Trends in the incidence of multi-trunking at 21 sites in Glenbranter Forest in western Scotland are reported. Monitoring began in 1978 and continued for 30 years except at five sites that were felled. Incidence varied greatly between sites, from 9% up to 67% of trees multi-trunked at age 15-16 years, but rates declined slowly at nearly all sites after this peak. Decline was partly due to trunk singling and partly to multi-trunked tree death; in generalised linear mixed model analyses, we found that singling showed a highly significant relationship to the girths of the main and second-ranked trunks, respectively positive and negative, and mortality showed a highly significant relationship to the difference between the main-trunk girth and plot mean girth. From observations at older sites monitored to felling, we predicted the final incidences of multi-trunking at three sites monitored since planting; for these sites, the trees predicted to remain multi-trunked had suffered substantially more leader browsing from deer when young than trees predicted to be finally single trunked. Sites planted in the 1970s are forecast to have final incidences of multi-trunking from 3 to 40%, with most expected to be in the range 20-30% multi-trunking. Hence appreciable losses in crop value are likely, and measures to combat multi-trunking are discussed.