Table 1.

Hypotheses to explain accelerated recent growth of forest stands

Hypothesis	Description
Increased temperature	Higher temperatures over the growing season (or growing degree day sums) can increase metabolic rates and therefore, lead to more rapid carbon sequestration.
Increased growing season	Longer growing seasons (especially earlier springs) has been observed in many systems. A longer growing season would allow stands earlier leaf-flush and therefore, increased carbon sequestration.
Increased CO2	Higher atmospheric CO2 can increase photosynthesis and lead to higher biomass accumulation.
Nutrient fertilization	Nitrogen and phosphorus fertilization from agricultural and urban runoff can increase tree growth. Atmospheric deposition of nitrogen has increased in urban and industrial regions in the United States in recent decades and could lead to soil fertilization as well.
Community composition	Some pioneer species tend to grow faster than others, especially sweet gum (Liriodendron styraciflua) and tulip poplar (Liquidambar tulipifera). An influx of these species into a plot or a preponderance of fast-growing species recently becoming dominant could lead to an increase in biomass accumulation.
Demographic stochasticity	Although a stand may follow the ensemble biomass-accumulation pattern, random deaths of large trees can lower the stand biomass, and rapid regeneration will quickly increase the biomass as if the plot were, through a death, moved to a younger state.