ENSO and frost codetermine decade-long temporal variation in flower and seed production in a subtropical rain forest

Flower and seed production of plants can be greatly influenced by both natural climatic oscillations and local weather extremes. However, owing to the rarity of long-term monitoring studies conducted at a sufficient temporal scale to capture climatic oscillations and the unpredictability of extreme weather events, evidence that demonstrates how these two external forcings act in concert to drive plant reproduction remains scarce. In addition, considerable variation in species' phenological responses to the external climatic forcings was often observed. Phylogenetic relationships may mediate this interspecific variation, but previous studies yielded inconsistent results when testing this hypothesis. We monitored the flower and seed production in a subtropical rain forest, Fushan, Taiwan (24°45′N, 121°35′E), for over 10 years (since September 2002). In March 2005, a record low temperature (-1.3 °C) occurred at Fushan and caused great frost damage to plants. We used weekly phenological records and long-term meteorological data to assess the effects of climatic fluctuations and extreme weather event on plant reproductive output. We show that the El Niño Southern Oscillation (ENSO) indices, which integrated local climatic variables at Fushan over several months, were strongly associated with flower and seed production. The 2005 spring frost also had long-lasting effects on the flower and seed production of several species. In particular, we detected phylogenetic signals in the relationships between phenological responses of flowering production and several climatic variables (maximum temperature, irradiance and ENSO34 index). By contrast, the relationships between seed production and climatic variables, as well as phenological responses to the frost event, did not exhibit a phylogenetic signal. Synthesis. Our findings add to the growing evidence that together the natural climatic oscillation (ENSO) and the extreme weather event (frost) determined the temporal variation in flower and seed production. In addition, phylogenetically closely related species resembled each other in their flowering responses to abiotic variation in this subtropical rain forest. Improved understanding of these abiotic and biotic interactions may help predicting population- and community-level phenological responses under future climate changes. Both natural climatic oscillations and extreme weather events greatly influence plant reproduction, but less is known about how these two external forcings act in concert. Our work demonstrated that together the natural climatic oscillation (ENSO) and the extreme weather event (frost) determined the temporal variation in flower and seed production in a subtropical rain forest. In addition, phylogenetically closely related species resembled each other in their flowering responses to abiotic variation in this subtropical rain forest. Improved understanding of these abiotic and biotic interactions may help predicting population- and community-level phenological responses under future climate changes.