PS 46-202 - Long-term floral synchrony across common dryland ecosystem types declined over the last 2 decades, although nonlinear relationships with climate suggest more complex interactions.

Abstract: Synchrony can mediate interactions within and among species and may shift under climate disruptions. Many animal-pollinated plant species have synchronized flowering phenologies triggered by environmental cues, such as temperature. Climate disruption could occur if synchrony among plant species increases under climate change. Amplified floral synchrony could reduce the consistency of resources available to pollinators throughout their growing season relative to less synchronous plant communities. In the Southwestern U.S., climate has become increasingly more arid and variable during the past century. To understand whether flowering phenology has shifted with climate, we used long-term phenology data for >200 plant species to ask, has flowering synchrony shifted during the last two decades, and do shifts differ among dryland ecosystem types? We evaluated which climate aspects best predicted floral synchrony/asynchrony to determine whether the climate drivers of floral synchrony differed among ecosystems. At the Sevilleta National Wildlife Refuge in Central New Mexico, we studied three plant communities: Chihuahuan Desert grassland, Chihuahuan Desert shrubland, and Plains-Mesa grassland, for which the proportion of plants in flower was recorded monthly for March-October from 2002-present by the Sevilleta Long-Term Ecological Research Program (SEV-LTER). We determined yearly synchrony values using the Loreau method, which compares the variance of aggregated species abundances with the summed variance of individual species. Temporally, floral synchrony decreased significantly in Plains grassland and Desert shrubland and marginally declined in Desert grassland (P=0.084). Monsoonal rainfall best predicted floral synchrony for both the Chihuahuan Desert ecosystems, but with differential sensitivities. Shrubland synchrony increased linearly with rainfall. In Desert grassland, synchrony increased nonlinearly with a convex increase in years with greater rainfall. In contrast, the Standardized Precipitation-Evapotranspiration Index was the best predictor of floral synchrony in the Plains grassland, with greater synchrony at both climate extremes: cool/wet and hot/dry. Climate predictions suggest that floral synchrony should decline in both Chihuahuan Desert ecosystems, as already observed. Although the nonlinear pattern in Desert grassland suggests synchrony will increase as climate becomes more variable, such increases do not appear long-term. In contrast, for Plains grassland, climate analysis predicted increases in floral synchrony with greater climate variability, which does not match the long-term trend and suggests that other environmental factors may be driving declines. Our analysis uncovered temporal declines in floral synchrony that match declines predicted under a drier mean climate. Reduced synchrony in flowering could buffer pollinators against climate change by increasing the constancy of floral resources across the season.