The age of trees in old-growth temperate rainforests on Vancouver Island, British Columbia, Canada

Background/Question/Methods

The coastal temperate rainforests of British Columbia (BC) are dominated by old-growth forests. Although a few studies have documented the age of specific areas or stands in detail, prior to this study there was no comprehensive examination of tree ages in BC’s old-growth. Knowledge of the historical development of these original forests, their characteristics and variability over the landscape can inform ecosystem-based approaches to forest management and conservation. Tree age is only one attribute of old-growth, yet it is important for both defining and understanding these forests. The key question addressed by this study was: What is the age of trees in old-growth forests on Vancouver Island?  Most of the data on the age of old growth in BC exists in unpublished inventories, ecological classification databases and graduate theses. We assembled individual tree age, height, diameter and site data in a common database representing 1,114 plots and 4,033 individual trees from over 20 individual projects established over the past 60 years. We examined these data for trends by species and landscape features including geographic location, biogeoclimatic ecosystem classification and elevation. We also characterized the age structure within old forests for selected areas with detailed stand data.

Results/Conclusions

Most dominant and co-dominant trees in old-growth forests on Vancouver Island are 150 to 450 years old. The median tree age was 289 years, with a slightly higher mean of 326 years. The frequency of trees over 350 years old diminishes rapidly, with trees over 850 uncommon. We observed higher median stand age in higher elevation biogeoclimatic units and in areas with less frequent fire and windthrow. Many stands in these coastal temperate rainforests can be considered “all-aged,” having a wide range of ages represented among dominant and co-dominant trees. The actual time since major disturbance is much greater than the mean or median stand age indicates, suggesting an important distinction between the age of a forest and the age of the trees within it. The major conifer species on Vancouver Island can be ranked from longest-lived to shortest-lived in the following order: yellow-cedar, western redcedar, mountain hemlock, Douglas-fir, western hemlock, amabilis fir, Sitka spruce, and lodgepole pine. These findings help improve our understanding of old-growth dynamics, particularly in relation to large- and small-scale disturbances and successional development.