In understory habitats, light quantity varies in time and space necessitating a range of adaptations that enable plants to utilise and withstand these variations. In a series of studies, the photosynthetic response to light was assessed is several species naturally growing in the understory of mixed-wood stands in west-central Alberta. Two trees of different successional positions (Picea glauca and Pinus contorta), a wide spread grass (Calamagrostis canadensis) and four shrubs differing in growth habit (Corylus cornuta, Lonicera involucrata, Rosa acicularis, and Rubus idaeus) were studied.

P. glauca is commonly found in the understory of hardwoods stands, yet also grows in the open under full light. P. contorta occasionally occurs in similar understory conditions, but usually grows in pure stands. After removal of the overstory (release) photosynthetic light response curves were developed to assess the short-term (days) photosynthetic response to the changed environment. P. glauca showed signs of damage to, and rapid recovery of, the photosynthetic system. P. contorta was mostly unaffected by release. Its photosystem appears adapted to high light. Long-term (years) effects of release in P. glauca included a large increase in photosynthetic area and photosynthetic rate. The season of release influenced shoot and crown morphology but the time of year of release had no immediate detrimental effects.

The shrub and grass species studied are common in the understory of boreal mixed-wood stands, yet thrive in open conditions created by disturbances. The photosynthetic response to light in these species indicated a limited carbon assimilation ability at understory light levels. These low photosynthetic rates are sufficient to enable them to persist in the interval between disturbances, but would restrict growth and expansion in the understory.

With the exception of P. glauca, the species studied have fire adapted reproduction mechanisms. The ability of the shrubs and C. canadensis to sustain an underground structure at understory light levels, and the serotinous cones in P. contorta, should ensure a presence on site immediately after a disturbance which will help to ensure rapid site re-vegetation. P. glauca has a range of adaptations to enable it to grow in low, high or changing light levels.