Structure and regeneration of old-growth Thuja plicata stands near Vancouver, British Columbia

In many old-growth stands on submontane (elevation <ca. 600 m) sites in the Very Wet Maritime Coastal Western Hemlock biogeocimatic subzone, Thuja plicata populations are considered to be declining. Thuja dominate the upper canopy of such stands, but are scarce or absent in the lower canopies and understories, and are believed to be in the process of being replaced by the more numerous Tsuga heterophylla and Abies amabilis. To assess Thuja population dynamics and to test the null hypothesis that Thuja populations are not sustained in old-growth forests, this study analyzed the structure of eight old-growth stands located near Vancouver, British Columbia, in which Thuja, Tsuga and Abies form the tree layer. The effect of stand-level fire disturbances were evident in the composition, age, and size structures of three of the study stands. However, small-scale processes, indicative of the oldgrowth stage of stand development, dominated the dynamics of seven of the stands. Regardless of stand histories, Thuja populations included trees of a range of sizes and ages and there were individuals in the understories in all study stands, indicating that the species is capable of regeneration in old-growth forests. Size and age structure analysis revealed that regeneration of Thuja was sporadic and is hypothesized to be disturbance-related, whereas regeneration of Tsuga and Abies appears to have been continuous. Spatial pattern analyses of seedlings and saplings revealed aggregate distributions for the three study species; the distribution of canopy trees was not different from random. The distribution of seedlings and saplings was highly associated with the location of exposed mineral soil and decaying wood substrates, which indicated the importance of disturbance to the successful establishment of all three species. Thuja and Tsuga regenerated most successfully on exposed mineral soil and decaying wood, while Abies regenerated most successfully on undisturbed forest floor and decaying wood. No consistent relationship was identified between the distribution of Thuja regeneration and canopy tree density, an indirect measure of canopy gaps, which suggested that canopy gaps may not be necessary to facilitate Thuja establishment. Temporal differences in the regeneration niches of the three species were revealed by the analysis of diameter distributions of seedlings and saplings. Seedling mortality of all three species was high, but mortality of established Thuja regeneration (diameter at ground level > 4 cm) appeared low, which implied that once established, the chance for successful recruitment was high. As few seedlings reached 4 cm in diameter, the recruitment and canopy density of Thuja remained low. This interpretation was consistent with the age structure analysis of canopy trees (dbh >10 cm) which suggested that mortality of Thuja trees was lower than that of Tsuga and Abies trees. The low recruitment - low density - low mortality population dynamic of Thuja is consistent with the storage effect ( i.e., long-lived, rare species may be sustained in spite of low establishment and recruitment). Increment cores from all live canopy trees were measured to provide ring-width series from which individual tree growth and stand development could be interpreted. Past gap events were identified by spatial pattern analysis of trees of similar age and time of release and through comparison of ring-width series of all Thuja and their neighbours. Comparison of tree growth by species, height class, and gap occurrence revealed interspecific differences in growth response to canopy gaps. Tsuga and Abies appeared dependent on gaps to recruit to the upper canopy. Their populations featured suppressed trees in the lower canopy, with low mean annual diameter increments although they often had released multiple times, and trees with significantly higher mean annual diameter increments in the upper canopy. The relatively low frequency of releases in Thuja and its constant mean annual diameter increment among height classes suggested Thuja was not dependent on canopy gaps to gain the upper canopy. Differences in the growth patterns and growth response to gaps of the three study species might be one mechanism that enables their coexistence. It was concluded that Thuja populations were not in decline in the study area. Differences in life history characteristics, including longevity, age-specific mortality rates, recruitment success, and adaptations and response to the old-growth understory light environment likely explain the coexistence of Thuja, Tsuga and Abies in the old-growth forest. The relative importance of these attributes to population and stand dynamics and quantification of these mechanisms and processes remain to be explored.

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