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Climate change will bring about changes in meteorological and ecological factors that are currently used in global-scale models to calculate biogenic emissions. By comparing long-term datasets of biogenic compounds to modeled emissions, this work seeks to improve understanding of these models and their driving factors. We compare speciated biogenic volatile organic compound (BVOC) measurements at the Virginia Forest Research Laboratory located in Fluvanna County, VA, USA, for the year 2020 with emissions estimated by the Model of Emissions of Gases and Aerosols from Nature version 3.2 (MEGANv3.2). The emissions were subjected to oxidation in a 0-D box model (F0AM v4.3) to generate time series of modeled concentrations. We find that default light-dependent fractions (LDFs) in the emissions model do not accurately represent observed temporal variability in regional observations. Some monoterpenes with a default light dependence are better represented using light-independent emissions throughout the year (LDFα-pinene=0, as opposed to 0.6), while others are best represented using a seasonally or temporally dependent light dependence. For example, limonene has the highest correlation between modeled and measured concentrations using an LDF =0 for January through April and roughly 0.74–0.97 in the summer months, in contrast to the default value of 0.4. The monoterpenes β-thujene, sabinene, and γ-terpinene similarly have an LDF that varies throughout the year, with light-dependent behavior in summer, while camphene and α-fenchene follow light-independent behavior throughout the year. Simulations of most compounds are consistently underpredicted in the winter months compared to observed concentrations. In contrast, day-to-day variability in the concentrations during summer months are relatively well captured using the coupled emissions–chemistry model constrained by regional concentrations of NOX and O3. 

Solar-induced chlorophyll fluorescence (SIF) is widely accepted as a proxy for gross primary productivity (GPP). Among the various SIF measurements, tower-based SIF measurements allow for continuous monitoring of SIF variation at a canopy scale with high temporal resolution, making it suitable for monitoring highly variable plant physiological responses to environmental changes. However, because of the strong and close relationship between SIF and absorbed photosynthetically active radiation (aPAR), it may be difficult to detect the influence of environmental drivers other than light conditions. Among the drivers, atmospheric dryness (vapor pressure deficit, VPD) is projected to increase as drought becomes more frequent and severe in the future, negatively impacting plants. In this study, we evaluated the tower-based high-frequency SIF measurement as a tool for detecting plant response to highly variable VPD. The study was performed in a mixed temperate forest in Virginia, USA, where a 40-m-tall flux tower has been measuring gas and energy exchanges and ancillary environmental drivers, and the Fluospec 2 system has been measuring SIF. We show that a proper definition of light availability to vegetation can reproduce SIF response to changing VPD that is comparable to GPP response as estimated from eddy covariance measurement: GPP decreased with rising VPD regardless of how aPAR was defined, whereas SIF decreased only when aPAR was defined as the PAR absorbed by chlorophyll (aPARchl) or simulated by a model (Soil Canopy Observation, Photochemistry and Energy fluxes, SCOPE). We simulated the effect of VPD on SIF with two different simulation modes of fluorescence emission representing contrasting moisture conditions, ‘Moderate’ and ‘Soil Moisture (SM) Stress’ modes. The decreasing SIF to rising VPD was only found in the SM Stress mode, implying that the SIF-VPD relationship depends on soil moisture conditions. Furthermore, we observed a similar response of SIF to VPD at hourly and daily scales, indicating that satellite measurements can be used to study the effects of environmental drivers other than light conditions. Finally, the definition of aPAR emphasizes the importance of canopy structure research to interpret remote sensing observations properly. 

Abstract. Emissions from natural sources are driven by various external stimuli such as sunlight, temperature, and soil moisture. Once biogenic volatile organic compounds (BVOCs) are emitted into the atmosphere, they rapidly react with atmospheric oxidants, which has significant impacts on ozone and aerosol budgets. However, diurnal, seasonal, and interannual variability in these species are poorly captured in emissions models due to a lack of long-term, chemically speciated measurements. Therefore, increasing the monitoring of these emissions will improve the modeling of ozone and secondary organic aerosol concentrations. Using 2 years of speciated hourly BVOC data collected at the Virginia Forest Research Lab (VFRL) in Fluvanna County, Virginia, USA, we examine how minor changes in the composition of monoterpenes between seasons are found to have profound impacts on ozone and OH reactivity. The concentrations of a range of BVOCs in the summer were found to have two different diurnal profiles, which, we demonstrate, appear to be driven by light-dependent versus light-independent emissions. Factor analysis was used to separate the two observed diurnal profiles and determine the contribution from each emission type. Highly reactive BVOCs were found to have a large influence on atmospheric reactivity in the summer, particularly during the daytime. These findings reveal the need to monitor species with high atmospheric reactivity, even though they have low concentrations, to more accurately capture their emission trends in models. 

Biogenic volatile organic compounds (BVOCs) contribute the majority of reactive organic carbon to the atmosphere and lead to aerosol formation through reaction with atmospheric oxidants including ozone and hydroxyl radicals. One class of BVOCs, sesquiterpenes, have a high reactivity with ozone but exist at lower concentrations compared to other BVOCs, and there are relatively few measurements of their concentrations in different environments or their importance in the atmospheric oxidant budget. To help close this knowledge gap, we examine concentrations of isomer-resolved sesquiterpene concentrations collected hourly at two sites in Virginia that are representative of different ecosystems in the southeastern US. Sesquiterpene concentrations are presented and discussed in relation to their diurnal patterns and used to estimate their contribution to reactivity with common gas-phase oxidants. Twenty-four sesquiterpenes were identified at the sites, eleven of which were observed at both sites. Total sesquiterpene concentrations were found to range between 0.8 and 2 ppt with no single isomer dominating throughout. Hydroxyl activity is similarly diverse, with no particular isomer dominating activity at either site. Ozone reactivity, however, was found to be dominated (∼3/4 total reactivity) by β-caryophyllene and humulene despite these compounds representing roughly only 10% of total sesquiterpene mass, highlighting their importance as the major driver of sesquiterpene-ozone reactivity. Average reaction rate constants for sesquiterpenes with ozone and hydroxyl radicals were calculated for both sites as a method to simplify future atmospheric modelling concerning sesquiterpenes. This work provides broad insight into the composition and impacts of sesquiterpenes, suggesting that sesquiterpene composition is relatively similar between sites. Furthermore, while the calculated average sesquiterpene-ozone reaction rate constants are at least an order of magnitude higher than that of more prevalent BVOC classes (isoprene and monoterpenes), their low concentrations suggest their impacts on atmospheric reactivity are expected to be limited to periods of high emissions. 

Abstract. Despite the significant contribution of biogenic volatileorganic compounds (BVOCs) to organic aerosol formation and ozone productionand loss, there are few long-term, year-round, ongoing measurements oftheir volume mixing ratios and quantification of their impacts onatmospheric reactivity. To address this gap, we present 1 year of hourlymeasurements of chemically resolved BVOCs between 15 September 2019 and15 September 2020, collected at a research tower in Central Virginiain a mixed forest representative of ecosystems in the Southeastern US.Mixing ratios of isoprene, isoprene oxidation products, monoterpenes, andsesquiterpenes are described and examined for their impact on the hydroxyradical (OH), ozone, and nitrate reactivity. Mixing ratios of isoprene rangefrom negligible in the winter to typical summertime 24 h averages of 4–6 ppb, while monoterpenes have more stable mixing ratios in the range of tenths of a part per billion up to ∼2 ppb year-round. Sesquiterpenes aretypically observed at mixing ratios of <10 ppt, but this representsa lower bound in their abundance. In the growing season, isoprene dominatesOH reactivity but is less important for ozone and nitrate reactivity.Monoterpenes are the most important BVOCs for ozone and nitrate reactivitythroughout the year and for OH reactivity outside of the growing season. Tobetter understand the impact of this compound class on OH, ozone, andnitrate reactivity, the role of individual monoterpenes is examined. Despitethe dominant contribution of α-pinene to total monoterpene mass, theaverage reaction rate of the monoterpene mixture with atmospheric oxidantsis between 25 % and 30 % faster than α-pinene due to thecontribution of more reactive but less abundant compounds. A majority ofreactivity comes from α-pinene and limonene (the most significantlow-mixing-ratio, high-reactivity isomer), highlighting the importance ofboth mixing ratio and structure in assessing atmospheric impacts ofemissions. 

Summary Plant resource allocation patterns often reveal tradeoffs that favor growth (G) over defense (D), or vice versa. Ecologists most often explain G–D tradeoffs through principles of economic optimality, in which negative trait correlations are attributed to the reconciliation of fitness costs. Recently, researchers in molecular biology have developed ‘big data’ resources including multi‐omic (e.g. transcriptomic, proteomic and metabolomic) studies that describe the cellular processes controlling gene expression in model species. In this synthesis, we bridge ecological theory with discoveries in multi‐omics biology to better understand how selection has shaped the mechanisms of G–D tradeoffs. Multi‐omic studies reveal strategically coordinated patterns in resource allocation that are enabled by phytohormone crosstalk and transcriptional signal cascades. Coordinated resource allocation justifies the framework of optimality theory, while providing mechanistic insight into the feedbacks and control hubs that calibrate G–D tradeoff commitments. We use the existing literature to describe the coordinated resource allocation hypothesis (CoRAH) that accounts for balanced cellular controls during the expression of G–D tradeoffs, while sustaining stored resource pools to buffer the impacts of future stresses. The integrative mechanisms of the CoRAH unify the supply‐ and demand‐side perspectives of previous G–D tradeoff theories. 

Abstract Life on Earth depends on the conversion of solar energy to chemical energy by plants through photosynthesis. A fundamental challenge in optimizing photosynthesis is to adjust leaf angles to efficiently use the intercepted sunlight under the constraints of heat stress, water loss and competition. Despite the importance of leaf angle, until recently, we have lacked data and frameworks to describe and predict leaf angle dynamics and their impacts on leaves to the globe. We review the role of leaf angle in studies of ecophysiology, ecosystem ecology and earth system science, and highlight the essential yet understudied role of leaf angle as an ecological strategy to regulate plant carbon–water–energy nexus and to bridge leaf, canopy and earth system processes. Using two models, we show that leaf angle variations have significant impacts on not only canopy‐scale photosynthesis, energy balance and water use efficiency but also light competition within the forest canopy. New techniques to measure leaf angles are emerging, opening opportunities to understand the rarely‐measured intraspecific, interspecific, seasonal and interannual variations of leaf angles and their implications to plant biology and earth system science. We conclude by proposing three directions for future research.