Growth patterns prior to mortality of mature Abies lasiocarpa in old-growth subalpine forests of southern British Columbia

Mortality of canopy trees is an important process in forest dynamics, and can be sudden without relationship to past events or the culmination of a long decline. For three Engelmann spruce ( Picea engelmannii Parry)-subalpine fir ( Abies lasiocarpa [Hook.] Nutt.) forests, we reconstructed growth of canopy-size A. lasiocarpa snags using dendrochronological procedures. We compared both the magnitude and rate of growth prior to death to a control (i.e., living) group of trees. Basal area increment averaged substantially less during the last 10 years before death than for control trees. Trees that died started diverging in growth, on average, 20–40 years before death. About 50% of trees that died had chronically slow growth, 30% had pronounced declines in growth, whereas 20% had good growth up to death. The rate of change in growth indicated a decline relative to the control group of trees but overlap was extensive between the two groups. Although some trees die abruptly without evidence in their ring patterns of impending mortality, overall our results indicate that mortality of canopy A. lasiocarpa in these old forests is a gradual process related to long periods of low growth or long-term declines in growth.