UC Santa Cruz

Abstract

Plant phenology, climate and climate change

Juliet Oshiro

Phenology, or the timing of life cycle events, is often cued by climate in plants. Many studies of plant phenology and climate use climate data from models, such as PRISM (prism.oregonstate.edu), which are generated from data collected at 2 m above the ground (above many plants), and are averaged over 4 km2 grid cells. We compared temperature recorded near-the-ground, at vegetation level, to PRISM’s temperature at ten study sites in Santa Cruz County representing grassland and sandhill habitats and a range of distances from the coast, elevations, and slope/aspects. Temperature near-the-ground was generally larger than PRISM’s, and the difference between the two sources varied with elevation, distance from the coast, and aspect. We reconstructed microclimate near-the-ground at the 10 sites from 1982-2016, and assessed how it has changed over time. Changes were minimal, but minimum temperature increased, and inter-annual variability in maximum temperature and winter rainfall changed. We then examined how these data have influenced plant phenology using historical records (1989-1998) and resurveys (2011-2015) of the 10 study sites. Higher minimum temperature and lower precipitation caused earlier first and last flowering dates in the resurveys (versus historical) and at the coast (versus inland), though variation among species was high.

We also examined how functional traits influenced species-specific phenological responses to climate. We hypothesized that phenological response to climate varied among species that avoided versus tolerated warm, dry summers, but our results did not support this. Responses varied among species with different leaf flatness, and whether the species was native or not native to California. The season in which species were sensitive to climate also influenced phenological response.

Finally, we manipulated minimum temperature, maximum temperature and precipitation in growth chambers. In higher maximum temperatures, plants began flowering earlier, and in drought and high maximum temperatures, plants stopped flowering later, and flowered for a shorter period. In these conditions plants also used water more efficiently, and therefore were smaller and had lower fitness.

Overall, this dissertation shows that phenological response to climate near the ground is trait based, and that phenological change impacts plant performance.