A new field instrument for leaf volatiles reveals an unexpected vertical profile of isoprenoid emission capacities in a tropical forest.

Both plant physiology and atmospheric chemistry are substantially altered by the emission of volatile isoprenoids (VI), such as isoprene and monoterpenes, from plant leaves. Yet, since gaining scientific attention in the 1950?s, empirical research on leaf VI has been largely confined to laboratory experiments and atmospheric observations. Here, we introduce a new field instrument designed to bridge the scales from leaf to atmosphere, by enabling precision VI detection in real time from plants in their natural ecological setting. With a field campaign in the Brazilian Amazon, we reveal an unexpected distribution of leaf emission capacities (EC) across the vertical axis of the forest canopy, with EC peaking in the mid-canopy instead of the sun-exposed canopy surface, and moderately high emissions occurring in understory specialist species. Compared to the simple interpretation that VI protect leaves from heat stress at the hot canopy surface, our results encourage a more nuanced view of the adaptive role of VI in plants. We infer that forest emissions to the atmosphere depend on the dynamic microenvironments imposed by canopy structure, and not simply on canopy surface conditions. We provide a new emissions inventory from 52 tropical tree species, revealing moderate consistency in EC within taxonomic groups. We highlight priorities in leaf volatiles research that require field-portable detection systems. Our self-contained, portable instrument provides real-time detection and live measurement feedback with precision and detection limits better than 0.5 nmolVI m-2 leaf s-1. We call the instrument ?PORCO? based on the gas detection method: photoionization of organic compounds. We provide a thorough validation of PORCO and demonstrate its capacity to detect ecologically driven variation in leaf emission rates and thus accelerate a nascent field of science: the ecology and ecophysiology of plant volatiles

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (Tleaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy Tleaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging Tleaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high Tleaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback

Isoprene emission structures tropical tree biogeography and community assembly responses to climate.

The prediction of vegetation responses to climate requires a knowledge of how climate-sensitive plant traits mediate not only the responses of individual plants, but also shifts in the species and functional compositions of whole communities. The emission of isoprene gas – a trait shared by one-third of tree species – is known to protect leaf biochemistry under climatic stress. Here, we test the hypothesis that isoprene emission shapes tree species compositions in tropical forests by enhancing the tolerance of emitting trees to heat and drought. Using forest inventory data, we estimated the proportional abundance of isoprene-emitting trees (pIE) at 103 lowland tropical sites. We also quantified the temporal composition shifts in three tropical forests – two natural and one artificial – subjected to either anomalous warming or drought. Across the landscape, pIE increased with site mean annual temperature, but decreased with dry season length. Through time, pIE strongly increased under high temperatures, and moderately increased following drought. Our analysis shows that isoprene emission is a key plant trait determining species responses to climate. For species adapted to seasonal dry periods, isoprene emission may tradeoff with alternative strategies, such as leaf deciduousness. Community selection for isoprene-emitting species is a potential mechanism for enhanced forest resilience to climatic change.Financial support for this study was provided to: T.C.T. and S.R.S. by grants NSF-PIRE #OISE-0730305, USDOE #3002937712, NASA #NNX17AF65G and the University of AZ Agnes Nelms Haury Program in Environment and Social Justice; to M.N.S. and S.R.S. by NASA-ESSF #NNX14AK95H; to C.V. by ERCStG-2014-639706-CONSTRAINTS; to I.S. by Grant Agency of the Czech Republic #16-26369S; and to P.M. by NERC # NE/ N006852/1 and ARC #DP170104091

The capacity to emit isoprene differentiates the photosynthetic temperature responses of tropical plant species

Experimental research shows that isoprene emission by plants can improve photosynthetic performance at high temperatures. But whether species that emit isoprene have higher thermal limits than non‐emitting species remains largely untested. Tropical plants are adapted to narrow temperature ranges and global warming could result in significant ecosystem restructuring due to small variations in species' thermal tolerances. We compared photosynthetic temperature responses of 26 co‐occurring tropical tree and liana species to test whether isoprene‐emitting species are more tolerant to high temperatures. We classified species as isoprene emitters versus non‐emitters based on published datasets. Maximum temperatures for net photosynthesis were ~1.8°C higher for isoprene‐emitting species than for non‐emitters, and thermal response curves were 24% wider; differences in optimum temperatures (Topt) or photosynthetic rates at Topt were not significant. Modelling the carbon cost of isoprene emission, we show that even strong emission rates cause little reduction in the net carbon assimilation advantage over non‐emitters at supraoptimal temperatures. Isoprene emissions may alleviate biochemical limitations, which together with stomatal conductance, co‐limit photosynthesis above Topt. Our findings provide evidence that isoprene emission may be an adaptation to warmer thermal niches, and that emitting species may fare better under global warming than co‐occurring non‐emitting species. That isoprene emission enhances the thermal tolerance of photosynthesis is supported by decades of experimental physiology. But whether isoprene differentiates the thermal niches of emitting from non‐emitting species remains untested in the real world. We provide evidence that isoprene‐emitting tropical woody plant species photosynthesize to higher maximum temperatures, and over a broader thermal range, compared with co‐occurring, non‐emitting species. Even accounting for the carbon cost of isoprene emissions, we find no substantial trade‐offs associated with this high‐temperature advantage

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (Tleaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy Tleaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging Tleaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high Tleaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback

Botanic gardens are an untapped resource for studying the functional ecology of tropical plants

Functional traits are increasingly used to understand the ecology of plants and to predict their responses to global changes. Unfortunately, trait data are unavailable for the majority of plant species. The lack of trait data is especially prevalent for hard-to-measure traits and for tropical plant species, potentially owing to the many inherent difficulties of working with species in remote, hyperdiverse rainforest systems. The living collections of botanic gardens provide convenient access to large numbers of tropical plant species and can potentially be used to quickly augment trait databases and advance our understanding of species' responses to climate change. In this review, we quantitatively assess the availability of trait data for tropical versus temperate species, the diversity of species available for sampling in several exemplar tropical botanic gardens and the validity of garden-based leaf and root trait measurements. Our analyses support the contention that the living collections of botanic gardens are a valuable scientific resource that can contribute significantly to research on plant functional ecology and conservation. This article is part of the theme issue ‘Biological collections for understanding biodiversity in the Anthropocene’

Empirical evidence for resilience of tropical forest photosynthesis in a warmer world

Tropical forests may be vulnerable to climate change1-3 if photosynthetic carbon uptake currently operates near a high temperature limit4-6. Predicting tropical forest function requires understanding the relative contributions of two mechanisms of high-temperature photosynthetic declines: stomatal limitation (H1), an indirect response due to temperature-associated changes in atmospheric vapour pressure deficit (VPD)7, and biochemical restrictions (H2), a direct temperature response8,9. Their relative control predicts different outcomes-H1 is expected to diminish with stomatal responses to future co-occurring elevated atmospheric [CO2], whereas H2 portends declining photosynthesis with increasing temperatures. Distinguishing the two mechanisms at high temperatures is therefore critical, but difficult because VPD is highly correlated with temperature in natural settings. We used a forest mesocosm to quantify the sensitivity of tropical gross ecosystem productivity (GEP) to future temperature regimes while constraining VPD by controlling humidity. We then analytically decoupled temperature and VPD effects under current climate with flux-tower-derived GEP trends in situ from four tropical forest sites. Both approaches showed consistent, negative sensitivity of GEP to VPD but little direct response to temperature. Importantly, in the mesocosm at low VPD, GEP persisted up to 38 °C, a temperature exceeding projections for tropical forests in 2100 (ref. 10). If elevated [CO2] mitigates VPD-induced stomatal limitation through enhanced water-use efficiency as hypothesized9,11, tropical forest photosynthesis may have a margin of resilience to future warming.6 month embargo; published 12 October 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Seasonal and drought‐related changes in leaf area profiles depend on height and light environment in an Amazon forest

Seasonal dynamics in the vertical distribution of leaf area index (LAI) may impact the seasonality of forest productivity in Amazonian forests. However, until recently, fine-scale observations critical to revealing ecological mechanisms underlying these changes have been lacking. To investigate fine-scale variation in leaf area with seasonality and drought we conducted monthly ground-based LiDAR surveys over 4 yr at an Amazon forest site. We analysed temporal changes in vertically structured LAI along axes of both canopy height and light environments. Upper canopy LAI increased during the dry season, whereas lower canopy LAI decreased. The low canopy decrease was driven by highly illuminated leaves of smaller trees in gaps. By contrast, understory LAI increased concurrently with the upper canopy. Hence, tree phenological strategies were stratified by height and light environments. Trends were amplified during a 2015-2016 severe El Niño drought. Leaf area low in the canopy exhibited behaviour consistent with water limitation. Leaf loss from short trees in high light during drought may be associated with strategies to tolerate limited access to deep soil water and stressful leaf environments. Vertically and environmentally structured phenological processes suggest a critical role of canopy structural heterogeneity in seasonal changes in Amazon ecosystem function

Functional composition of epiphyte communities in the Colombian Andes

We identify changes in the functional composition of vascular epiphytes along a tropical elevational gradient with the aim of quantifying the role of climate in determining the assembly of epiphyte communities. We measured seven leaf functional traits (leaf area, specific leaf area, leaf dry‐matter content, leaf thickness, force to punch, stomatal density, and potential conductance index) in the 163 most abundant epiphyte species recorded across 10 sites located along an elevational gradient between 60 and 2,900 m above sea level in the Colombian Andes. We grouped the epiphyte species into seven hierarchical functional groups according to their most characteristic leaf traits. Along the elevational gradient, the two main independent leaf trait dimensions that distinguished community assemblages were defined primarily by leaf area‐photosynthetic (LAPS) and mass‐carbon (LMCS) gradients. Mean annual temperature was the main determinant of species position along LAPS. In contrast, local changes in specific leaf area due to variation in the epiphytes’ relative height of attachment was the main determinant of their position along the LMCS. Our findings indicate that epiphytic plant leaves have evolved to optimize and enhance photosynthesis through a leaf area–based strategy and carbon acquisition through investments in construction costs of leaf area per unit of biomass that aim to regulate light capture and tissue development. Given that most studies of plant functional traits neglect vascular epiphytes, our quantification of the multiple dimensions of epiphyte leaf traits greatly augments our understanding of vascular plant function and adaptation to changing environments