The incidences of stem breakage and forking at ages five and eight years in low and high elevation Pinus tecunumanii were assessed in multilocation provenance-progeny tests comprising 16 provenances of both low and high elevation P. tecunumanii. Stem breakage and forking were found to be more severe at the highest test site (1760m a.s.l.), and progressively declined with decreasing altitude. The mean stem breakage at individual sites for the two ages ranged from 0.7% to 19.5% in the low elevation P. tecunumanii, while that of the high elevation P. tecunumanii was between 0% and 12.9%. The mean percent of forked trees ranged between 3.9% and 46.8% in the low elevation P. tecunumanii and from 5.3% to 27.5% in the high elevation P. tecunumanii. The wide provenance variation in stem breakage and forking suggests that genetic factors related to seed source influence these traits. The low elevation P. tecunumanii had significantly higher incidences of stem breakage and forked trees compared to the high elevation P. tecunumanii. Overall, provenances with very low levels of stem breakage and forking included San Jerónimo, Montana Sumpul, Las Piedrecitas and Jocón. Although tested on a limited number of sites, the genotype* environment interaction at both provenance and family level appeared to be inconsequential. The mean percent of trees with stem breakage and forks increased between the two assessment ages, with changes ranging from small to more than double, implying that decision on selection against stem breakage and forking should be avoided at five years or earlier. The results of this study showed that the challenges of stem breakage and forking in plantations can be managed by one or a combination of site and seed source selection, while intensive breeding and selection could also be used as a long term strategy.
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