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Phenology refers to the timing of specific biological events (for example, first openings of leaf and flower buds, insect hatchings and bird migration) in relation to changes in season and climate. Phenological responses to seasonal changes (such as variation in day-length, temperature, and rain or snowfall) can be ideal indicators of the impact of local climate change.
Recent studies, such as Primack et al. (2004) and Lavoie and Lachance (2006), have demonstrated that the flowering times of plants can be a useful indicator of local climate change. Historical phenological records are relatively rare, therefore, herbarium specimens are an excellent resource to get a glimpse into the past to see when specific plants were in flower. The SD Herbarium has a large collection of plants collected in San Diego County that are just waiting to be used in research, particularly in studies on local climate change.
In addition to studies that focus on historical data, the USA-National Phenological Network had developed the Species Observation Program in order to provide observers with a selection of native plants in their region of the country that are representative of the local/regional flora, but also abundant enough to make them easy to identify and observe. There are several species that are listed in their “Mediterranean ecoregion” that are found in our local flora including: California Box-Elder (Acer negundo), Brittlebush (Encelia farinosa), Ocotillo (Fouquieria splendens), Wood Strawberry (Fragaria vesca), Ironwood (Olneya tesota), Pacific Ponderosa Pine (Pinus ponderosa), and Desert Mallow (Sphaeralcea ambigua). We will include these species in our study of historical phenological data and also try to identify some different species that could be potential indicators of climate change in San Diego County.
Some recent papers that focus on using herbarium specimens to assess climate change:
Primack et al.(2004) compared flowering time of plants growing at the Arnold Arboretum in Boston, MA with 372 records of flowering times from 1885-2002 using herbarium specimens of the same plants. Boston experienced a 1.5°C increase in mean annual temperature over since 1885. The study shows that the plants studied are flowering 8 days earlier as a response to changing temperatures.
Lavoie and Lachance (2006) used herbarium specimens to determined that Coltsfoot (Tussilago farfara) is blooming 15-31 days earlier now than in the first part of the 20th century in large urban areas near Montreal and Quebec City. They attribute the earlier blooms with warming trends recorded in the area, especially during the last three decades when the month of April became warmer than in the past.
Lavoie, C., D. Lachance. April 2006. A new herbarium-based method for reconstructing the phenology of plant species across large areas. American Journal of Botany 93(4): 512-516.
Primack, D., C. Imbres, R.B. Primack, A.J. Miller-Rushing, and P. Del Tredici. 2004. Herbarium specimens demonstrate earlier flower times in response to warming in Boston. American Journal of Botany 91(8): 1260-1264.
USA-National Phenology Network: http://www.uwm.edu/Dept/Geography/npn/index.html