High mortality levels observed in old-growth spruce stands of west-central British Columbia

Background/Question/Methods

The stability and resilience of old forests, especially in the face of climate change and other stressors, remain largely unknown. Climax forests in equilibrium with their environment are largely self-perpetuating, and this concept is integral to a definition of old-growth forest based on tree population demographics and cohort dynamics. On the one hand, long-lived, wide-niched species such as boreal and sub-boreal trees are expected to be robust in dominating a site once established. On the other hand, trees with a marginal carbon balance, near the end of their life expectancies, may be particularly vulnerable to added stress. Nine old forest stands with large a component of spruce (natural hybrids of Picea engelmannii x glauca) that had been measured in 1991 (published in Journal of Vegetation Science 8:615-626) were re-evaluated in 2011. These stands were 144 to 363 years old since initiated by the last stand-replacing fire. Observations were made on the status (live or dead), size (DBH and height), and vigour of individual tagged trees (>7.5 cm DBH) in 30 m x 30 m plots. Results were summarized by plot and compared with values from 20 years ago. The apparent causes of stress or mortality were recorded and tallied.

Results/Conclusions

Of the plots which could be relocated, one had been burned by a severe wildfire, one had been completed logged, one was partially logged, and two had logging damage at their edges. Large numbers of trees, especially large spruce, had died and/or fallen in the 20 years since their previous measurement. In the remaining intact plots, 73% of the lodgepole pine (Pinus contorta var. latifolia) had succumbed to mountain pine beetle (Dendroctonus ponderosae) attack in recent years. Forty-two percent of the spruce were dead, reflecting attack by spruce beetle (Dendroctonus rufipennis), windthrow, and several root rot and stem rot fungi. Thirty-four percent of the subalpine fir (Abies lasiocarpa) trees were dead due to various largely undiagnosed causes. While most plots, especially the older ones, showed recruitment from the sapling cohort into the measureable size classes, the net result of these changes was a 17% reduction in stand density, a 31% reduction in live basal area, and a corresponding 29% increase in the density of standing snags.  These changes are not significantly associated with stand age or cohort-based old-growth status, but are greater than would be expected over a 20-year window in equilibrial, climax forests.