Temporal analysis of reproduction distributed in space illuminates the climate-change resiliency of toyon (Heteromeles arbutifolia)

Premise: Toyon, Heteromeles arbutifolia, is an iconic and ecologically important member of chaparral and oak woodland communities in the California Floristic Province. Toyon's habitat faces changing wildfire regimes, widening variation in annual rainfall, and competition by invasive introduced species. We used a new modeling pipeline, temporal analysis of reproduction distributed in space (TARDIS) to examine how recent climate trends have shaped habitat suitability for toyon. Methods: As input data for TARDIS, we annotated flowering and fruiting in images from 4,105 observations of toyon contributed to the iNaturalist crowdsourcing platform. From these records we trained Bayesian additive regression tree models relating weather to toyon flowering intensity. We used a trained model to hindcast flowering each year back to 1900, and examined trends in hindcast flowering. For comparison, we also modeled changing habitat suitability using a conventional species distribution model (SDM) relating toyon presence to 30-year climate averages. Key results: Toyon flowering is associated with greater winter precipitation and warmer fall and winter temperatures. Our hindcast predicts that mean flowering intensity has been stable to slightly increasing since 1900, with greater increases in intensity at higher elevations, but also at lower latitudes. Annual variation in flowering intensity has also increased, especially at lower latitudes. Hindcast trends in flowering are positively correlated with changes in SDM-predicted suitability. Conclusions: TARDIS recovers biologically realistic predictors of toyon flowering, and hindcast changes in flowering intensity indicate the species' range remains suitable after 125 years of changing climate. Overall our results indicate toyon populations remain healthy, but may have limited opportunity to migrate northwards as climate change continues.