Arthur Johnson has lived in New Orleans' Lower 9th Ward for nearly three decades, long enough to appreciate the trees that filter pollution from the big ships traveling the nearby Mississippi River and that offer shade on sweltering summer days.

When Hurricane Katrina roared through two decades ago, it wiped out 200,000 trees across the city, including many in Johnson's neighborhood and several in his own yard. The city has struggled ever since to restore its tree canopy.

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