I read that abstract twice and still can't parse it, but the 30% number comes from the noaa.gov website quote:

"A new study published in the April 6 edition of the journal Nature concludes that as emissions of carbon dioxide from burning fossil fuels have increased since the start of the 20th century, plants around the world are utilizing 30 percent more carbon dioxide (CO2), spurring plant growth."

...by the way perhaps not more than we thought (dunno?), but more than the 19th century.

> 29–100% increase [in volume]reduces to 20–76% [increase in mass].

30% is a decent point estimate for 20-76%.