A flag form tree often occurs at timberline. These individuals are characterized by an upright trunk that extends above a krummholzlike mat. Branches on the trunk generally grow only along the leeward side of the trunk, giving the plant a flaglike appearance. The krummholz form is typical of alpine areas above timberline. In these areas, because of cold temperatures and severe winds, subalpine fir grows in dwarfed, shrubby mats along the ground, and is often much broader than it is tall.
Subalpine fir is monoecious.
Single or small bunches of erect female cones occur in the upper part of the crown on the upper side of young branches. Nearly all cones are produced on the uppermost part of the crown. After dispersal, numerous small rodents and birds consume seeds from the ground. Because Engelmann spruce requires a mineral soil seedbed, subalpine fir seedlings usually outnumber spruce seedlings in the understory of spruce-fir stands. Thus, even though it is short-lived, many ecologists consider subalpine fir better able to regenerate under climax conditions than Engelmann spruce.