Evaluation of Selfing Effects on Western Redcedar Growth and Yield in Operational Plantations Using the Tree and Stand Simulator (TASS)
Seed lots of western redcedar (Thuja plicata) from both wild stands and seed orchards contain a relatively large proportion of self-pollinated seeds compared to other tree species. Selfed seeds do not exhibit early inbreeding depression, and thus have the same probability of producing acceptable seedlings for field planting as outcrossed ones. However, selfed trees show, on average, about 10% reduction in 9-year height after outplanting. To evaluate the potential negative effects of selfing on productivity at harvest, growth and yield of stands comprising selfs were simulated using TASS (tree and stand simulator) with various planting sites and initial planting densities in this study. Height distributions of selfed and outcrossed individuals were also compared and modeled, which provided a solid basis for accurate TASS simulations and some inside view of selfing in this species. Our results indicate that effects of selfing are smaller on stand final productivity (8% on average) than on 9-year-young trees due to natural selection (competition) against slower-growing trees after crown closure, resulting in more selfs being purged than outcrossed trees. Using better planting sites with higher initial planting densities can substantially intensify the selection and reduce the negative effects of selfing on final productivity.
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