## 4 Discussion
### Effects of N fertilization on ET, WUE and LUE  
N application showed no consistent effect on ET among the three stands. N fertilization slightly decreased ET at HDF88 in the last three years and had no clear effect on ET at HDF00 over the four post-fertilization years (Table 3). On the other hand, increased nutrient availability (e.g., N fertilization) had significantly positive effects on WUE at HDF88 and HDF00 due to its strong positive effect on GPP \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Stand** & **Flux** & **Study** & **2007** & **2008** & **2009** & **2010** \\ \hline \multirow{8}{*}{DF49} & \multirow{3}{*}{NEP} & This study (ANN) & 253 & 289 & 272 & 383 \\  & & [PERSON] _et al._[53] & 168 & 78 & & \\  & & [PERSON] _et al._[52] & 250 & & & \\  & & This study (ANN) & 152 & 175 & 121 & 214 \\  & & [PERSON] _et al._[53] & 203 & 139 & & \\  & & [PERSON] _et al._[52] & 157 & & & \\  & & This study (ANN) & \(-\)101 & \(-\)114 & \(-\)152 & \(-\)170 \\  & & & [PERSON] _et al._[53] & 35 & 61 & & \\  & & [PERSON] _et al._[52] & \(-\)93 & & & \\ \hline \multirow{8}{*}{HDF88} & \multirow{3}{*}{NEP} & This study (ANN) & 249 & 360 & 429 & 478 \\  & & [PERSON] _et al._[53] & 182 & 179 & & \\ \cline{1-1}  & & This study (ANN) & 416 & 269 & 256 & 260 \\ \cline{1-1}  & & [PERSON] _et al._[53] & 615 & 366 & & \\ \cline{1-1}  & & This study (ANN) & 168 & \(-\)91 & \(-\)173 & \(-\)218 \\ \cline{1-1}  & & [PERSON] _et al._[53] & 433 & 187 & & \\ \hline \multirow{8}{*}{HDF00} & \multirow{3}{*}{NEP} & This study (ANN) & 166 & 222 & 226 & 279 \\ \cline{1-1}  & & [PERSON] _et al._[53] & \(-\)3 & \(-\)82 & & \\ \cline{1-1}  & & This study (ANN) & 435 & 340 & 324 & 439 \\ \cline{1-1}  & & [PERSON] _et al._[53] & 305 & 329 & & \\ \cline{1-1}  & & This study (ANN) & 269 & 118 & 98 & 160 \\ \cline{1-1}  & & [PERSON] _et al._[53] & 308 & 411 & & \\ \hline \end{tabular} Three different-aged Douglas-fir stands: DF49, HDF88 and HDF00, established in 1949, 1988, and 2000, respectively.  
\end{table} 
Table 4: N-induced changes (= measured – modeled) in annual gross primary productivity (GPP, g\(\cdot\)C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)), ecosystem respiration (\(R\), g\(\cdot\)C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) and net ecosystem productivity (NEP, g\(\cdot\)C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) from different study results in the three West Coast Douglas-fir stands during 2007 to 2010.  
compared with its small effect on ET, but had no significant effect at DF49. [PERSON] _et al._[47] found that N addition increased both WUE and biomass production. They concluded that this was mainly due to the positive response of photosynthesis to N availability with it having no impact on either stomatal conductance or transpiration. Consequently, improved tree nutrition (e.g., N fertilization) could cause an increase in forest productivity [78] without leading to an increase in demand for water. Furthermore, N fertilization caused an increase in the 4-year mean WUE of 0.34 g\(\cdot\)C (kg\(\cdot\)water)\({}^{-1}\) for DF49, implying a relatively conservative response to N fertilization, and increases of 0.78 and 1.19 g\(\cdot\)C (kg\(\cdot\)water)\({}^{-1}\) for HDF88 and HDF00, respectively, likely due to their younger stand ages and less developed root system [56]. [PERSON] _et al._[79] suggested that low or appropriate N application might improve the survivability of _Sophora davidi_ seedlings under water deficit conditions, resulting in an increase in WUE and stimulating growth and biomass production. Our results indicated that the supply of 60 kg N ha\({}^{-1}\) applied to individual trees at HDF00 may be more efficient in terms of WUE compared with the higher application of 200 kg N ha\({}^{-1}\) at HDF88 and DF49, especially in the latter stand. N fertilization in our study had positive impacts on LUE in all three stands in the four post-fertilization years, especially at HDF00 followed by HDF88 with the smallest increase occurring in 2009 in all three stands accompanied by the highest annual PAR.