Survival dynamics of mechanically topped Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) snags in Douglas-fir plantations, Oregon, USA

Abstract

Structural enrichment in commercial tree plantations is a potential tool to increase snag numbers but relatively little information is available about how species, size, and spatial distribution of created snags are associated with longevity of these structures. We created 1197 snags in 31 harvest units from 1997 to 1999 in the Cascade and Coast Ranges, Oregon, USA, by topping live trees with harvesting equipment. We used an experimental design to distribute created snags at three densities and as either single or clumped created snags. We fit Weibull and log-logistic Accelerated Failure Time models and found that the median failure time was insensitive to the choice of distribution. We found a small positive effect of diameter at breast height (DBH) and a slight negative effect of increasing distance between created snags on survival. Assuming Weibull and log-logistic distributions at mean observed values of DBH and distance between snags, median survival times for Douglas-fir (Pseudotsuga menziesii) were 21.0 (95% confidence interval: 19.3, 22.8) years and 21.2 (19.7, 22.8) years, respectively. For western hemlock (Tsuga heterophylla), median survival times were 13.0 (11.9, 14.2) years and 12.5 (11.4, 13.7) years, respectively. Although the two failure distributions had similar median failure times, the log-logistic implies a higher survival probability over time for snags that remained standing at the end of the study period. Created snags can be a useful supplement for harvest units rotated at ∼45 years and Douglas-fir will be available for longer as standing structures. For example, under the log-logistic model, a predicted 5% of Douglas-fir snags are retained to rotation age, so that 40 snags per hectare would be required at harvest to maintain 2 snags per hectare through stand rotation. Snags created from western hemlock will provide an early rotation pulse but are unlikely to last longer than 20 years. Our results suggest that longevity can be increased by maximizing the snag size within the safety constraints of harvesting equipment. Scattering of snags may have a slightly negative effect on snag survival but this outcome should be weighed against potential ecological benefits of variation in snag distributions.