# The Observed Impacts of Wind Farms on Local Vegetation Growth in Northern China
## 3 Results  
First, the spatial coupling of the layout of WFPs and LAI anomaly differences are calculated to analyze the effect of the WFs on vegetation growth. Figure 3a-c show LAI anomaly differences at pixel scale in the summer between pre- and post-turbine construction periods (2012-2014 minus 2003-2005), and the probability distributions of LAI anomaly differences between WFPs and NNWFPs. The results show that negative LAI anomaly differences are spatially coupled with wind turbines, with 86.2% of WFPs corresponding to negative LAI anomaly differences. The average of LAI anomaly differences is \(-\)0.11 \(\pm\) 0.10 and 0.01 \(\pm\) 0.13 for WFPs and NNWFPs, a significant difference (\(p<\) 0.01) according to the \(t\)-test statistics. This result indicates that WFPs inhibit vegetation growth compared with NNWFPs. The spatial distributions of individual year average LAI anomaly differences are very similar to those of multi-year averages (Figure 3b,d), with 84.1% of WFPs corresponding to the negative LAI anomaly differences. The average of LAI anomaly differences is \(-\)0.12 \(\pm\) 0.13 and 0.01 \(\pm\) 0.13 for WFPs and NNWFPs, respectively, which is also a significant difference (\(p<\) 0.01). The anomaly differences for NDVI and EVI are also analyzed in this paper. The results for NDVI and EVI are found to be very similar to those for LAI, which indicates that the WFs inhibit vegetation growth in summer (Figure S3). Note that, to the northeast of the WFs, there is a small area with zero anomalies in the vegetation index and net vegetation productivity. The reason for this observation is that the land cover type in this area is a lake, and the MODIS LAI and the vegetation productivity present no data for this area. Figure 3: Summer MODIS LAI anomaly differences and the corresponding LAI anomaly differences probability distribution of WFPs and NNWFPs between post- and pre-turbine-construction years (**a**,**c**) (2012–2014 minus 2003–2005 averages) and (**b**,**d**) (2012 minus 2004). WFPs and NNWFPs are defined as in Figure 2. Plus symbols represent pixels with one or more wind turbines. Then, the effects of WFs on vegetation productivity are analyzed. Figure 4 shows the summer GPP and annual NPP differences at pixel scale between the pre- and post-turbine construction periods (2012-2014 minus 2003-2005) in the study area. The results show that 80.4% negative summer GPP anomaly differences are spatially coupled with the wind turbines (Figure 4a). The average of GPP anomaly differences is \(-\)16.51 \(\pm\) 21.10 (\(-\)0.42 \(\pm\) 33.59) g C m\({}^{-2}\) JJA\({}^{-1}\) for WFPs (NNWFPs), which are significantly different (\(p\) < 0.01) according to the \(t\)-test statistics. The annual NPP anomaly differences over WFPs (\(-\)8.36 \(\pm\) 13.08 g C m\({}^{-2}\) year\({}^{-1}\)) are significantly smaller than that over NNWFPs (0.19 \(\pm\) 19.02 g C m\({}^{-2}\) year\({}^{-1}\), \(p\) < 0.01), with 71.9% of WFPs corresponding to negative NPP anomaly differences (Figure 4b). We also calculated average summer GPP and annual NPP of WFPs and NNWFPs in 2003-2005 and 2012-2014 (Figure 4c,d). From 2003 to 2005, the average summer GPP over WFPs is 179.03 g C m\({}^{-2}\) JJA\({}^{-1}\) and significantly larger than the GPP (175.26 g C m\({}^{-2}\) JJA\({}^{-1}\)) over NNWFPs (\(p\) < 0.01). From 2012 to 2014, the average summer GPP over WFPs (NNWFPs) increased to 291.78 (304.10 g C m\({}^{-2}\) JJA\({}^{-1}\)). Vegetation productivity in WFPs has a smaller rate of increase in summer GPP, becoming significantly smaller than that in NNWFPs (\(p\) < 0.01). From 2003 to 2005, the average annual NPP over WFPs (236.18 g C m\({}^{-2}\) year\({}^{-1}\)) is significantly larger than that over NNWFPs (232.68 g C m\({}^{-2}\) year\({}^{-1}\), \(p\) < 0.05). From 2012 to 2014, the average annual NPP over WFPs (NNWFPs) decreased to 222.67 (227.72 g C m\({}^{-2}\) year\({}^{-1}\)). Vegetation productivity in the WFPs has a larger rate of decrease of NPP, becoming significantly smaller than that in NNWFPs (\(p\) < 0.01).