Figure 4.
Correlations between isoprene emission and (a) PPFD (photosynthetic photon flux density), (b) air temperature, and (c, d) CO 2 fluxes in July 2015. Dots are the monthly averaged half‐hourly values of each day; bars represent the standard error (n = 31 days) of each half‐hourly mean value. The data correspond to daytime (05:30 to 20:30), when isoprene flux was higher than zero. The inset in (a) emphasizes the hysteresis of the isoprene emission as a function of PPFD. GPP, gross primary production; and NEE, net ecosystem exchange. (***) denotes a significant correlation at the level of P < 0.001.