The notation yr⁻¹ means “per year.”
- yr stands for “year.”
- The ⁻¹ (negative exponent) means “per” or “inverse of.”
So, m yr⁻¹ means “meters per year” (m/year), indicating the rate at which something moves or spreads annually.
In your sentence, it refers to the migration rate of trees, meaning they spread between 115 and 385 meters per year, depending on how their seeds are dispersed.