## 3 Results
### The Change of Crown CO\({}_{2}\) Exchange with Environmental Variables
#### 3.4.2 The Change of Crown CO\({}_{2}\) Exchange with _Pak_ under Different _Vpd_ Ranges  
Similarly, we used light response curves (Equation (4)) to assess the influence of _PAR_ on daytime _Ne_ values under different _Vpd_ ranges and nitrogen addition levels. As can be seen from Figure 9a-d, the absolute value of _Ne_ in MN and HN were much higher than that of LN when 0 kPa \(<\)_Vpd_\(<\) 1 kPa, and there was little difference between MN and HN. The carbon assimilation decreased under different nitrogen addition treatments when _Vpd_\(>\) 1 kPa. Similarly, the absolute value of _Ne\({}_{max}\)_ in different nitrogen treatments linearly declined with the increasing of _Vpd_ during the growing season (_p_\(<\) 0.01), especially the MN and HN treatments, which were about twice as large when _Vpd_ was low (0 kPa \(<\)_Vpd_\(\leq\) 1 kPa) as those were when _Vpd_ was high (_Vpd_\(>\) 1.5 kPa) (Figure 9e). However, the daytime respiration linearly increased with increasing _Vpd_ under different nitrogen addition levels (_p_\(<\) 0.01) (Figure 9f). From Figure 9g, we found \(\alpha\) changed irregularly as _Vpd_ increased under different nitrogen addition levels, which ranged from 0.015 to 0.033 \(\upmu\)mol\(\cdot\)umol\({}^{-1}\).  
Figure 7: Cumulative values of gross primary productivity (_GPP_). (**a**) crown respiration (_R_), and (**b**) net crown carbon exchange (_Ne_), (**c**) after nitrogen addition and the control during the growing season (from June to September). CK, LN, MN and HN represent control (no nitrogen addition), low nitrogen addition (23 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)), medium nitrogen addition (46 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)) and high nitrogen addition (69 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)), respectively. Letters (a, b, or c) suggest significant differences between treatments at \(\alpha\) = 0.05.  
Figure 8: _Ne - PAR_ relations from June to September (**a–d**). Dots represent hourly daytime _Ne_ values; curves are Michaelis–[PERSON] equation fits (Equation (4)). The solid lines, dashed lines, dot-dash lines, and dotted lines show the _Ne - PAR_ curves under CK (control, no nitrogen addition), LN (low nitrogen addition, 23 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)), MN (medium nitrogen addition, 46 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)), and HN (high nitrogen addition, 69 kgN\(\cdot\)hm\({}^{-2}\).a\({}^{-1}\)) treatments, respectively. Equation fitting parameters for these months are presented in (**e–g**): maximum net photosynthetic rate of crown (_Nemax_), the daytime respiration of crown (_R\({}_{d}\)_), and initial quantum efficiency (_a_).  
Figure 9: _Ne \(-\) PAR_ relations for different \(VPD\) ranges (0 kPa \(<\)\(VPD\)\(\leq\) 1 kPa, 0.5 kPa \(<\)\(VPD\)\(\leq\) 1.5 kPa, 1 kPa \(<\)\(VPD\)\(\leq\) 2 kPa and \(VPD\)\(>\) 1.5 kPa) and nitrogen addition levels (**a–d**) during the growing season. Dots represent hourly daytime _Ne_ values and lines are Michaelis–Menten equation fits (Equation (4)). The solid lines, dashed lines, dot-dash lines, and dotted lines show the _Ne \(-\) PAR_ curves under CK (control, no nitrogen addition), LN (low nitrogen addition, 23 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)), MN (medium nitrogen addition, 46 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)), and HN (high nitrogen addition, 69 kgN\(\cdot\)hm\({}^{-2}\cdot\)a\({}^{-1}\)) treatments, respectively. Equation fitting parameters for these months are presented in (**e–g**): maximum net photosynthetic rate of crown (_Nemax_), the daytime respiration of crown (_Rd_), and initial quantum efficiency (\(\alpha\)). The black lines represent the linear regression across all N treatments, and the light grey shading represents the 95% confidence intervals.