## 4 Discussion
### Remotely Sensed Phenology  
With the remote sensing product chosen in this study, including the products shown in the supplementary material, most of the six remote sensing methods in this study show relatively poor performance compared with the ground-observed phenology, regardless of which VI is used. The discrepancy can be explained by several possible reasons, within which the difference in definition perhaps having the largest contribution. For example, the onset retrieved using DERIVATIVE is later than BBRK30 (Figure 3a) because the maximum of the first derivative of VI time series represents the time when VI increases the fastest, which usually responds to the period of fast leaf expansion after allbuds have broken. LOGISTIC2 and MIDPOINT arbitrarily define the onset as the time when 50% of the amplitude between the minimum and maximum of either fitted or normalized VI time series is reached, which is expected to be later than the time when buds break. LOGISTIC1 and CAMELBACK are both based on the second derivative of VI time series. The local maximum of the second derivative tends to capture the subtle change in the VI, which is too sensitive to the growth of understory that occurs earlier than the development of canopy [22, 49]. The onset retrieved using LOGISTIC1 can also be affected by the curve fitting, because the maximum rate of change in the curvature is determined by the shape of the fitted curve (_i.e._, the parameters of logistic function), and the shape is controlled by how the VI changes when it starts increasing as well as when it reaches the peak. For the offset, MOVING and CAMELBACK use a process symmetrical to the onset. Because the onset derived using these methods well represents the period when LAI starts to increase from the minimum value, the offset tends to represent the period when LAI drops to the minimum value, which is much later than when LCOLOR20 is reached. LOGISTIC2, MIDPOINT and DERIVATIVE also produce dates later than LCOLOR20 as they tend to capture the period when VI drops the fastest, which usually corresponds to the fast change in leaf color and decrease in LAI rather than the beginning of offset. Although LOGISTIC1 is trying to capture the period when VI starts to decrease, there is still a difference between the offset derived using LOGISTIC1 and that based on ground observation, because the VI does not change synchronously with LAI. Moreover, similar to the onset, the offset derived using LOGISTIC1 can be affected by the shape of the fitted curve, which is controlled by the change in VI around the period when VI drops to the minimum. Regardless of remote sensing method, offset is later when using NDVI than with EVI. This is likely because NDVI tends to saturate when the LAI is high (3 or more for a pure forest pixel) [50, 51] so that it is not as sensitive as EVI to the drop in LAI. Generally, EVI is more responsive to the canopy structural variation, such as LAI, and NDVI is more sensitive to chlorophyll [52]. There is a difference between the phenological dates retrieved using LOGISTIC1 with EVI, and those from MCD12Q2, which was produced using LOGISTIC1 as well with NBAR EVI. This highlights the fact that the phenology retrieved can also be affected by factors other than the method, such as the choice of data source (see Supplementary Material) [28] and data processing. On the other hand, the similar patterns shown by the two results suggest that the general features of a certain method are relatively independent of data source. Another issue related to the satellite methods is the parameterization. The width of the moving window used in MOVING, DERIVATIVE and CAMELBACK as well as the 50% amplitude used in LOGISTIC2 and MIDPOINT can be considered as parameters. Because those parameters were chosen based on the input data used when developing the method, when the data source changes, they may no longer be optimal and can contribute to the discrepancies between the remotely sensed phenology and the ground observation.