3.2. Photosynthesis and carbon investment in isoprenoid emissions
Photosynthesis significantly decreased in most species and habitats during the dry season, while trends varied among species in total isoprenoid emissions and their associated carbon investments (Fig. 3). Photosynthesis was significantly lower in the dry than wet season in most habitats for E. grandiflora and P. hebetatum , but not for E. coriacea . Total C emitted in the form of isoprenoids varied among habitats, being significantly greater from E. grandiflora in AR than Up habitats in the dry season, and greater fromP. hebetatum in AR and Up than WS habitats in the wet season (mainly due to the trend in sesquiterpene emissions shown in Fig. 2). Total C investment in emissions as a percent of photosynthesis was mostly < 2% and not distinguishable between habitats in either season, though it reached an average 6.1% in P. hebetatumin the Up habitat in the dry season. In P. hebetatum , isoprenoid mass investment was significantly greater in AR and Up than WS habitats in both wet season (attributable to greater monoterpene emissions, Fig. 2) and dry season (attributable to greater sesquiterpene emissions, Fig. 2), and was significantly greater in the dry than wet season in AR and Up habitats (Fig. 3).
When aggregated across forest habitats and species, seasonal variation was strongest in photosynthesis and isoprenoid mass investment, and less pronounced in total isoprenoid emissions and carbon emitted as a percent of photosynthesis (Fig. 4). Statistical significance reflects paired t-tests for individuals measured in both seasons (alpha = 0.05), while Figure 4 visualizes the dry season values as a factor of wet season values measured from the same individuals (‘dry/wet factor’). Photosynthesis was non-significantly lower in the dry season in E. coriacea (mean dry/wet factor = 0.94), significantly lower in E. grandiflora (mean dry/wet factor = 0.73) and P. hebetatum (mean dry/wet factor = 0.43), and in all species combined (mean dry/wet factor = 0.62). Total emission was non-significantly higher in the dry season (mean dry/wet factor for all species combined = 1.47), even though emissions were strongly suppressed in many individual trees, except inE. coriacea which showed significantly higher emissions (mean dry/wet factor = 2.97). Carbon emitted as a percent of photosynthesis was non-significantly higher in the dry season overall (mean dry/wet factor for all species combined = 3.22) but highly variable among trees, except in E. coriacea which showed a significantly higher percentage (mean dry/wet factor = 3.43). Isoprenoid emission mass investment in the dry compared to wet season was non-significantly greater in E. coriacea (mean dry/wet factor = 1.41) and E. grandiflora (mean dry/wet factor = 1.07), significantly greater inP. hebetatum (mean dry/wet factor = 1.65), and significantly greater in all species combined (mean dry/wet factor = 1.41). Total emissions were positively correlated with photosynthesis in E. grandiflora (linear regression on all measurements combined, p < 0.01, r2 = 0.37), while there was no detectable correlation in the other two species (Fig. S3).