Abstract
The El Niño Southern Oscillation (ENSO) affects tropical environmental conditions, potentially altering ecosystem function as El Niño events interact with longer-term climate change. Anomalously warm equatorial Pacific Ocean temperatures affect rainfall and temperature throughout the tropics and coincide with altered leaf flush phenology and increased fruit production in wet tropical forests; however, the understanding of mechanisms underlying this pattern is limited. There is evidence that increases in tropical tree reproduction anticipate El Niño onset, motivating the continued search for a global driver of tropical angiosperm reproduction. We present the solar-wind energy flux hypothesis: that physical energy influx to the Earth's upper atmosphere and magnetosphere, generated by a positive anomaly in the solar wind preceding El Niño development, cues tropical trees to increase resource allocation to reproduction. We test this hypothesis using 19 years of data from Luquillo, Puerto Rico, correlating them with measures of solar-wind energy. From 1994 to 2013, the solar-wind energy flux into Earth's magnetosphere (Ein ) was more strongly correlated with the number of species fruiting and flowering than the Niño 3.4 climate index, despite Niño 3.4 being previously identified as a driver of interannual increases in reproduction. Changes in the global magnetosphere and thermosphere conditions from increased solar-wind energy affect global atmospheric pressure and circulation patterns, principally by weakening the Walker circulation. We discuss the idea that these changes cue interannual increases in tropical tree reproduction and act through an unidentified mechanism that anticipates and synchronizes the reproductive output of the tropical trees with El Niño.
Highlights
We do not attempt to identify the specific cue that may trigger flowering but suggest that such a cue would be adaptive for species that could anticipate the positive changes that an ENSO event might have on tree reproduction
Tropical trees may be adapted to trade off the timing of leaf flush and fruiting phenologies to maximize the exploitation of solar insolation by new leaves with the investment of sugars into fruits under dry conditions to avoid drought stress (Detto et al, 2018)
In regard to reproductive effort, individual trees are likely insensitive to the benefits of density-dependent effects acting of offspring (Salisbury, 1942; Crawley and Long, 1995; Connell and Green, 2000), and the development time for most tropical fruits is several to many months (Zimmerman et al, 2007), suggesting a separate abiotic cue that allows them to forecast the onset of ENSO
Summary
Interannual fluctuations in global climate, such as ENSO (Moy et al, 2002; McPhaden et al, 2006; Vecchi et al, 2006; Power et al, 2013), influence tropical forest energy flux (e.g., change in forest temperature or rates of nutrient cycling; Malhi and Wright, 2004; Levine et al, 2018) including biomass accretion (Phillips et al, 1998), carbon dynamics (Brienen et al, 2015), and the reproductive phenologies of tropical trees (Chang-Yang et al, 2016; Wright and Calderόn, 2006; Zimmerman et al, 2007; Pau et al, 2018; Zimmerman et al, 2018; Sakai and Kitajima, 2019). Lagged time-series correlations between the community phenological response (the number of species in fruit or flower) for two extensively-studied neotropical forests, Barro Colorado Island (BCI), Panama, and Luquillo, Puerto Rico have revealed negative lags, with respect to temperature (Wright and Calderόn, 2006; Zimmerman et al, 2007, 2018).
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