Climate lags and genetics determine phenology in quaking aspen (Populus tremuloides).

Spatiotemporal patterns of phenology may be affected by mosaics of environmental and genetic variation. Environmental drivers may have temporally lagged impacts, but patterns and mechanisms remain poorly known. We combine multiple remotely-sensed, physically modeled, and genomic datasets to determine the spatiotemporal patterns and drivers of canopy phenology in quaking aspen, a widespread clonal dioecious tree species with diploid and triploid cytotypes. We show that over 391 km 2 of southwestern Colorado, (1) greenup date, greendown date, and growing season length vary by weeks, and differ across sexes, cytotypes, and genotypes; (2) phenology has high phenotypic plasticity and heritabilities of 31-61% (interquartile range), and (3) snowmelt date, soil moisture, and air temperature predict phenology, at temporal lags of up to 3 years. Our study shows that lagged environmental effects are needed to explain phenological variation, and that the effect of cytotype on phenology is obscured by its correlation with topography. Phenological patterns are consistent with responses to multi-year accumulation of carbon deficit or hydraulic damage.