UC Berkeley

The timing of plant growth and reproduction in the grasslands of California follows patterns driven by the cool wet and hot dry seasons of the Mediterranean-type climate. However, within this framework there is a great deal of timing complexity. In my dissertation research I explore phenology dynamics in California grasslands, from local to species range scales. Climate change is causing shifts in phenology (the timing of yearly life cycle events). These timing shifts are disrupting interactions between species, and can lead to timing asynchrony between plants and their pollinators. Evidence indicates that a lengthened duration of flowering could buffer some of the negative impacts that occur with timing shifts. Therefore, longer flowering duration may contribute to the adaptive capacity of systems as the climate changes. In Chapter 1, I explore the potential for an extended duration of resources as a potential conservation technique to respond to phenological asynchrony, setting the stage for the three empirical chapters that follow.

In Chapter 2, I examine microclimate influences on the duration of flowering resources in a northern California grassland. To do this, I recorded the flowering time of all species on paired north and south aspects during four successive spring growing seasons (2015-2018). I evaluated flowering time differences of pollinator resource species between paired aspects at the community level, within species, and between genotypes of Lasthenia gracilis (Asteraceae). I found that temperature on the landscape was a strong driver of phenology, and that aspect differences resulted in complementary timing of flowering resources across sites. Differences in timing between flowering on north and south aspects served to extend the flowering season by an average of 4-8 days (8-15%), depending on the year. This extension was due to both within-species timing responses as well as species turnover. These findings indicate that both heterogeneous topography and species diversity can extend overall flowering duration.

In Chapter 3, I examine population differentiation for flowering time in common goldfields (Lasthenia gracilis, Asteraceae). To do this, I measured variation in flowering time under common growth conditions in a greenhouse. I found that populations of L. gracilis exhibit differentiation in flowering time, with earlier flowering in populations from warmer and drier locations (approximately 1 day earlier per 1 ˚C difference in mean growth season temperature). The differences in population flowering time in the common environment growth conditions were similar to field flowering records in response to site conditions, and were associated with climate variables in the same direction but with a shallower slope. This pattern of response reveals that both environmental and genetic differences influence flowering time and duration, and that these influences are aligned (i.e. co-gradient variation). Due to the existence of population differentiation in flowering traits, planting diverse genotypes may extend flowering duration.

In Chapter 4, I assess how planting date and competition removal influence the timing of flowering. To do this I planted another goldfields species (Lasthenia californica, Asteraceae) three times during the wet season (November, January, and March) into plots with and without competition removal treatments, in a serpentine grassland in Northern California. Planting date and competition removal treatments both significantly impacted flowering time, growth, and reproduction. Later planting dates and competition removal delayed flowering time. Later planting resulted in lower inflorescence production, revealing strong abiotic controls on flowering time and fitness in the spring. These results suggest that heterogeneous planting time as well as competition reduction can extend the duration of flowering.

Taken together, these chapters examine the abiotic and biotic influences on flowering time in California grasslands, and provide a conceptual framework for responding to concerns of phenological asynchrony by extending flowering duration. The benefit of conserving and restoring for an extended flowering duration is that longer flowering seasons and increased native floral resources benefit pollinators whether or not a phenological asynchrony occurs. The empirical chapters offer initial tests of techniques that could be applied in landscape restoration contexts to maintain and extend flowering duration, including utilization of abiotic heterogeneity, genetic and species diversity, and alteration of population timing.