*3.2. Relationships between SPEI and Vegetation Indices in Wild Blueberry Fields of Maine*

Based on the relationships of both the short-term and long-term average SPEI with the EVI and NDVI (Figures 6 and 7, and Table S1), the long-term SPEI showed a stronger influence on the vegetation indices (EVI in Figure 6 and NDVI in Figure 7) of wild blueberries compared to the short-term SPEI. While analyzing the impact of the short-term SPEI (SPEI\_1\_Year in Figures 6 and 7, and SPEI\_1 to SPEI\_11 in Table S1) on the EVI (Figure 6a–c) and NDVI (Figure 7a–c) during the growing season (May–September), no significant relationship was observed for the studied wild blueberry fields in Maine.

On the contrary, while observing the impact of the long-term SPEI (2 to 4 consecutive years) on both the EVI and NDVI of the wild blueberry fields during the growing season, we found both significant linear and quadratic relationships between the SPEI and the EVI (Figure 6d–l) as well as between the SPEI and the NDVI (Figure 7d–l). Among the significant linear and quadratic relationships between an average SPEI of 2 consecutive years (SPEI\_2\_Year) and vegetation indices for the Airport (Figures 6d and 7d), Baxter (Figures 6e and 7e), and studied wild blueberry fields in Maine (Figures 6f and 7f), the coefficient of determination (R2) was higher for the quadratic relationships. Moreover, the strength (R2 values) of both the linear and quadratic relationships was higher when considering more consecutive years, such as SPEI\_3\_Year (Figures 6g–i and 7g–i) and SPEI\_4\_Year (Figures 6j–l and 7j–l) compared to the SPEI\_2\_Year (Figures 6d–f and 7d–f). Although both the relationships between the SPEI and EVI (Figure 6) as well as the SPEI and NDVI (Figure 7) were significant when considering the long-term SPEI, the coefficient of determination (R2) was higher for the relationships between the SPEI and EVI compared to the relationships between the SPEI and NDVI. Because of the stronger relationship between the SPEI and EVI, we further analyzed the impact of the short- and long-term water conditions (SPEI) on wild blueberry yield, as well as the influence of monthly water conditions (SPEI) during the growing season on EVI and yield. Interestingly, when considering the impact of the monthly SPEI (different temporal SPEI in Table S1) during the growing season, the SPEI of the early season (April–June) showed more impacts on the EVI of the wild blueberry fields compared to the SPEI later in the season (July–August).

**Figure 6.** Average enhanced vegetation index (EVI) of wild blueberry fields during the growing season (May to September) for three different study zones: Airport (irrigated), Baxter (non-irrigated), and the major wild blueberry production region in Maine in relation to (**a**–**c**) SPEI\_1\_Year; (**d**–**f**) SPEI\_2\_Years (average SPEI of two consecutive years); (**g**–**i**) SPEI\_3\_Years (average SPEI of three consecutive years); (**j**–**l**) SPEI\_4\_Years (average of SPEI of four consecutive years). Here, SPEI refers to SPEI\_6 of September and it represents the SPEI (water conditions) of the growing season (April–September). A positive SPEI value represents wet conditions, while a negative SPEI value indicates dry conditions. The blue solid lines indicate significant (*p* < 0.05) and blue dashed lines indicate marginally significant (*p* < 0.10) linear relationships. The dashed red lines indicate significant (*p* < 0.05) or marginally significant (*p* < 0.10) quadratic relationships. The time period of the EVI and SPEI data was from 2000 to 2020.
