2.4. Emission Metrics and Statistical Analysis
Isoprenoid emission and photosynthetic rates were analyzed in common
units of μgC g-1 h-1. This enabled
us to produce integrated metrics related to the leaf carbon balance. We
analyzed total isoprenoid emissions both as the sum of all C emitted in
the form of isoprenoids, and as a percentage of photosynthetic C
assimilation rates. The three compound classes vary in mass (5C
isoprene, 10C monoterpenes, 15C sesquiterpenes). We developed an
‘isoprenoid mass investment’ metric to assess the partitioning of C
among the different classes of emitted isoprenoids, calculated by
multiplying the emission rates of each compound (in μgC
g-1 h-1) by their respective masses
(number of C atoms), and dividing by the combined total emission rate
(i.e. the emission-rate weighted mean mass of emitted compounds). The
mass investment metric reflects how much carbon is allocated to lighter
versus heavier compounds. For example, the average mass investment of
pure isoprene emission is 5C. Since sesquiterpenes contain three times
as much C as isoprene, a 2:1 molar ratio of sesquiterpenes to isoprene
yields an average mass investment of (2(15C × 3) + 1(5C × 1)) / 7 =
13.6C. Variation in metrics among habitats within species and seasons
was analyzed by Tukey Honestly Significant Difference (HSD) test (alpha
= 0.05). Variation between dry and wet seasons was analyzed by paired
t-test (alpha = 0.05) on measurements taken from the same individuals in
both seasons.