Using herbarium specimens to study phenological trends in San Diego County California

Background/Question/Methods For over 130 years, the San Diego Natural History Museum has conducted field explorations and collections-based research to document the biodiversity of southern California and the peninsula of Baja California. The Museum’s herbarium specimen collection from San Diego County represents an important long-term dataset stretching back to 1874. To build on this resource, data from historical San Diego County plant collections records were obtained from a number of different institutions and combined for the first time into one dataset along with contemporary collections data being generated for the San Diego County Plant Atlas. Several papers have recently described the importance of herbarium specimens in documenting biological response to climate change. The associated data records include dates, locations, and reproductive status enabling changes in geographic range, distribution, and timing of plant reproduction to be examined. The project goal was to investigate  how information (such as phenology) captured in herbarium specimen records could be extracted and used to study and document the effects that climate change may be having on the county’s floristic resources.

Results/Conclusions In 2003, the Museum’s herbarium had approximately 37,000 historic San Diego County specimens dating back to 1874. All of these data have now been databased and georeferenced with funding provided by the Institute of Museum and Library Services. Since 2005 the Plant Atlas project has trained several hundred volunteers who have contributed another 37,000 plant specimens from across the county. An additional 33,000 specimen records were obtained from the Consortium of California Herbaria, resulting in a dataset totaling over 100,000 records.  Challenges encountered while compiling data from different institutions included reconciling taxonomy, interpreting and collating phenological data (which were not necessarily recorded consistently, and were not always included in the electronic records) and dealing with different ways of recording dates and geographic coordinates. From this dataset, approximately 1500 records of a perennial flowering shrub in the genus Ceanothus (Rhamnaceae) were extracted for detailed phenological analysis. These data represented 17 taxa which had differing patterns of distribution and flowering.  Only 428 of those records included a combination of valid dates, geographic coordinates, elevations, and phenological status. Precipitation and temperature records covering the same time period as the collections also were available for analysis, allowing correlations between climate variables and plant flowering to be examined.