# The Observed Impacts of Wind Farms on Local Vegetation Growth in Northern China
## 3 Results
The results of summer GPP and annual NPP indicate that, compared with NNWFPs, WFPs inhibit vegetation productivity. The spatial coupling of the layout of the WFPs and vegetation index and productivity anomaly differences, are quantified by comparing the SCI values of 5%, 10%, and 15% pixels of the smallest  
Figure 4: (**a**) Summer MODIS GPP (g C m\({}^{-2}\) JJA\({}^{-1}\)) and (**b**) annual MODIS NPP (g C m\({}^{-2}\) year\({}^{-1}\)) anomaly differences between post- and pre-turbine construction years (2012–2014 minus 2003–2005 averages). (**c**) Average summer GPP and (**d**) annual NPP of WFPs and NNWFPs in 2003–2005 and 2012–2014. vegetation index as well as GPP, and NPP anomaly differences located in WFPs with the corresponding SCI threshold. Generally, the larger the SCI value, the stronger the spatial coupling. The SCI values for LAI, NDVI, EVI, GPP, and NPP in WFPs range from 9.5% to 35.5%, and 14 of the 15 are larger than the threshold (10.8%), indicating a strong spatial coupling between the layout of WFPs and vegetation anomaly differences. In other words, WFs can inhibit vegetation growth and productivity (Table 1). To analyze the quantitative effects of the WFs on vegetation growth and productivity for 2003-2014 in the study area, the area mean time series of differences in VI, GPP, and NPP between WFPs and NNWFPs (WFPs minus NNWFPs) are analyzed (Figure 5). Although the time series is short, the LAI, EVI, and NDVI differences all present significant downward trends, which are as follows: \(-0.15/11\) years (\(p<0.01\)), \(-0.04/11\) years (\(p<0.01\)) and \(-0.05/11\) years (\(p<0.01\)). The vegetation productivity also shows downward trends and the decreasing trends are \(-20.93\) g C m\({}^{-2}/11\) years (\(p<0.01\)) for summer GPP and \(-10.19\) g C m\({}^{-2}/11\) years (\(p<0.01\)) for annual NPP. Based on the vegetation index and productivity differences between post- and pre-turbines construction periods (2012-2014 minus 2003-2005), the effects of WFs on vegetation growth are quantified. From 2003 to 2014, the inhibiting effects of the WFs on vegetation growth are \(-0.12\) (LAI), \(-0.04\) (EVI), and \(-0.04\) (NDVI), or \(14.5\%\), \(14.8\%\), and \(8.9\%\), respectively, compared with the climatology values of 0.83 (LAI), 0.27 (EVI), and 0.45 (NDVI) for WFPs. The inhibiting effects of the WFs on vegetation productivity are \(-16.08\) g C m\({}^{-2}\) JJA\({}^{-1}\) (GPP) and \(-8.55\) g C m\({}^{-2}\) year\({}^{-1}\) (NPP), representing a change of 8.9% and 4.0% of the climatology value 180.81 g C m\({}^{-2}\) JJA\({}^{-1}\) and 214.86 g C m\({}^{-2}\) year\({}^{-1}\) over the WFPs. Note that the VI differences and vegetation productivity differences both remain relatively stable after 2012. The main reason for this trend may be that the wind turbine construction was essentially complete after 2012 and, as a result, the differences in VI, GPP and NPP between WFPs and NNWFPs gradually become stabilized. However, during the first few years, the number of wind turbines in WFPs gradually increased and, therefore, the differences between WFPs and NNWFPs gradually become larger (Figure 1a). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number of Pixels (\% of Total Pixels)** & & **WFPs** & **NNWFPs** & **UWFPs** & **DWFPs** \\ \hline Random & SCI threshold & 10.5 & 14.9 & 5.3 & 6.3 \\ \hline \multirow{6}{*}{231 (5\%)} & LAI & **31.2** * & 14.7 & 2.2 & 6.1 \\  & EVI & **35.5** * & 10.0 & 0.9 & **6.9** * \\  & NDVI & **29.4** * & 13.4 & 0.4 & **8.7** * \\  & GPP & 9.5 & **19.5** * & 1.3 & **8.2** * \\  & NPP & **19.0** * & **20.3** * & 0.4 & **10.0** * \\ \hline \multirow{6}{*}{693 (15\%)} & LAI & **30.5** * & 14.1 & 2.4 & **8.1** * \\  & EVI & **29.9** * & 10.6 & 1.7 & **7.4** * \\  & NDVI & **26.0** * & 12.8 & 1.9 & **7.1** * \\  & GPP & **13.0** * & **18.4** * & 0.6 & **10.0** * \\  & NPP & **19.0** * & **17.1** * & 1.1 & **10.8** * \\ \hline \hline \end{tabular}
\end{table}