Take what I'm about to say with a grain of salt as my research is focused on the Acadian and Borel Forest regions. The increase in length of the growing seasons doesn't necessarily equate to an increase in biomass, especially for native species. This is due to the increase in likelihood and severity of climate disturbance (wind, drought,..ect) and shifts in yearly temperatures and precipitation having a stronger negative impact compared to the position impact of a longer growing season.