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.