Phenological responses to prior-season defoliation and soil-nutrient availability vary among early- and late-flushing aspen (Populus tremuloides Michx.) genotypes

•Prior-season defoliation and low soil nutrient treatments delayed aspen budbreak.•Magnitudes of budbreak delay varied among aspen genotypes.•Budbreak delays were largest among early-flushing aspen genotypes.•Both treatments reduced intraspecific variation in aspen budbreak phenology.•Findings enhance understanding of factors that mediate phenological relationships.