## 4 Discussion
### Coresponse of Canopy COS and CO\({}_{2}\) Fluxes to Variations in Environmental Conditions
The LRU and \(\rm g_{s}\) were found to respond in opposite ways to the changes in environmental conditions in MXF and \(\rm CPL_{C3}\). Moreover, the LRU was found to differ between species. In the NLF and \(\rm CPL_{C3}\) groups, the LRU decreased with increasing \(\rm F_{C,COS}\) and decreasing \(\rm F_{C,CO_{2}}\). In contrast, increases in both \(\rm F_{C,COS}\) and \(\rm F_{C,CO_{2}}\) were found in GRA in which the LRU was found to decrease. In the process of changing environmental conditions, under a minor increase in light intensity, \(\rm F_{C,COS}\) decreased. Furthermore, synchronization was observed between the decrease in F\({}_{\text{C,CO}_{2}}\) caused by warming and the decrease in the LRU caused by an increase in light. In contrast, g\({}_{\text{s}}\) increased when the light intensity increased. Recent studies noted that pathway conductance and CA enzyme activity collimated F\({}_{\text{C,COS}}\)[73]. Furthermore, the response of stomata to temperature was not very sensitive [151], as the CO\({}_{2}\) concentration in the subcavities of the mesophyll and the activity of enzyme CA colimited conductance throughout the whole pathway [138]. This study found a linear trade-off relationship only between F\({}_{\text{C,CO}_{2}}\) and temperature. However, there was no correlation between F\({}_{\text{C,CO}_{2}}\) and g\({}_{\text{s}}\) because GPP and ER were not partitioned from F\({}_{\text{C,CO}_{2}}\). Meanwhile, water shortage resulted in decreased F\({}_{\text{C,CO}_{2}}\) (consistent with the decreases in rH and VPD). In the diurnal cycling groups, there was a synergetic correlation between F\({}_{\text{C,CO}_{2}}\) and the variation in the rH or C\({}_{\text{A,CO}_{2}}\). However, F\({}_{\text{C,CO}_{2}}\) exhibited a trade-off with temperature or C\({}_{\text{a,COS}}\). Then, stomata closure was controlled by the C\({}_{\text{a,COS}}\) decrease [51,124]. Moreover, enhanced friction wind caused COS emission from the canopy and induced a quadratic correlation between C\({}_{\text{a,CO}_{2}}\) and F\({}_{\text{C,COS}}\). Therefore, the variation in F\({}_{\text{C,COS}}\) was smaller than that in F\({}_{\text{C,CO}_{2}}\)[96]. However, no significant correlation was found between the LRU and either F\({}_{\text{C,COS}}\) or F\({}_{\text{C,CO}_{2}}\). Environmental conditions such as light intensity, VPD, C\({}_{\text{a,COS}}\) and C\({}_{\text{a,CO}_{2}}\) were the key factors influencing LRU variation. As evidenced by the results from NLF, MXF and CPL\({}_{\text{C3}}\), C\({}_{\text{a,COS}}\) and C\({}_{\text{a,CO}_{2}}\) increased in the diurnal cycling groups in non-wetting biomes. During the diurnal variation, the canopy COS absorption flux mainly occurred in the daytime. First of all, a minor increase in light intensity enhanced g\({}_{\text{s}}\), and enhanced friction wind caused COS emission from the canopy gap; furthermore, F\({}_{\text{C,COS}}\) decreased. Furthermore, as the intercomparison of global studies revealed, as C\({}_{\text{a,CO}_{2}}\) increased by 10 to 40 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), F\({}_{\text{C,COS}}\) decreased to its lowest and rebounded. The LRU was influenced by both C\({}_{\text{a,COS}}\) and C\({}_{\text{a,CO}_{2}}\). In contrast, as the stomata closed, the COS diffusion pathway was inhibited due to water shortage at midday; even if C\({}_{\text{a,COS}}\) and C\({}_{\text{a,CO}_{2}}\) increased, F\({}_{\text{C,COS}}\) was still decreased. Finally, F\({}_{\text{C,COS}}\) was the lowest when the stomatal openings were small. Notably, the increase and decrease in F\({}_{\text{C,COS}}\) could both be related to an increase in g\({}_{\text{s}}\). The regulation mechanism is important in formulating the LRU time series and in upscaling the LRU into the ecosystem relative uptake (ERU) and the atmospheric relative uptake (ARU) [122].