3.1. Isoprenoid emission variation among habitats and seasons
Isoprenoid emissions and their habitat and seasonal associations varied
among plant species (Fig. 2, Table 2). Isoprene emissions were mostly
indistinguishable among habitats, except that emission capacities were
higher from P. hebetatum in the ancient river terrace (AR) and
upland (Up) forest habitats than white-sand (WS) forest in the dry
season. Isoprene emission capacities were generally lower in the dry
than wet season (Fig. 2), though only significantly so for P.
hebetatum in two habitats. When aggregating all trees together,
isoprene emissions were significantly lower in the dry than wet season
(a factor of 0.33, paired T-test p < 0.05, Fig. S1, Supporting
Information). Among the subset of trees sampled during the dry-to-wet
transition season (December 2018), isoprene emissions were significantly
higher than in the dry or wet seasons (Tukey HSD, p < 0.05),
averaging 3.3 and 2.4 times higher than wet season emissions forE. coriacea (n = 5 trees) and P. hebetatum (n = 3 trees)
(Fig. S2).
Monoterpene emission capacities were highly variable in magnitude and
chemical diversity among individuals within species, even in the same
habitat and season (Table 3). Total monoterpene emission capacities were
mostly indistinguishable among habitats, except for higher rates fromE. grandiflora in AR than Up in the wet season, tracking patterns
of isoprene emission. The species that emitted most monoterpenes wasP. hebetatum , with emissions frequently exceeding 15 μgC
g-1 h-1. Comparing monoterpene
emission capacities between seasons, we observed that E. coriaceaonly emitted monoterpenes during the dry season. No significant seasonal
differences were detected within habitats (Fig. 2) or when aggregating
by trees and species (Fig. S1). However, the number of chemical species
of monoterpenes increased from the wet season to the dry season, both in
WS and AR habitats (Table 3). During the dry-to-wet transition season,
no monoterpene emissions were detected from E. coriacea orP. hebetatum .
Sesquiterpene emissions were only detected from P. hebetatum ,
reaching rates comparable to isoprene (when analyzed in units of C
emitted, but not moles emitted). These emissions only occurred during
the dry season, and significantly increased from WS to AR and Up
habitats (Fig. 2). No sesquiterpene emissions were detected during the
dry-to-wet transition season. For the chemical species of monoterpenes
and sesquiterpenes, see Table 3.