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 <WSD>pioneer</WSD> in this field. 
Meaning of the word marked with <WSD> tags:  A person or other entity who is first or among the earliest in any field of inquiry, enterprise, or progress.