Figure 7. A survey of cottonwood mortality on the Colorado Plateau after the drought year of 2002 revealed higher survival of hybrids than parental species. Forty-six stands were chosen, including 20 narrow-leaf cottonwood stands, 15 Fremont cottonwood stands (broadleaf) and 11 stands in the hybrid zone where upper and lower elevation species are both found along with their F₁ type hybrids (narrow-leaf n = 628 trees; F₁ type n = 100; Fremont (broadleaf) cottonwood n = 574). The first 30 standing trees encountered when walking transects perpendicular to the river’s edge were counted and identified based on leaf morphology. Individuals were defined as being >2 m tall and included all resprouting that connected to the main trunk above the ground level. Death was defined as the complete mortality of a single individual, and a tree was considered live if there was evidence of basal resprouting from the trunk. The taxonomic status of dead trees was based upon dried leaves, tree structure and placement in relation to other trees. When a tree could not be clearly identified, it was not included in the survey. In order to capture the effect of the current drought on tree stands, only recently dead trees were counted (i.e. standing trees with intact bark and small diameter branches present). F₁ type mortality never exceeded 8% for a single stand, while mortality for pure species ranged between 0 and >50% for individual stands. Average F₁ mortality was 4% and differed significantly from mortality of parental species (χ² = 14.889; P = 0.0006).