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The Sierra Madre Occidental in the Mexican State ofDurango, is home to about five million ha of species-richforest ecosystems. Many people live in or near theseforests and depend on them for their livelihood. Thepreservation of the species richness of this uniqueresource requires improved understanding of individualspecies functioning, effective modeling and advancedmethods of monitoring. Identifying similarities and differencesbetween individual tree species is a key toachieving these aims. Accordingly, the purpose of thisstudy is to define species cohorts based on Durango’sextensive network of permanent observational studies.We review different approaches of simplifying speciesrichforest communities. A simple height-growth ordenation,that had been used in several previous studies, providedthe motivation for an improved new method ofspecies grouping. The new approach is based on the differentiationbetween canopy species and permanent subcanopyspecies. The canopy group is further subdividedinto mature and immature individuals using the relationshipbetween diameters and heights within a bivariatemixed normal distribution. The main contribution ofthis work is the specific approach of vertical stratificationbased on height /diameter ratios and bivariate clusters.This, to our knowledge, is new and potentiallyimportant for developing more advanced methods of harvestcontrol in the forests of the Sierra Madre Occidental,and perhaps also in other multi-species forests thatare subject to management.