Girdling: an effective and practical treatment for enhancing seed yields in Douglas-fir seed orchards

Abstract

Results of three girdling experiments in operational Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seed orchards, spanning a period of 12 years are summarized. Saw-cut girdles were superior to band girdles for stimulating a cone crop because they were easier to perform, healed faster, and resulted in lower abortion rates in the stimulated crop. Both annual and biennial saw-cut girdle regimens resulted in improved cone yields relative to untreated controls in four successive years. Over that same period of time, cumulative response to annual girdling was greater than to biennial girdling, but in certain response years biennial girdling was best. Girdling had no significant effect on filled seed per cone, filled seed weight, and germination of existing or stimulated crops. There was some indication that repeated girdling increased abortion rates in existing crops but decreased the rate of abortion in the stimulated crop. The long-term effect of repeated girdling on vegetative health and vigor was minor. Flower promotion treatments such as girdling probably improve the genetic quality of orchard seed by creating near-random mating conditions and by preempting unwanted pollen sources.