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).