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Warming temperatures are advancing the timing of seasonal vegetation development in the extratropics, altering plant-animal interactions and increasing the risk of trophic asynchrony. Forest understories are critical yet under-observed ecosystems in which phenological patterns are both altered and obscured by overstory trees. We address the challenge of observing phenological dynamics in the understory by exploiting the physiological relationship between plant phenology and temperature accumulation, a horticultural principle we show to be preserved across spatial scales through a combination of field and growth-chamber observations. These observations provide the foundation for a spaceborne thermal-observation framework, which can trace the discrete phenophases of forest understory plants in near-real time. The thermal basis of this framework also enables the prediction of understory phenology for future climates, which we demonstrate here using Shepherdia canadensis, a widespread fruiting shrub of western North America that has important trophic connections to frugivores. Our approach enables researchers to assess the regional-scale impacts of climate change on bottom-up forest ecosystems and to monitor emerging trophic mismatches.