USA National Phenology Network

The first extensive USA phenological observation networks began in the late 1950s with a series of (U.S. Department of Agriculture, USDA) regional agricultural experiment station projects, designed to employ phenology to characterize seasonal weather patterns and improve predictions of crop yield (Schwartz 1994). J. M. Caprio at Montana State University began the first of these projects in 1956, and it eventually included around 2500 volunteer observers distributed throughout 12 Western states (Caprio 1957).

Common purple lilac plants (Syringa vulgaris) were observed initially, with two cloned honeysuckle cultivars (Lonicera tatarica ‘Arnold Red’ and L. korolkowii ‘Zabeli’) added in 1968. Lilacs are not invasive, and thus they are acceptable for widespread distribution in a phenology network. However, honeysuckles are considered invasive, and no new plants have been distributed as part of the USA phenology networks since the mid-1990s.

Most observations in the Western USA network ended in 1994; however, a few observers have again reported data since the later 1990s (Cayan et al. 2001). Initially, only the dates of “first bloom”, “full bloom”, and “end of bloom” were recorded, with dates of “first leaf” and “95% or full leaf” added in 1967. These events were all precisely defined for the observers with verbal descriptions and photographs.

Encouraged by the success of Caprio’s program in the Western USA, similar projects were started in the central U.S. in 1961, and in the northeastern U.S. in 1965. All five events (phenophases) described above were recorded for plants in the Eastern USA networks from the start. Both of these networks observed cloned plants of the lilac cultivar Syringa chinensis ‘Red Rothomagensis’ and the same two honeysuckle cultivars used in the Western states project. The cloned lilacs are sterile (in that they do not produce seeds) and non-invasive.

In 1970, the Eastern networks were combined and expanded to about 300 observation sites. Between 1975 and 1986 observations continued under several additional projects, but the Eastern network lost funding and was terminated by at the end of 1986. After the “decommissioning” of the Eastern USA network operations by the USDA, M. D. Schwartz corresponded with the most recent network supervisors, who granted him permission to contact the observers and to invite them to continue participating in an “interim” network, pending new funding (Schwartz 1994). Approximately 75 observers responded to a renewed survey form sent out in March 1988, returning data for 1988 and in many cases 1987 as well. From that time to 2004, Schwartz continued to operate this interim “Eastern North American Phenology Network” with approximately 50 observers reporting lilac and/or honeysuckle event dates each year.

Starting in 2005, these existing lilac observation stations were reorganized into an “Indicator Observation Program” and combined with a new “Native Species Observation Program” to form the prototype “Plant Phenology Programs” of the developing USA-National Phenology Network. The historical lilac phenology data from these networks (see figure) are publicly available on-line as part of the USA National Climatic Data Center’s “World Data Center for Paleoclimatology archive”, which can be accessed at: http://www.ncdc.noaa.gov/paleo/phenology.html

Years of Observations for North American Lilac Phenology Stations over the period 1956-2003 (the pattern for honeysuckle locations is similar)

Station density is highest in the western and northeastern portions of the country, allowing more detailed analyses of patterns in those regions. Phenology network stations are not present in the southeastern USA, as lilacs and honeysuckles do not receive sufficient chilling to grow successfully in that region.

Much of the work accomplished using these phenology data involved the construction of models, which when driven by climate data can compensate for spatial and temporal gaps in network coverage. However, such models are then no longer independent of the climate data, and are less flexible in determining new or changing relationships between climate and phenology. Also, the advantage of clonal plants limits their ability to directly address interesting questions related to how native plants differentially respond to changing climate.

The USA-NPN will address both of these issues by expanding the network of clonal plant observation sites across the continental USA and Alaska (lilacs don’t grow in Hawaii) as well as instituting similarly widespread observations of native plant phenology to allow the pursuit of more detailed questions of plant responses to global warming at a national scale.

In summary, the historic lilac-honeysuckle network provides spatially extensive spring phenology information from the late 1950s in the Western USA and early 1960s in the Eastern USA. Despite varying lengths of station observation records over this period, these data are sufficient to establish the general relationship among spring phenology, spring temperatures, and spring runoff (Schwartz & Reiter 2000; Cayan et al. 2001; Schwartz et al. 2006).

Additional information about the scientific findings and applications of the historic lilac-honeysuckle observation network are available here.

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