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A growing body of research documents how the El Niño Southern Oscillation (ENSO) results in short-term changes in terrestrial environmental conditions, with the potential to drive ecosystem processes as the duration and severity of ENSO events increases with anthropogenic climate change. An ENSO positive phase results in anomalous patterns of rainfall and temperature throughout the tropics that coincide with leaf flush and increased fruit production in tropical forests worldwide. However, our understanding of possible mechanisms underlying this natural phenomenon is limited. Furthermore, flowering in tropical trees anticipates ENSO development, motivating the continued search for a global phenological cue for tropical angiosperm reproduction. We propose the solar energy flux hypothesis: that a physical energy influx in the Earth’s upper atmosphere and magnetosphere generated by a positive anomaly in the solar wind preceding ENSO development, cues tropical trees to increase allocation of resources to reproduction. We show that from 1994-2013, the solar wind energy flux into the Earth’s magnetosphere (Ein) is more strongly correlated with the number of trees in fruit or flower in a Puerto Rican wet forest than the Niño 3.4 climate index, despite Niño 3.4 being a previously identified driver of interannual increases in reproduction. We discuss the idea that changes in the global magnetosphere and thermosphere conditions via solar wind-effects on global atmospheric circulation, principally a weaker Walker circulation, cue interannual increases tropical tree reproduction. This may be a mechanism that synchronizes the reproductive output of the tropical trees to changes in environmental conditions that coincide with ENSO. Thus, space weather patterns may help explain terrestrial biological phenomena that occur at quasi-decadal scales.