4.1. Isoprenoid emission capacity from hyperdominant tree species distributed along a topographic and edaphic environmental gradient
We found that on average, isoprene emission capacities did not significantly vary across tree populations of E. grandiflora in different habitats, but did vary among populations of P. hebetatum . Although it is known that the isoprene trait is conserved within plant species, emission quantities may vary significantly with photosynthetic capacity, carbon and nutrient investment tradeoffs, habitat and the environment (Harrison et al. 2013). The species investigated here occur in forest types that vary in edaphic properties, soil water holding capacity, species richness and below- and above-ground biomass (Andreae et al. 2015). These changes in soil and vegetation attributes can influence plant performance and, in intraspecific processes, it is common to observe increases in both plant growth and defense secondary metabolites as resource availability increases (e.g. greater nutrient availability in uplands) (Agrawal 2020). Yet, although we observed the highest isoprene emission fromP. hebetatum at upland forest,  this was not observed in E. grandiflora and that was probably due to the large variability within individuals at the same forest type.
The plant intraspecific variability was even more pronounced within emissions of monoterpenes – either in diversity of chemical compounds or in total amount of monoterpenes – when comparing individuals distributed in the different forest types. This group of compounds is known to have two different processes of emission. After being synthesized, many monoterpenes are commonly stored in leaves and their inducible emission results mostly from biotic stress; nonetheless, when plant species lack storage structures, most hydrophobic monoterpenes that were synthesized can accumulate in the leaf lipid phase and be constitutively emitted to the atmosphere (Ormeño, Goldstein & Niinemets 2011).  Although the functional role of the constitutive emission of monoterpenes has been reported as being similar to isoprene emission (Loreto et al. 1996) – namely, leaf thermal protection and leaf excess energy dissipation (Rosenstiel, Ebbets, Khatri, Fall & Monson 2004; Sanadze 2004; Sharkey & Monson 2017)  –  the main functional role of monoterpenes is attributed to plant defense against pathogens and herbivores (Fineschi & Loreto 2012), and that may result in better success in plant competition and colonization (Salazar et al.2018). Thus, the high variability of monoterpene emission within species may be related to a plant defense strategy.
In contrast to isoprene and monoterpenes, sesquiterpene emission was only observed from P. hebetatum , from trees in all three forest types. To the best of our knowledge, this is the first time that constitutive emission of sesquiterpenes has been observed for an Amazonian tree species that also presented variation across populations.P. hebetatum was measured for isoprenoid emission in another Amazonian forest site, but the authors only found emission of isoprene and the monoterpene cis-β-ocimene (Jardine et al. 2020).Protium is a genus that is widespread across the whole Amazon basin (ter Steege et al. 2013; Fauset et al. 2015) and it has been hypothesized that its great abundance is probably related to its high diversity of chemical defenses, which provides a strategy against a large number of enemies that consistently attack plant species (Salazar et al. 2018). This hypothesis is indeed reinforced by the fact that P. hebetatum occurs in all three forest types of this study, in contrast to the other two hyperdominant species in our study, which occurred in only one or two forest types. Furthermore, given the fact that the upland forest has the highest plant species richness in this experimental site (Andreae et al. 2015), a high demand for chemical compounds for herbivory defense is expected  in this environment. Habitats with high plant richness are commonly followed by high richness of herbivores, meaning that constant and high potential plant losses to herbivores are diminished with high investments in defenses (Bixenmann, Coley, Weinhold & Kursar 2016). This could explain why emission rates and chemical diversity of sesquiterpenes peaked in the upland forest (Table 3).