*3.1. Historical Changes in SPEI, Climate Variables, EVI, and Productivity of Wild Blueberry Systems in Maine, USA*

During the last 71 years (1950–2020), the drought index (SPEI, Figure 3a,b) and precipitation (Figure 3c,d) tended to increase marginally (Figure 4a–d; Table 1) in the studied wild blueberry fields in Maine (Figure 3a,c and Figure 4a,c), as well as in two specific fields (Airport/Baxter) at Deblois, Maine (Figure 3b,d and Figure 4b,d). However, the mean atmospheric temperature increased significantly in the wild blueberry fields in Maine overall (Figure 3e,f; Table 1), and in the two fields in Deblois, ME. These patterns were also supported graphically by the upward UF curve (forward trend) mostly being >0.0 and UB (backward trend) curve mostly being <0.0 (Figure 4e,f).

**Figure 3.** Historical (1950 to 2020) patterns of the (**a**,**b**) SPEI\_6 of September; (**c**,**d**) mean precipitation (average of May–September); (**e**,**f**) average temperature (average of May–September) throughout the major wild blueberry production region in Maine as well as at the Airport/Baxter wild blueberry fields in Deblois, ME. A positive SPEI value represents wet conditions, while a negative SPEI value indicates dry conditions. Here, mean precipitation refers to an average of the monthly total precipitation (mm), and mean temperature refers to an average of the monthly air temperature at 2 m from the surface for the growing season.

**Figure 4.** Sequential Mann–Kendall test statistics (UF and UB values) calculated from the (**a**,**b**) SPEI\_6 of September; (**c**,**d**) mean precipitation (average of May–September); (**e**,**f**) average air temperature (average of May–September) throughout the major wild blueberry production region in Maine, as well as at the Airport/Baxter wild blueberry fields in Deblois, ME. Here, the upward UF curve (forward trend) mostly being > 0.0, UB (backward trend) curve mostly being < 0.0, and UF and UB not intersecting with each other indicate the significant increasing trends of the mean temperature. The intersections of the curves with the 0.0 line as well as with each other represent the non-significant changing (increasing/decreasing) trends of the SPEI and precipitation.

Based on the yield data from the crop years (every alternate year from 1993–2019), the wild blueberry yield of the Airport field (irrigated) had no significant change (; Figure 5a; Table 2). No significant changes in the EVI during the growing season (April–September) of the Airport field were detected over the last 21 years (2000–2020) (Figure 5d; Table 2). In contrast, both the yield (Figure 5b) and the EVI (Figure 5e) showed significant increments in the Baxter field (non-irrigated; Table 2). No significant changes in yield were observed from the studied wild blueberry fields of Maine over the last 21 years (2000–2020) (Figure 5c). A significant increase in the EVI during the growing season (April–September) was observed over the last 21 years for fields of Maine as a whole (Figure 5f; Table 2).

**Table 1.** Sequential Mann–Kendall trend analysis of the standardized precipitation-evapotranspiration index (SPEI), precipitation and mean temperature (Tmean) at different wild blueberry study zones: Airport/Baxter wild blueberry fields (Deblois, ME), and Maine wild blueberry fields (Washington and Hancock counties, ME). Here, the SPEI refers to SPEI\_6 of September. It represents the SPEI of the growing season (April–September) and indicates water conditions and drought severity. Tmean represents the average air temperatures during the growing period (May–September).


**Figure 5.** (**a**,**d**) Historical values of yield and SPEI ((**a**): 1993–2019), and of the EVI and SPEI ((**d**): 2000–2020) for the Airport (irrigated) field, Deblois, ME. (**b**,**e**) Historical values of yield and SPEI ((**b**): 1993–2019), and in the EVI and SPEI ((**e**): 2000–2020) for the Baxter (non-irrigated) field, Deblois, ME. (**c**,**f**) Historical values of yield and SPEI ((**c**): 2000–2020), and in the EVI and SPEI ((**f**): 2000–2020) for the major wild blueberry production region in Maine. Here, orange dashed lines indicate SPEI, blue solid lines indicate yield, green solid lines indicate EVI. Here, SPEI refers to SPEI\_6 of September. It represents the SPEI of the growing season (April–September) and indicates water conditions. A positive SPEI value represents wet conditions, while a negative SPEI value indicates dry conditions.


**Table 2.** Sequential Mann–Kendall trend analysis of yield and enhanced vegetation index (EVI) at three different wild blueberry study zones: Airport (irrigated field, Deblois, ME), Baxter (non-irrigated field, Deblois, ME), and Maine wild blueberry fields (Washington County and Hancock County, ME).
