Phenological shifts in flowering in southern California under El Niño conditions (1976)

Background/Question/Methods

Unusual weather frequently affects plant phenology, and unusual weather patterns in southern California are frequently governed by the El Niño/Southern Oscillation. High rainfall in August and September, typically drought months in this Mediterranean-climate system, are often brought by tropical storms associated with El Niño conditions. The impact of these early storms on plant phenology in California’s chaparral has not been well studied.

In organizing the herbarium at California State University, San Bernardino, we rediscovered a collection made by Roy W. Martin in the San Gabriel Mountains between 1975 and 1978. This period included rains from Tropical Storm Kathleen, a 160+ year event that was associated with the 1976-1977 El Niño. In September 1976, this storm dropped more than 13 cm of rain over the collection area, an area dominated by chaparral, conifer- and oak-woodlands, and riparian vegetation. Some specimens collected in subsequent months appeared to have flowered out of season.

To assess shifts in plant phenology that year, we examined the specimens collected over the three months following the storm, noted whether they were flowering or not, and compared the collection dates of flowering specimens to the typical flowering dates for those species as described in standard floras. 

Results/Conclusions

Of 32 species that were flowering when collected in the three months after the storm, 15 were flowering outside their normal flowering period. Half of those were flowering substantially (3-4 months) outside their normal flowering period. These species included perennial herbs, deciduous shrubs and evergreen shrubs. The few annual species collected during those months were species that typically flower in late summer and fall. Although not all elevated autumn rainfall that is associated with El Niño events may cause flowering, the apparent response to the extreme, early rainfall in 1976 indicates that some chaparral species, particularly perennial species, can shift their phenology dramatically in response to altered rainfall patterns.