## 3 Results
### NDVI Trend Intercomparison  
The trends of \(N_{p,max}\) and NDVI phenology are compared to determine how effective the long-term datasets are in detecting vegetation trends. Figure 5 shows the interannual trends of the maximum annual NDVI (\(N_{p,max}\)), start of season, end of season, and duration of season for NDVI3g. Trends near the border with Canada in western North Dakota and eastern Montana as well as trends in the Corn Belt of southern Minnesota, northern Iowa, Illinois and much of northern Mexico suggest a positive trend in \(N_{p,max}\) (Figure 5a). Past studies analyzing long-term changes in NDVI-based vegetation abundance across the area vary based on the dataset being used as well as the method of estimation. For instance, [PERSON] _et al._[27] using NDVIg shows a strong significant increase in \(N_{p,max}\) over areas of N. Central Mexico that are not apparent in NDVI3g from Figure 5a or VIP and STAR (not shown). Several studies agree with increasing annual NDVI in the border region of North Dakota, eastern Montana and Canada [36, 60, 61]. Furthermore, [PERSON] _et al._[36] found large disagreements between various AVHRR-based long-term NDVI datasets over the Southeast US and lower Missouri Valley region, suggesting that care should be taken in interpreting trends in these areas. [PERSON] _et al._[61] showed that nearly all pixels across US/Mexico have seen at least one major change in trend. Figure 4: Matrix of plots showing the time-series of the spatial regression slopes of NDVI (blue) and GVF (red) for each 10 day period annually averaged from 1982 to 2008 comparing each combination of four datasets to each other. Also shown are the regression slopes between NDVI of Terra and both NDVIg and NDVI3g in the VIP row (green) for each 10 day period from 2001 to 2008. A significant trend in start of season is seen in the Sierra Madre Occidental and Southwestern Mexico where the season is beginning up to 10 days earlier per decade (Figure 5b). Also, there is a small trend towards delayed start over the Upper Midwest. Large areas of the northern US and Southeast US show significant delays in the end of season with the exception being over the Corn Belt of Northern Iowa and Illinois (Figure 5d). Trends vary over Mexico, suggesting an earlier end of season in southwestern Mexico and a delayed end of season over the northeastern portion of Mexico. The combined results of trends in start and end of season show that deciduous broadleaf forests and mixed forests of the Lower Mississippi Valley and Southeast US are seeing a longer season (Figure 5c). There are also trends in the western Great Lakes region and Dakotas outside of the corn belts towards a longer greening period as well, whereas over the Corn Belt the season is decreasing in duration. Over Mexico, there are no obvious areas of significant trends but most significant pixels suggest an increase in season duration. Results from [PERSON] _et al._[59] suggested that over North Dakota and South Dakota the maximum NDVI is increasing while the greening period duration is decreasing due both to delayed start and earlier end. These results are similar for the eastern Dakotas in our results in the northwestern portion of the Corn Belt. This might be the result of expansion of corn into this area which generally has a  
Figure 5: Trends mean (**a**) maximum annual NDVI (per decade); (**b**) start of season (days per decade); (**c**) duration of season (days per decade); and (**d**) end of season (days per decade) for NDVI3g from 1982 to 2008. The nine point locations indicated by circles are used in Figure 1.  
shorter greening period than natural areas. [PERSON] _et al._[62] depicted a delayed onset over the Upper Midwest and central Plains, which is similar to the results presented here. However, our results tend to be more restrictive in the significance test masking more areas. For end of season most temperate areas over the US except the Corn Belt show delayed end of season and the duration of season is getting longer in the areas of delayed end of season.