Sentence:  As towns continue to grow, replanting vegetation has become a form of urban utopia and green roofs are spreading fast. Last year 1m square metres of plant-covered roofing was built in France, as much as in the US, and 10 times more than in Germany, the pioneer in this <WSD>field</WSD>. 
Meaning of the word marked with <WSD> tags:  (course of study or domain of knowledge or practice) A domain of study, knowledge or practice.