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

Abstract

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 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 51 tropical tree species, revealing moderate consistency in EC within taxonomic groups. Our self-contained, portable instrument provides real-time detection and live measurement feedback with precision and detection limits better than 0.5 nmol m-2leaf 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

“…In contrast to VOCs stored in oils that are released passively by heat and wounding, such as the monoterpenes responsible for pine scent, light‐dependent emissions are linked to photosynthetic substrate supply and are dynamically tuned to environmental and metabolic conditions as a component of metabolic regulatory processes (Laothawornkitkul et al ., 2009; Riedlmeier et al ., 2017; Lantz et al ., 2019; Monson et al ., 2021). Due to its much higher emission rate and relative ease of detectability, isoprene has received far more study in the field, although both isoprene and monoterpenes are expressed by many angiosperms and gymnosperms across all biomes (see Taylor et al ., 2021, and references therein).…”

“…Fewer than half of tree species express significant light‐dependent isoprene emissions (Kesselmeier & Staudt, 1999; Taylor et al ., 2018), such that vertical variation in emissions is influenced by bothy vertical variation in species compositions and by plasticity in emission rate capacities (Taylor et al ., 2021). Within species, isoprene emission tends to increase toward brighter and hotter microenvironments (Niinemets, 2007), and across landscapes, emitting species are more abundant in hotter climates, exceeding half of trees in warm tropical forests (Taylor et al ., 2018).…”

“…Within species, isoprene emission tends to increase toward brighter and hotter microenvironments (Niinemets, 2007), and across landscapes, emitting species are more abundant in hotter climates, exceeding half of trees in warm tropical forests (Taylor et al ., 2018). However, a recent study found a contrasting interspecific vertical structuring of emission capacities, with more emitting species and higher species‐maximum emission rates in the midcanopy of an Amazonian forest (Table 1; Taylor et al ., 2021). Similarly, within European beech crowns, monoterpene emissions were highest in semishaded leaves beneath the canopy surface (Table 1; Šimpraga et al ., 2013).…”

Seasonal shifts in isoprenoid emission composition from three hyperdominant tree species in central Amazonia

“…Knowing that not all plant species emit isoprene (Monson et al 2013), the aforementioned study aimed to quantify the isoprene emission capacity for multiple plant species from different Amazonian regions, and a method was created to impute the isoprene trait to other non-measured trees by using species identification and phylogenetic proximity; after which the results were used to upscale the isoprene emission capacity to the ecosystem level based on the fraction of tree isoprene emitters. Subsequent studies further expanded the number of species measured (Jardine et al 2020;Taylor et al 2021). Recent work has derived more mechanistic approaches to scaling isoprene emission across the landscape by determining how the fraction of emitters relates to mean climate conditions (Taylor et al 2018) due to differences in performance between isoprene-emitting and non-emitting species (Taylor et al 2019).…”

“…2020; Taylor et al . 2021). Recent work has derived more mechanistic approaches to scaling isoprene emission across the landscape by determining how the fraction of emitters relates to mean climate conditions (Taylor et al .…”