Effects of Low Levels of Dispersed Retention on the Growth and Survival of Young, Planted Douglas-Fir

Abstract

Three large-scale, experimental, dispersed residual tree sites established in coastal British Columbia, Canada were measured for planted Douglas-fir tree growth and survival five to six years after planting. The dispersed trees were predominantly large diameter (60 cm+) Douglas-fir left with a range of 0% to 30% of the original forest stand basal area (0 m ). Two sites had 0%, 5% and 15% retention, while one site had 0%, 5%, 10% and 30% retention. The trees were measured in sector plots established to randomly sample the range of microsites in each treatment. There was no detectable difference between height and basal diameter growth or mortality rates between the retention treatments over the measurement period, except for a reduction of basal diameter growth at the 30% retention level (p < 0.05). Thus a statistically significant impact on growth was demonstrated for the 30% retention compared to the 0% retention level. We expected intermediate growth rates between the 0% and the other lower retention levels but were unable to demonstrate this due to the low statistical power of the test (10 observations) and high site variability for these very young trees.

Level and spatial pattern of overstory retention impose trade‐offs for regenerating and retained trees

“…Model simulations of low‐density stands have demonstrated similar loss of productivity (Birch and Johnson 1992, Long and Roberts 1992, Hansen et al 1995). In contrast, others have failed to detect growth suppression in planted seedlings under low levels of dispersed retention (Mitchell et al 2007, Smith and Beese 2012). Our model results also illustrate the consequences of allocating growing space to forest aggregates.…”

“…Whether it is possible to achieve multiple, often competing objectives in the longer term is less clear, as these objectives may be tied to the fates of retained trees or to the dynamics of regeneration. Managers face two fundamental challenges: (1) balancing the benefits of live‐tree retention for biodiversity and other ecological values (Franklin et al 1997, Gustafsson et al 2012) with potential loss of productivity in the regenerating cohort (Rose and Muir 1997, Acker et al 1998, Zenner et al 1998, Smith and Beese 2012) and (2) ensuring sufficient regeneration (via natural seeding or planting) while promoting spatial variability in the developing forest (Franklin et al 2002, Donato et al 2012, Lindenmayer et al 2012).…”