Herbarium specimens provide a critical source of phenological data that can be used to identify the direct and indirect drivers of variation in flowering date within and among species. Specimen-based phenological research in California has been accelerated by digitization efforts such as the California Phenology Network, which has scored and archived the phenological status of over 1.4 million specimens to date. Using this new data source in the Consortium of California Herbaria's CCH2 data portal, we obtained data from 993 specimens of the iconic California Poppy, Eschscholzia californica Cham., along with climate data representing all collection sites. Our goal was to determine how long-term and interannual climate variation affect flowering dates, and whether the magnitude of phenological sensitivity to climate varies across the species' range. We found that specimens collected from chronically warm or dry sites flowered relatively early, and flowering date was more sensitive to long-term mean temperature than to long-term mean precipitation. Independent of these effects of long-term conditions, flowering date in E. californica was sensitive to interannual variation in seasonal precipitation, but the direction of this effect depended on the season in which the precipitation occurred. Specimens sampled from sites experiencing warmer-than-average springs in the year of collection flowered 2.7–3.3 days earlier for every 1°C increase in spring temperature relative to long-term mean spring temperature. The magnitude of these effects, however, varied across the range of E. californica, with greater sensitivity to temperature in relatively cooler regions and no discernible sensitivity in relatively warm regions. Consistently, California Poppies exhibited significant phenological advancement over the last 120 years, but this advancement was restricted to the cooler portions of its range. Our results provide one of the first accounts of intraspecific variation in both phenological sensitivity to climate and the magnitude of phenological shifts over time, and demonstrate that, for a single species, location- or population-specific estimates of phenological sensitivity or of temporal trends in phenology might not accurately predict phenological responses to climate change in other locations throughout its range. In this study, we highlight the utility and promise of herbarium specimens for addressing novel questions about the phenological responses of plants to climate trends.

Calochortus plummerae Greene (Liliaceae) is a mariposa lily that is restricted to five counties in southern California. Although rarely observed in late seral chaparral during most years, it flowers abundantly after fires. In the spring of 2004, a population of C. plummerae was discovered in recently burned chaparral on the campus of California State University, San Bernardino. Long-term study plots were established to monitor year-to-year variation in flowering of C. plummerae, and to explore potential causes of this variation. We explored the relationship between annual precipitation and flowering of C. plummerae. We also examined the relationship between precipitation and growth of the strap leaf. Finally, we tested effects of experimental herbivory (leaf cutting) on current-year flowering. Flowering was monitored for 10 years, and leaf characteristics (leaf size and leaf loss to herbivory) were monitored for six years. During the driest year of our study, no plants flowered, but a substantial number flowered during wet years. Plants also produced narrower leaves during the driest years of the study. Leaf loss (experimental removal of most of a strap leaf) resulted in failure to flower. Overall, flowering between fires appears to be suppressed by factors that reduce current-year carbon gain: drought and leaf loss to herbivory. Although our data on herbivory were too incomplete to assess its importance in driving the dynamics of flowering in years after fire, the remaining two factors we studied (time since fire and annual rainfall) were sufficient to predict the number of C. plummerae plants that flowered each year.

Climate change impacts the severity and frequency of droughts in California. As a result, native plants will likely face changes in soil moisture as temperature and weather patterns shift. Lasthenia gracilis (DC.) Greene, commonly known as Needle Goldfields, is a native California wildflower distributed across broad climate gradients from northern to southern California. Therefore, this species is ideal for testing how water availability affects populations from contrasting locations across its range. In a greenhouse experiment, we measured the effects of differing water treatments on the growth and reproductive fitness of this species to explore if there is variation in response between northern and southern populations. We also measured days to germination, germination rate, and days to flowering to further compare responses among the geographically distinct populations. Individuals from most populations exhibited faster growth rates when exposed to more water. The response of reproductive output (number of inflorescences) to the treatments was reduced in populations historically exposed to drought conditions, suggesting that the southern populations have lower population plasticity for this trait. Further, individuals in five out of six populations produced inflorescences later and flowered for longer when exposed to the highest watering treatment, revealing that phenology is significantly impacted by differing water treatments. Studying the effects of limited water availability on growth and reproductive ability in plant populations across a species' geographic range can provide more complete insight into how climate change may impact a species. This study indicates the presence of relative population plasticity in response to drought, which may be important to consider during restoration planning.

Modifications in the timing of life-history events can alter the biotic and abiotic environments experienced during an organism's lifetime. In plants, germination timing plays a critical role in relation to seasonal environmental conditions, pollinator availability, competitive dynamics, etc. Individuals can compensate for a change in timing of germination by modifying their growth and flowering time. However, biotic and abiotic factors may affect these compensatory responses. In this study we assess how biotic and abiotic differences due to planting date influence the timing of flowering, survival, and reproduction. To do this we manipulated germination timing of the annual California Goldfields (Lasthenia californica DC. ex Lindl., Asteraceae) in a serpentine grassland in northern California, by seeding three times during the growing season (November, January, and March). Neighbor removal plots were compared with control plots to examine influences of close neighbors and seasonal priority effects (due to early individuals pre-empting resources) on flowering time and duration, growth, and reproductive success (survival to reproduction and inflorescence production). Both planting date and neighbor removal treatments significantly impacted flowering time and duration, growth, and reproduction in this species. Later planting dates did delay flowering time, but this delay was minimal as flowering time was constrained within set biotic and abiotic boundaries. In addition, we find that a mixture of planting times and levels of neighbor removal can extend the duration of flowering on the landscape. Inflorescence production and survival declined with later planting dates, but neighbor removal counteracted this decline. We find that L. californica exhibits graded growth allocation, as well as plasticity in flowering time in response to planting date. This study has implications for the timing of restoration projects, as planting time influences both the timing of flowering as well as the overall reproductive success of planted individuals. Our results suggest that practitioners should aim to plant earlier in the season, but that neighbor removal may counteract some of the costs of late planting.

To date, most herbarium-based studies of phenological sensitivity to climate and of climate-driven phenological shifts fall into two categories: detailed species-specific studies vs. multi-species investigations designed to explain inter-specific variation in sensitivity to climate and/or the magnitude and direction of their long-term phenological shifts. Few herbarium-based studies, however, have compared the phenological responses of closely related taxa to detect: (1) phenological divergence, which may result from selection for the avoidance of heterospecific pollen transfer or competition for pollinators, or (2) phenological similarity, which may result from phylogenetic niche conservatism, parallel or convergent adaptive evolution, or genetic constraints that prevent divergence. Here, we compare two widespread Clarkia species in California with respect to: the climates that they occupy; mean flowering date, controlling for local climate; the degree and direction of climate change to which they have been exposed over the last ∼115 yr; the sensitivity of flowering date to inter-annual and to long-term mean maximum spring temperature and annual precipitation across their ranges; and their phenological change over time. Specimens of C. cylindrica were sampled from sites that were chronically cooler and drier than those of C. unguiculata, although their climate envelopes broadly overlapped. Clarkia cylindrica flowers ∼ 3.5 d earlier than C. unguiculata when controlling for the effects of local climatic conditions and for quantitative variation in the phenological status of specimens. However, the congeners did not differ in their sensitivities to the climatic variables examined here; cumulative annual precipitation delayed flowering and higher spring temperatures advanced flowering. In spite of significant spring warming over the sampling period, neither species exhibited a long-term phenological shift. Precipitation and spring temperature interacted to influence flowering date: the advancing effect on flowering date of high spring temperatures was greater in dry than in mesic regions, and the delaying effect of high precipitation was greater in warm than in cool regions. The similarities between these species in their phenological sensitivity and behavior are consistent with the interpretation that facilitation by pollinators and/or shared environmental conditions generate similar patterns of selection, or that limited genetic variation in flowering time prevents evolutionary divergence between these species.

Flowering time in plants is a highly variable trait that influences species' resource use and exchange of pollen with con- and heterospecifics. Levin (2009) suggested that habitat shifts within species might cause plastic shifts in flowering phenology, reducing pollen exchange across habitats. Coupled with divergent selection across habitats, diverged flowering time might thus pave the way towards ecological speciation. Some of these ideas may apply across species as well. If close heterospecific relatives share phylogenetically conserved flowering times and negatively affect each other's fitness, habitat shifts to microallopatry might provide a means for local coexistence by close relatives by reducing resource competition, shared enemies, or negative interactions via pollination. Habitat shifts might also select for diverged flowering time, or cause flowering time divergence, if phylogenetically conserved cues arrive at different times across habitats. Here, we ask if flowering phenology is phylogenetically conserved for 208 species at our coastal field site in northern California, whether flowering phenology differs systematically across habitat types, and whether habitat shifts are associated with phenological separation, especially in congeners. Because annuality and perenniality have been shown to be associated with habitat traits and flowering time, we included life history in our analyses as well. We also explore the frequency of habitat shifts between congener and noncongener pairs. We use both field observations and data from Jepson eFlora/Jepson Manual 2 (Baldwin et al. 2012) to explore patterns in flowering phenology. The two data sources were well-correlated across 59 species. Phylogeny, habitat, and life history all influenced flowering time, and habitat and life history were also phylogenetically conserved across 208 spp. Congeners differed in habitat more often than noncongener pairs, and also overlapped more in flowering time. Habitat shifts were not associated with shifts in flowering time in congeners, despite mean peak flowering time differences across habitats, and phylogenetic conservatism in habitat use. Congeners that differed in both habitat use and life history, however, did have the greatest difference in peak flowering dates. Habitat shifts likely play a role in local coexistence of close relatives, but our data do not support habitat-mediated changes in phenology as a possible mechanism. Experimental approaches may elucidate the role of phenology, resource competition, pollinators, and other associates in mediating coexistence of congeners at our coastal California field site.

Weather and climate may influence the phenology of Dirca occidentalis A.Gray (Thymelaeaceae) in ways that impact reproductive success. Dirca occidentalis blooms during winter, when the likelihood of entomophily may be low. Based on preliminary observations that the timing of dormancy release and growth resumption varies over years and among shrubs within years, we quantified fruit set among flowers that formed at different times and examined whether annual variation in autumnal precipitation and temperature during autumn and winter are associated with phenology. Fruit set was determined during 2007–2008 through 2011–2012 by tracking 37,461 flowers near or at anthesis early, midway, and late within the blooming period of D. occidentalis. In addition, measures of phenology of 18 individual shrubs were made each December 29, January 26, and February 23 of the five blooming periods, and fruit set of these shrubs was determined. Fruit set was low (<5%) among flowers present early (December 26–January 2), but increased significantly in all blooming periods, to as high as nearly 30%, among flowers at anthesis later. Phenology ratings, and lengths of newly formed stems and leaves, on December 29 increased linearly as the amount of precipitation from October 1–December 15 of the same year increased. Phenology ratings on February 23 increased linearly with increasing air temperature from November 1–February 23. Rankings of phenology of the 18 shrubs were highly correlated over years, and fruit set of individual shrubs over years was 1% to 52% and increased linearly as growth resumption and flowering became later. Our results demonstrate that low autumnal precipitation is associated with delayed growth resumption and flowering, which corresponds with increased fruit set of this rare species.

Oaks (genus Quercus) often display a large range of phenotypic variation across many of their traits. The contribution of genetic and environmental sources, and their interaction, to this variation can be partitioned experimentally using common garden plantings in which several genotypes are grown in a single location and the phenotype of interest is measured. Due to their slow growth and complex genetic structure, oaks have rarely been grown to maturity in experimental conditions that would allow partitioning phenotypic variation in this traditional way. Here we present results from trees growing in two established experimental gardens of Quercus douglasii Hook. & Arn. planted in 1991. Data from these common gardens are combined with additional data collected at the original provenance populations that served as acorn source locations. We surveyed phenological progression through the spring at both garden and field sites and found significant associations between fall and spring minimum temperatures and spring phenology, represented here as the date of bud break. A genetic component of phenological variation associated with the provenance sites was identified at both common gardens and accounts for 16.4% of the total variation observed among trees, while 68.2% of the variation can be attributed to environmental plasticity, plus a genetic × environmental interaction that accounts for about 1% of bud break variation. We discuss the implications of these components of phenological variation in Blue Oak, especially with respect to climate change, local adaptation, restoration, and assisted gene flow.

We studied the timing of budburst of valley oak (Quercus lobata Née) at Hastings Reservation, central coastal California. Similar to other taxa, budburst was advanced by warmer temperatures. Over the 30-year study period, however, there were no significant trends in either air temperature or the timing of budburst, except during the 2014–2016 drought, during which the earliest budburst dates were advanced. Several individual tree characteristics correlated with budburst timing, including access to ground water, soil available phosphorus, and elevation, the effects of which were in turn correlated with winter microclimatic conditions of individual trees. Budburst timing was significantly related to both subsequent acorn production and radial growth; trees leafing out on or near the population mean for the year experienced greater radial growth and produced larger acorn crops than trees leafing out earlier or later than the mean. Differences in acorn production were due to both differences in phenology among trees and plasticity in the phenology of individual trees across years, while differences in radial growth were primarily due to plasticity in individual tree phenology. Valley oak phenology exhibits considerable variability; the extent to which this plasticity will help this keystone California species adapt to future climate change remains to be seen.

Deciduous trees leaf out in the spring beginning with bud burst. Proximally, the timing of that process is triggered by temperature, but natural selection on bud burst timing may have acted in local populations through additional factors, such as frost damage, insect herbivores and fungal pathogens. Valley Oak, Quercus lobata Née, is a deciduous California endemic tree species that shapes the ecosystems where it is found. We examined the phenology of spring bud burst in trees collected from 674 maternal families across the species range, grown in two replicate common gardens. We found significant differences among the families for timing of bud burst, and also found that onset of bud burst differed between gardens, presumably due to climate. The differences among families were associated with the climate of origin where the trees were collected suggesting that some extent of genetic differentiation in bud burst is due to local adaptation. Given predicted changes in climate in California for the future, understanding patterns of bud burst will help inform the selection of seed sources for reforestation efforts.

Understanding the phenological sensitivity of keystone plant species, such as oaks, to climate variables provides a foundation for assessing the impacts of changing climate on ecosystem resilience, biodiversity, and carbon sequestration. This study assessed the responsiveness of bud burst, flowering, and fruiting phenophases of five native California oaks to climate variables using 2012–2019 USA National Phenology Network data, which are contributed by scientists and trained volunteers. Climate data included seasonal measurements of precipitation and maximum and minimum temperatures. Phenophase data for five oak species: Quercus agrifolia Née and Quercus kelloggii Newb. from Quercus section Lobatae (red oaks), Quercus douglasii Hook. & Arn., Quercus garryana Douglas ex Hook., and Quercus lobata Née from Quercus section Quercus (white oaks) were analyzed. The majority of the trees in the study were located in California, although a small number of sites were beyond their normal distribution ranges. No significant differences were found among species for bud burst and fruiting onsets using ANOVA and Tukey HSD tests. However, significant differences were identified between the flowering onsets of the white oaks Quercus lobata and Quercus douglasii and between Quercus lobata and the red oak Quercus agrifolia. Multiple regression models identified the strongest climate predictors of oak phenophase onset as: (1) winter precipitation, (2) mean accumulated precipitation, and (3) maximum winter temperature, so that winter precipitation and temperature have been found to be the main climate drivers of vegetative growth and reproductive potential for these native California oaks.

Flowering phenology in five chaparral species was investigated using more than a century of data obtained from herbarium collections. Three species examined were from the genus Arctostaphylos (Ericaceae) and two from Ceanothus (Rhamnaceae). Collections of these species were examined relative to climate change data during the same time period. For all the species, no change in average flowering time occurred during the past century. Considerable variability was found in flowering phenology and this variability was explored using generalized linear (GLM) and generalized linear mixed models (GLMM) and different dimensions of temperature and precipitation timing. While the genera performed differently, both required combinations of precipitation, temperature, and their interactions to predict flowering date. Arctostaphylos responded the most to precipitation interactions, while Ceanothus responded the most to temperature interactions and the previous growing season's precipitation. In both genera, regression coefficients were combinations of both positive and negative variables, indicating that flowering dates are complex interactions among the different dimensions of precipitation and temperature.

The southwestern U.S. is a global hotspot of climate change. Models project that temperatures will continue to rise through the end of the 21st century, accompanied by significant changes to the hydrological cycle. Within the Sonoran Desert, a limited number of studies have documented climate change impacts on the phenology of native plant species. Much of this phenological work to understand climate change impacts to phenology builds on research conducted nearly three decades ago to define flowering triggers and developmental requirements for native keystone Sonoran Desert woody species. Here we expand on the drivers and explore recent phenological trends for six species using a unique 36-year observational data set. We use statistical models to determine which aspects of climate influence the probability of flowering, and how flowering time may respond to climate change. We move beyond traditional models of phenology by incorporating different metrics of moisture availability in addition to temperature, weather, and climate at several time scales, including daily, weekly, seasonal, and antecedent conditions. Our results provide evidence of a trend towards earlier flowering (on the order of 1–4 days per decade) for five of the six species analyzed, and no trend for one species. The species we evaluated had contrasting phenological responses to different aspects of climate, suggesting individualistic changes in phenology and the potential of divergent plant community flowering patterns under future climate change. Understanding recent changes in flowering phenology and their climatic triggers is important to anticipating whether plant species can attract pollinators, reproduce, and persist within the community under continued climate change.

Plants with persistent fleshy fruits that last throughout fall and into winter and spring are an important source of nutrition for animals and people in boreal, subarctic, and arctic regions, but little information on fruit retention or loss is available for these regions. We evaluated fruit loss for four species across Alaska using data from our Winterberry community science network. Plants of Rosa acicularis Lindl., Viburnum edule (Michx.) Raf., Vaccinium vitis-idaea L., and Empetrum nigrum L. were monitored on a weekly basis throughout fall until snow cover and again after snow melt in 24 communities in six ecoregions in 2016–2020. Observers counted fruits and classified them into “unhealthy” (dried, rotten, or damaged) or “healthy”. Number of fruits lost per day (absolute loss rate) decreased over the course of the fall, but percent of fruits lost per day (relative loss rate) was constant for all species except E. nigrum, where it declined throughout the fall. Rates of loss were similar across ecoregions and climatic gradients, although for V. vitis-idaea the two most southern sites had the lowest relative loss rates and for E. nigrum the sites warmest in summer had the lowest loss rates. Fruit loss pulse events (>15% fruits lost in one week) were uncommon (<5% of weekly observations). At the time of persistent winter snow cover, plants retained 25–50% of fruits, with higher retention in more southern ecoregions. During winter, both relative fruit loss and absolute fruit loss rates dropped compared to fall, but in spring they rebounded to fall levels. Low proportions of unhealthy fruits in E. nigrum and V. vitis-idaea were in part due to rapid abscission of unhealthy fruits, while the other two species tended to retain unhealthy fruits. We estimate that vertebrate frugivores obtain 6–45 × as many fruits in fall as do decomposers / invertebrates. The higher loss rates during the snow-free seasons and constant rates of fruit loss for most of the focal species and locations suggest that longer falls and earlier fruit ripening will lead to lower fruit availability to animals in winter and spring.