## 4 Discussion
### The Relationship between BVOCs and NEE  
The contributions of BVOC emissions (including isoprene and monoterpenes) to NEE and GPP (BVOCs/NEE, BVOCs/GPP) were 3.5% and 0.8%, respectively, from 2013-2016, according to the hourly average estimates of BVOC emissions [27] and GPP and the observed NEE. The ratio of BVOCs/NEE varies from a few percent in most forests to 25% in a propane savanna woodland, Botswana (19\({}^{\circ}\)54 S, 23\({}^{\circ}\)33 E) [31; 32; 33; 35]. The ratio of BVOCs/GPP is 0.6% at a ponderosa pine plantation [34] and close to but slightly smaller than that at the Qianyanzhou conifer plantation. The ratios of BVOCs/NEE were 3.5% from 2013-2016, which was an updated value compared to the previous 1.5% using the annual NEE in 2003 [27], 4.0% in a ponderosa pine plantation (38\({}^{\circ}\)53\({}^{\prime}\) N, 120\({}^{\circ}\)37\({}^{\prime}\) W) [34], less than 1.0% in the Changbai Mountains temperate forest (42\({}^{\circ}\)24\({}^{\prime}\) N, 128\({}^{\circ}\)6\({}^{\prime}\) E) from June-September (except for 1.4% in September) [70] and 0.63% at a poplar short-rotation plantation (51\({}^{\circ}\)64\({}^{\prime}\)47\({}^{\prime}\) N, 3\({}^{\circ}\)51\({}^{\prime}\)2\({}^{\prime}\) E), Belgium [101]. In the Inner Mongolia grassland (43\({}^{\circ}\)26\({}^{\prime}\)-44\({}^{\circ}\)08\({}^{\prime}\) N, 116\({}^{\circ}\)04\({}^{\prime}\)-117\({}^{\circ}\)05\({}^{\prime}\) E), isoprene/NEE was 2.8-4.3% during the growing seasons of 2002 and 2003 [69]. It is speculated that BVOCs/NEE is higher in the tropical zone, followed by a decline in subtropical and then temperate zones. However, grasslands may have a larger ratio than forests, and further confirmation is needed. It should be noted that BVOCs are necessary compositions of carbon emitted by vegetation into the atmosphere; although they make up small fractions in forests and grasslands, they play significant roles in O\({}_{3}\) and aerosol formation as well as cloud formation through chemical and photochemical reactions, influencing solar radiation transfer and distribution in the atmosphere and global warming [5; 31; 38; 43; 93; 102; 103]. In addition, as BVOC emissions are significant to carbon balance on a global scale [31; 37], total amounts of BVOCs emitted into the atmosphere should be considered in regional and global carbon balance [34]. Strong linear and nonlinear relationships were found between BVOCs/NEE and climate zones for the ecosystems except the Inner Mongolia grassland and the savanna woodland (Figure S5). The responses of BVOCs (using the REA technique and taking isoprene as a representative) to changes in each driving factor (i.e., PAR, E, S/Q) were generally much larger than those for GPP, which was observed and confirmed in a subarctic wet heath by [104] that warming increases gross ecosystem production, but these increases are much lower than those for BVOCs, and the responses to PAR and E (E also represents air temperature) were much higher than those to S/Q for BVOCs and GPP [27] (Tables S6 and S7, in Supplementary Material). The nonlinear relationship is more reasonable for revealing intrinsic interactions between BVOCs, GPP and their controlling factors described in EMBE and EMGPP models. These mechanisms are the main reasons for the strong relationship between BVOCs/NEE and climate zones, and BVOCs/NEE values are higher in tropical climates with high PAR and water vapor and lower in temperate climates with low PAR and E. Based on this mechanism (BVOCs/NEE is mainly dependent on PAR and water vapor), it is speculated that BVOCs/NEE is the largest in the tropical climate and the smallest in the Arctic/subarctic, and more evidence is needed. A similar mechanism but for a positive and nonlinear relationship between isoprene/CO\({}_{2}\) and leaf temperature ranging from 20 \({}^{\circ}\)C to 40 \({}^{\circ}\)C is observed [31].