Original: One way in which the eucalyptus, mainly the blue gum E. globulus, proved valuable in California was in providing windbreaks for highways, orange groves, and other farms in the mostly treeless central part of the state.
Simplified: One way in which the eucalyptus, mainly the blue gum,  proved valuable in California was in providing windbreaks for highways, orange groves, and other farms in the mostly treeless central part of the state.