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Structure and regeneration of old-growth Thuja plicata stands near Vancouver, British Columbia Daniels, Lori D.

Abstract

In many old-growth stands on submontane (elevation 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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