Cottonwood Seed Dispersal Phenology across North America and Worldwide: Tracking ‘Summer Snow’ through an Internet Search

Abstract

Cottonwoods, Populus trees, provide the foundation for riparian woodlands around the Northern Hemisphere but have declined partly due to river damming and water withdrawal. Cottonwood conservation requires periodic seedling replenishment, which depends upon coordinating river flows with the limited seed dispersal interval. Tracking seed dispersal phenology is laborious and we recognized a complementary approach with photographs and videos of prolific cottonwood seed release on internet websites. We searched the internet for ‘cottonwood seeds’ or ‘summer snow’, and about 800 images were posted since 2004. The 337 reports from North America with dates and/or locations revealed sequential seed dispersal in the American Southwest, the Pacific Northwest, the Great Lakes Region, and finally Alaska. Mean dispersal was June 3, with two-thirds of reports in the flanking four week interval. Dispersal was later at higher latitude and elevation, and a two-factor regression model provided 85% accuracy with reported peak dispersal from nine previous studies across western North America. The 49 Eurasian reports also revealed later seed dispersal with increasing latitude. This study revealed the utility of internet searching for tracking cottonwood seed dispersal, which could facilitate environmental flows for riparian restoration. More broadly, internet reports could reveal spatiotemporal patterns in other natural phenomena.