*3.3. Relationships between SPEI and Yield of Wild Blueberry Fields in Maine*

The impacts of the short-term and long-term SPEI on the wild blueberry yield (Figure 8 and Table S2) were different from the relationships between the SPEI and EVI during the growing season (April–September) (Figure 6 and Table S1). A significant and positive linear relationship was found between the short-term SPEI (SPEI\_1\_Year) and yield for the non-irrigated Baxter field (Figure 8b), whereas the relationship between the short-term SPEI (SPEI\_1\_Year) and yield was non-significant at the 95% confidence level (marginally significant at the 90% confidence level, *p* = 0.058) for the irrigated Airport field (Figure 8a). For the wild blueberry fields in Maine as a whole, we found a marginally significant (*p* < 0.1)

and positive linear relationship between the short-term drought index (SPEI\_1\_Year) and the wild blueberry yield (Figure 8c). We found a significant quadratic relationship between the short-term SPEI (SPEI\_1\_Year) and yield for the non-irrigated Baxter field (Figure 8b), but not for the irrigated Airport field or the studied wild blueberry fields of Maine as a whole. When considering the impact of monthly water conditions (different temporal SPEI in Table S2) during the growing season, the correlation between the SPEI and yield was significant for the non-irrigated Baxter field, whereas it was not significant for the irrigated Airport field.

**Figure 7.** Average normalized difference vegetation index (NDVI) 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 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, whereas 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.

**Figure 8.** Average yield (lbs./acre) in a year for three different study zones: Airport, Baxter, 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). The SPEI data represent SPEI\_6 of September. 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 numbers mentioned in the *Y*-axis were shortened by dividing the yield (lbs./acre) by 1000. 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.

While analyzing the impact of the long-term SPEI (2 to 4 consecutive years) on the yield of the irrigated Airport field and the non-irrigated Baxter field, no significant linear or quadratic relationships were found (SPEI\_2\_Year in Figure 8d,e; SPEI\_3\_Year in Figure 8g,h; SPEI\_4\_Year in Figure 8j,k). However, stronger relationships were observed between the long-term SPEI during the growing season and yield when considering the wild blueberry fields of Maine as a whole (Figure 8i–l). We found significant positive linear and quadratic relationships between the average yield of the wild blueberry fields in Maine and the longterm SPEI (Figure 8i–l), except for the SPEI\_2\_Year (Figure 8f), where the linear relationship

was marginally significant (R <sup>2</sup> = 0.36, *p* = 0.08) and the quadratic relationship was not significant. In fact, the quadratic relationships were stronger between yield and the average SPEI of 3 and 4 consecutive years (Figure 8i–l). Moreover, while considering the cumulative impacts for more consecutive years, both the linear and quadratic relationships were observed to be stronger for the wild blueberry fields in Maine as a whole (Figure 8f,i,l).
