GPP), narrow SIF emission spectrum around the atmospheric oxygen absorption windows ((similar to) 761 nm and (similar to)687 nm) has been widely used to represent SIF and to derive GPP. As SIF is a continuous spectrum, deriving the full broadband SIF emission spectrum over 640-850 nm provides an opportunity to fully explore the potential of SIF in estimating GPP. Using high-frequency measurements of canopy carbon flux and SIF emissions at the atmospheric absorption bands, we reconstruct the full SIF spectrum from SIF signals at the absorption bands, and then analyze correlations between the GPP and the selected single SIF bands and their combinations using both linear regression (LR) and Gaussian processes regression (GPR). Our results indicate that (1) the red SIF bands (640-700 nm) shows low correlation with GPP due to the strong (re)absorption of red SIF emissions by leaf chlorophyll; (2) the individual bands in near-infrared area (at 720 nm, 740 nm, and 761 nm) can determine about 60% and 62% of the variance in GPP with hourly scale by LR and GPR, respectively, and the combination of those SIF bands provide increased predictive power, explaining 66% and 76% variance in GPP with hourly scale by LR and GPR, respectively; (3) the solar radiation saturation, fraction of direct solar radiation, air temperature and leaf area index may have negative impacts on the SIF-GPP correlations when they are beyond optimal thresholds; and (4) the temporal aggregation of SIF-GPP (daily scale) enhances the correlations as compared with the hourly scale: the daily combination of SIF bands (at 687 nm, 720 nm, and 761 nm) can account for 80% and 93% of variance in daily daytime GPP by LR and GPR, respectively, suggesting that the combination of these three bands of SIF on the daily scale is the best proxy for GPP. Our results provide a new approach to analyze the SIF-GPP correlations using the ground-based full broadband SIF emission, suggesting that multi-bands SIF has a stronger capacity in predicting plant GPP than traditionally used signal band SIF data.

Temperature is a crucial factor in determining the rates of ecosystem processes, for example, leaf respiration (R) - the flux of plant respired CO2 from leaves to the atmosphere. Generally, R increases exponentially with temperature and formulations such as the Arrhenius equationare widely used in earth system models. However, experimental observations have shown a consequential and consistent departure from an exponential increase in R. What are the principles that underlie these observed patterns? Here, we demonstrate that macromolecular rate theory (MMRT), based on transition state theory (TST) for enzyme-catalyzed kinetics, provides a thermodynamic explanation for the observed departure and the convergent temperature response of R using a global database. Three meaningful parameters emerge from MMRT analysis: the temperature at which the rate of respiration would theoretically reach a maximum (the optimum temperature, T-opt), the temperature at which the respiration rate is most sensitive to changes in temperature (the inflection temperature, T-inf) and the overall curvature of the log(rate) versus temperature plot (the change in heat capacity for the system, Delta C-P(double dagger)). On average, the highest potential enzyme-catalyzed rates of respiratory enzymes for R are predicted to occur at 67.0 +/- 1.2 degrees C and the maximum temperature sensitivity at 41.4 +/- 0.7 degrees C from MMRT. The average curvature (average negative Delta C-P(double dagger) was -1.2 +/- 0.1 kJ mol(-1) K-1. Interestingly, T-opt, T-inf and Delta C-P(double dagger) appear insignificantly different across biomes and plant functional types, suggesting that thermal response of respiratory enzymes in leaves could be conserved. The derived parameters from MMRT can serve as thermal traits for plant leaves that represent the collective temperature response of metabolic respiratory enzymes and could be useful to understand regulations of R under a warmer climate. MMRT extends the classic TST to enzyme-catalyzed reactions and provides an accurate and mechanistic model for the short-term temperature response of R around the globe.

Nitrogen (N) and phosphorus (P) are essential nutrients for plant metabolism, and their availability often limits primary productivity. Whereas the effects of N availability on photosynthetic capacity are well established, we still know relatively little about the effects of P availability at a foliar level, especially in P‐limited tropical forests. We examined photosynthetic capacity, leaf mass per area (LMA) and foliar P fractions in five woody plant species after 6 years of N and P fertilization in a lowland tropical forest. Foliar N:P ratios indicated P limitation of the unfertilized plants; accordingly, photosynthetic P‐use efficiency (PPUE) and LMA decreased with P addition, and foliar N and P concentrations increased, whereas N addition had little effect on measured foliar traits. However, P addition enhanced photosynthetic capacity only in one species and not in other four species. We then assessed plant acclimation to low P availability by quantifying four fractions of foliar P representing different functional pools: structural P, metabolic P (including inorganic P), nucleic acid P, and residual P. We found that P addition enhanced the concentrations of metabolic, structural, and nucleic acid P fractions in all species, but the magnitude of the effect was species‐specific. Our findings indicate that tropical species acclimate to low P availability by altering allocation of foliar P to meet the demand of P for photosynthesis. Importantly, species typical of lowland tropical forests in East Asia maintained their photosynthetic rate under low P availability. We conclude that P limitation of leaf photosynthetic capacity may not be as common as previously assumed due to plant acclimation mechanisms in low‐P tropical forests. Species‐specific strategies to allocate P to different foliar fractions represent a potentially important adaptive mechanism for plants in P‐limited systems.

Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, South 7 China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, P.R. China; 8 South China Agricultural University, Guangzhou, 510650, P.R. China; 9 Xiaoliang Research Station for Tropical Coastal Ecosystems, Chinese Academy of Sciences, 10 Maoming 525029, P.R. China; 11 Lancaster Environment Center, Lancaster University, Lancaster LA1 4YQ, UK; 12 Smithsonian Tropical Research Institute, P.O. Box 0843-03092, Balboa, Ancon, Panama 13 School of Biological Sciences, The University of Western Australia, Crawley (Perth) WA 6009, 14 Australia 15 The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, 02543, USA; 16 Department of Biology, Macalester College, Saint Paul, MN 55105 17 Agro-Environmental Protection Institute, Tianjin 300191, P.R. China; 18

Uncertainty in the estimation of daytime ecosystem carbon cycling due to the light inhibition of leaf respiration and photorespiration, and how these small fluxes vary through the growing season in the field, remains a confounding element in calculations of gross primary productivity and ecosystem respiration. Our study focuses on how phenology, short-term temperature changes and canopy position influence leaf-level carbon exchange in Quercus rubra L. (red oak) at Harvard Forest in central Massachusetts, USA. Using leaf measurements and eddy covariance, we also quantify the effect of light inhibition on estimates of daytime respiration at leaf and ecosystem scales. Measured rates of leaf respiration in the light and dark were highest in the early growing season and declined in response to 10-day prior air temperatures (P < 0.01), evidence of within-season thermal acclimation. Leaf respiration was significantly inhibited by light (27.1 ± 2.82% inhibited across all measurements), and this inhibition varied with the month of measurement; greater inhibition was observed in mid-summer leaves compared with early- and late-season leaves. Increases in measurement temperature led to higher rates of respiration and photorespiration, though with a less pronounced positive effect on photosynthesis; as a result, carbon-use efficiency declined with increasing leaf temperature. Over the growing season when we account for seasonally variable light inhibition and basal respiration rates, our modeling approaches found a cumulative 12.9% reduction of leaf-level respiration and a 12.8% reduction of canopy leaf respiration, resulting in a 3.7% decrease in total ecosystem respiration compared with estimates that do not account for light inhibition in leaves. Our study sheds light on the environmental controls of the light inhibition of daytime leaf respiration and how integrating this phenomenon and other small fluxes can reduce uncertainty in current and future projections of terrestrial carbon cycling.

The balance of photosynthesis and respiration, their responses to a changing environment, and predictive models of these responses continue to be an active body of research. While photosynthesis is robustly described by a long-standing, scalable biochemical model (Farquhar et al., 1980), a similar mechanistic model of respiration remains an ongoing challenge. Respiration encompasses multiple cellular processes in the mitochondria and cytosol that drive energy and carbon skeleton production for plant growth and maintenance. Through glycolysis (cytosol), the tricarboyxlic acid (TCA) cycle (mitochondrial matrix), the electron transport chain/oxidative phosphorylation (mitochondrial inner membrane), and other associated pathways, metabolic products of photosynthesis are transformed into energy in the form of ATP, oxygen is consumed, and carbon dioxide is produced. Unlike its metabolic foil, photosynthesis, mitochondrial respiration takes place in all plant tissues, in all cells, at all times. Its ubiquity as an energy source in plants, its role promoting and maintaining efficient photosynthesis, and its contribution to the terrestrial carbon cycle warrant accurate quantification for scaling leaf-level fluxes of carbon. New strategies for measuring and modeling plant respiration across systems and scales are necessary to robustly characterize how carbon flows through terrestrial environments. Developments in measurement techniques, comprehensive field-based data sets, and cross-scale research collaborations are directly addressing environmental sensitivities and biochemical nuances and, in turn, advancing how respiration is considered at the leaf and ecosystem levels. This essay covers current areas of plant respiration research and their integration into the broader terrestrial carbon cycle. Mitochondrial respiration is often termed “dark respiration” because leaves must be darkened for measurement of carbon efflux to eliminate the co-occurring signal of photosynthetic carbon assimilation. However, leaves not only continue to respire during daylight, but many of the cellular pathways of respiration are altered by light and result in inhibition of oxygen uptake and carbon release. This phenomenon, first reported in algal suspensions in the mid-20th century (Kok, 1948), has recently been the focus of increased attention due to its potential to impact calculations of ecosystem primary productivity (Wehr et al., 2016). For example, ecosystem estimates of respiration are often based on above-canopy eddy covariance measurements made at night or made in darkened chambers that eliminate a photosynthetic signal. Applying these approaches equates daytime and nighttime respiration fluxes, neglecting the known inhibition of respiration by light, and in turn, overestimating the gross flux of carbon into the ecosystem (Fig. 1). The degree of this overestimation may vary across systems. For example, overestimation may be greater in ecosystems with high values of canopy leaf area index (i.e., tropical rainforests), where leaf respiration is likely to comprise a large proportion of total ecosystem respiration. The sources of the light inhibitory effect and the controls of the degree of inhibition are complex and include environmental, developmental, and biochemical signals (Tcherkez et al., 2017a). Studies at the subcellular level have identified a reorganization of the TCA cycle in light and link inhibition of mitochondrial respiration with the co-occurring process of photorespiration (Tcherkez et al., 2008). In field and greenhouse studies, variability in the degree of inhibition is also associated with environmental growth conditions of atmospheric CO2, temperature, and soil nitrogen availability (Crous et al., 2017). Leaf developmental age and seasonality can also impact the inhibition of leaf respiratory fluxes; inhibition increases with the progression of the growing season in evergreen and deciduous species in arctic tundra (Heskel et al., 2014), though it is unaltered in a Mediterranean forest (Turnbull et al., 2017). With overlapping, and sometimes inextricable, biological and environmental controls regulating the daytime respiratory flux, minimizing other sources of variation and potential measurement error is critical. Within the past year, two studies delved into the methodological complexity of capturing accurate measurements of respiration in the light while accounting for sensitivity to cellular carbon dioxide concentrations (Buckley et al., 2017; Farquhar and Busch, 2017). Overall, the study of the light inhibition of respiration—for many decades considered a niche within a niche of plant physiological research—is rapidly expanding at both the biochemical and canopy scales and exemplifies the importance and need of cross-scale collaborations for accurate ecosystem and global carbon accounting. The amount of carbon respired by leaves increases with temperature in an exponential-like response within the normal growth temperature range (<45°C) in the time frame of seconds to days (Heskel et al., 2016). As leaf temperatures exceed ~50°C, respiration rates increase at a slower rate, eventually reach a maximum rate, and plummet at higher temperatures. The inherent shape of this response, the derivative of which exhibits a declining rate of change (often termed Q10) as temperatures increase, is conserved across functionally and climactically diverse species (Heskel et al., 2016). This apparently universal response suggests a deeply phylogenetically shared metabolic response to temperature, which may be explained by enzyme thermodynamics (Liang et al., 2017). A consistent and quantitatively (relatively) simple short-term temperature response of respiration across broad plant functional types provides great utility for regional and global carbon models, which generally rely on the oft-applied, yet inaccurate, fixed Q10 of 2. The apparent universal shape of the short-term response of respiration to temperature allows for easy calculation of leaf carbon efflux across a wide range of temperatures when a reference value of respiration exists. A recently assembled global database of leaf respiration values, GlobResp, comprised of nearly 900 species’ values of respiration representing 100 climactically diverse terrestrial sites that span the globe, provides an extensive library of reference respiration values for those seeking to model fluxes (Atkin et al., 2015). This collection of leaf respiration values, along with trait, climate, and plant functional group information, creates opportunities for empirically driven improvements of how respiration is represented in dynamic global vegetation models. Field-based values of leaf respiration can have significant impacts on estimated carbon fluxes from leaves—substituting GlobResp values in place of standard respiration parameters may increase estimates of whole-plant respiration by up to 30% (Huntingford et al., 2017). In agreement with meta-analyses where data spans diverse species and ecosystems, GlobResp data reveal thermal acclimation responses to growth temperatures, where leaves grown at colder temperatures respire at higher rates than leaves grown at warmer temperatures (when measured at a common reference temperature) (Vanderwel et al., 2015). In many plant species, the long-term response of respiration to warmer growth temperatures results in thermal acclimation, the decreasing adjustment of metabolic rates that occurs on the timescale of days to years (Atkin and Tjoelker, 2003). When acclimated, a leaf grown under elevated temperatures will release less carbon through respiration compared to a leaf grown under ambient conditions when measured at the same reference temperature. By contrast, photosynthesis often acclimates to elevated growth temperature by increasing its optimum temperature, and over longer-term warming, could result in more carbon assimilation and less carbon efflux at higher temperatures (Fig. 2). Integrating thermal acclimation observed at the leaf level into Earth Systems Models has shown a reduction in carbon efflux to the atmosphere and an increase in carbon stored in plants and soils (Lombardozzi et al., 2015). However, acclimation should not be hailed as a potential “silver-lining” to plants experiencing warmer growth temperatures due to climate change; the interactive impacts of individual species, drought, and soil nutrient variability, as well as acclimation of photosynthesis and plant hydraulic processes will factor greatly in the future carbon exchange of terrestrial ecosystems. These interactions and their resulting effects on respiratory acclimation and whole-plant carbon storage is an area of expanding, active leaf-level research from the tropics to the tundra (Slot et al., 2014). Modeling the variability in spatial, species-level, and environmental controls on long-term temperature responses of respiration and integrating these responses into Earth System Models will further refine our understanding of how carbon flows through ecosystems under current and future conditions. So, what is next? Will a robust, predictive model of respiration emerge from empirical studies on environmental and biological controls and become anchored in Earth System Models? The past few years’ expansive collection of field and lab studies, novel analyses, available databases, and cross-scale collaborations suggest a productive and insightful future for the study of leaf respiration and its impact on whole-plant and ecosystem carbon fluxes. Momentum from recent New Phytologist Workshops (Atkin et al., 2014; Tcherkez et al., 2017b), increased dialogue between ecosystem, organismal, leaf, and systems scientists, and inclusive cross-scale collaborations that promote and encourage new perspectives on this “small flux” will drive research forward toward a more complete understanding of plant respiration and its role in the global carbon cycle. I thank the Rosenthal Family Foundation and The Ecosystems Center of the Marine Biological Laboratory for sponsoring my postdoctoral fellowship and the reviewers for their comments on this essay.

Abstract Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (>25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming‐induced fertilization. In addition, we compared our measured ecosystem CO 2 flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO 2 exchange with nutrient addition. We observed declines in abundance‐weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%–50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization—over an order of magnitude or more than warming‐induced rates—significantly alter the capacity for tundra CO 2 exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming‐related nutrient availability may impact ecosystems differently than single‐level fertilization experiments.