Spruce-Aspen-Spruce Cycle

Young spruce growing among white-trunked aspen The Spruce-Aspen Cycle
Spruce and fir produce an acidic soil solution that increases the rate of leaching of nutrient bases and nitrates. Eventually the soil environment is too nutrient-poor to support their seedlings and replacement of older trees ends.
As the older spruce die, the canopy opens allowing sun-loving species like aspen to invade. Since aspen are associated with nitrogen-fixing bacteria, they are not limited by the lack of nitrogen in the soil.
Aspen come to dominate the community; they build up bases and nitrates in the soil. Increased nutrient levels and the shade of the mature aspen create an ideal habitat for spruce seedlings.
Aspen are short-lived and their seeds do not germinate well in shade. As the older aspen die off, spruce once again come to dominate (1).

The entire cycle takes some 200 years. The spruce-fir community exists through the greater part of this time.


Young spruce growing among white-trunked aspen	The Spruce-Aspen Cycle
Spruce and fir produce an acidic soil solution that increases the rate of leaching of nutrient bases and nitrates. Eventually the soil environment is too nutrient-poor to support their seedlings and replacement of older trees ends. As the older spruce die, the canopy opens allowing sun-loving species like aspen to invade. Since aspen are associated with nitrogen-fixing bacteria, they are not limited by the lack of nitrogen in the soil. Aspen come to dominate the community; they build up bases and nitrates in the soil. Increased nutrient levels and the shade of the mature aspen create an ideal habitat for spruce seedlings. Aspen are short-lived and their seeds do not germinate well in shade. As the older aspen die off, spruce once again come to dominate (1). The entire cycle takes some 200 years. The spruce-fir community exists through the greater part of this time.