主句是concern is spreading，其它都是修饰。

Following+名词性从句 (what一直到U.S.，名词性从句，做follow的宾语，history drought带一个定语。what表示xxx的事情/情形，你用the thing/situation that 代替what就很清楚)what is on track to be the hottest year on record and historic drought )(定语that's stricken much of the U.S.),( 主句concern is spreading) (介词+名词，做状语regarding the future of the nation's more than a billion-dollar Christmas tree industry。

Following what is on track to be the hottest year on record and historic drought that's stricken much of the U.S. 状语
concern 主语
is spreading 谓语
regarding the future of the nation's more than a billion-dollar Christmas tree industry 状语