MULTISCALE INFLUENCES OF GOPHERS ON ALFALFA YIELD AND QUALITY

Pocket gophers (Thomomys bottae) in alfalfa (Medicago sativa L.) stand s have been associated with yield loss, but such damage has not been r econciled with their potential improvements of soil attributes that en hance plant growth. At spatial scales appropriate for identifying mech anism and effects of management, alfalfa yield was related to gopher b urrows among 40 alfalfa stands in the Sacramento Valley, California. F orage yield among fields did not relate to gopher densities of less th an 1 per 100 m of transect, but it declined as gopher density increase d beyond this threshold. Gopher density increased logistically through time, while yield declined linearly. Within fields, yield and gopher density correlated positively across seven stands, and negatively acro ss five, depending on whether the gopher density gradient happened to match the yield gradient. Gopher densities shifted spatially across th e stands through time, however, and yield improved where gopher densit ies had been greatest the year before. Yield was usually greater on ve rsus off gopher burrows, but paired-sample t-tests detected no signifi cant differences between mean yields. Within the spatial extent of ind ividual burrows, yield increased with soil moisture at the 60 to 80 cm depth, and yield and soil moisture both decreased with distance from soil mounds. Plant nitrogen content, which relates to forage quality, generally was not affected by gopher burrows, but it was related to yi eld. Results and interpretation of gopher influences on alfalfa depend on observation scale. Data must be collected at scales representing b oth individual animals and populations to understand and effectively r espond to this pest. Negative associations between gophers and yield m ay occur, but the gopher's overall impacts will have to be weighted wi th respect to soil and wildlife benefits they provided to the agroecos ystem.