JIA-2018-09

2103 WANG Di et al. Journal of Integrative Agriculture 2018, 17(9): 2096–2106 profile, where the corresponding sampling unit sizes were all less than or equal to 3000 m, decreased steadily as the cultivated land fragmentation increased. This was because the higher the cultivated land fragmentation of the sampling unit, the more scattered were the cultivated land plots within the unit. Therefore, the degree of crop clustering of planting became lower, reducing spatial autocorrelation. Three kilometers may be a threshold size for the efficient estimation of the spatial autocorrelation between the sampling units for crop acreage estimation. In addition, when the sampling unit size was less than 3 000 m, although the Moran’s I value of maize and rice distribution decreased as the level of cultivated land fragmentation increased, this value still ranged from approximately 0.9 to 0.5 and FRG remained quite strongly autocorrelated. This implied that the level of cultivated land fragmentation, at all sampling sizes, had little effect on decreasing the degree of spatial autocorrelation. 3.4. Influence of ground slope stratification on spatial autocorrelation among crops acreage in the sampling units To explore the spatial autocorrelation of ground slope, Fig. 12 shows the Moran’s I values for five ground slope levels in maize and rice sampling units. For maize, when the sampling unit size was less than or equal to 1000 m, the Moran’s I value was the highest in the 0.87–1.01% class, and less at lower or higher slope values. This was because the average value of the ground slope of all sampling units with maize was 0.99%, and this value just fell within the range belonging to the second class of the ground slope. Fig. 9 Moran’s I and Z -scores for different sampling unit sizes. Fig. 10 Moran’s I values for different cropping intensities in maize (A) and rice (B) sampling units. Seven solid lines denote different sampling unit sizes. y =0.8373 x –0.16 R 2 =0.998 y =0.9358 x –0.152 R 2 =0.9832 y =302.45 x 0.8075 R 2 =0.9992 y =338.04 x 0.8158 R 2 =0.9994 0 500 1 000 1 500 2 000 2 500 3 000 3 500 4 000 4 500 5 000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 200 400 600 800 1 000 1 200 1 400 1 600 1 800 2 000 2 200 2 400 2 600 2 800 3 000 3 200 3 400 3 600 3 800 4 000 4 200 4 400 4 600 4 800 5 000 Z -score Moran’s I Sampling unit size (m) Maize Rice Z -score_maize Z -score_rice –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 A B 0–20 20–40 40–60 60–80 80–100 Moran’s I Cropping intensity (%) 200 m 500 m 1 000 m 2 000 m 3 000 m 4 000 m 5 000 m –0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 0–20 20–40 40–60 60–80 80–100 Moran’s I Cropping intensity (%)