A Legacy of Phenological Observations

The USA Lilac and Honeysuckle Observation Networks

For over 50 years cooperators in the United States and Canada have assisted phenological researchers by reporting event dates for lilac and honeysuckle. Cloned plants (genetically identical individuals) have been employed to help minimize response variations between locations. Most recently in eastern North America, three indicator species, Syringa chinensis ‘Red Rothomagensis’ (lilac), Lonicera tatarica ‘Arnold Red’ (honeysuckle), and Lonicera korolkowii ‘Zabeli’ have been observed. Lilacs were used in the original study in the western United States, and honeysuckles were later added because they are adapted to a larger geographical range than the lilac. These plants were also selected because of their well-defined phenological events, cold hardiness, and resistance to heat and drought. Due to their invasive nature, honeysuckles have not been distributed to observers since 1987, nor does USA-NPN actively support their distribution.

USA-NPN honeysuckle observation support on these pages is directed toward legacy plants that began to grow at their present locations prior to 1987. In contrast, lilacs are not invasive, and the cloned Syringa chinensis 'Red Rothomagensis' lilacs are particularly well-suited as non-invasive scientific instruments because they are sterile and do not produce seeds. More detailed descriptions of the historic lilac and honeysuckle networks, and their utility.
Links to lilac and honeysuckle species descriptions and sampling protocols:

• Syringa vulgaris – common lilac
• Syringa X chinensis – Chinese or Rouen lilac
• Lonicera tatarica – Tatarian honeysuckle

Research Results

Initially phenological studies using the lilac and honeysuckle data were aimed at agricultural applications. For example, phenology of these plants can be related to the growth of crops and insects in an attempt to predict crop yields and bloom dates (for more efficient management), and insect and disease infestations (for more economical and efficient control). Recently, phenology has been identified as a crucial contributor to global change research (Schwartz 1994, 1998; Schwartz et al. 2006). Understanding the interaction between the atmosphere (weather and climate) and the biosphere (living organisms) is a necessary part of efforts to improve models of Earth's physical systems and monitor the impact of global climate change. Observations of cloned plants (such as the lilac) over large geographical regions serve as a vital link between satellite measurements and native plant phenology across regions and in local areas.

Mark D. Schwartz and collaborators have conducted a comprehensive and long-running research program, based on data from the lilac networks, that has furthered understanding of continental-scale phenological processes through: 1) development and refinement of phenological simulation models, optimized for continental-scale studies and providing a ready means to process climate data into a form comparable with satellite sensor-derived and conventional phenological data (Schwartz 1997; Schwartz et al. 2002, 2006); and 2) evaluation of the effects of springtime plant development on energy exchange, mass exchange, and measured characteristics of the lower atmosphere (Schwartz 1992, 1996; Schwartz & Crawford 2001). Results from this work have: 1) provided spring phenological models and a suite of associated measures (derived from daily maximum-minimum air temperature data) that allow a first assessment of the possible impacts of climate change on mid-latitude phenology at the local, regional, and global scale (Schwartz et al. 2006); 2) increased understanding of the magnitude and impact of spring plant growth initiation in mid-latitude seasonal climates on lower atmospheric temperature, moisture, sensible-latent heat balance, and carbon balance (Schwartz & Crawford 2001), and 3) produced measures of phenological development over large areas that can be compared to and provide a means to assess the accuracy of satellite sensor-derived phenological products (Schwartz et al. 2002).

The Future: Lilacs AND Native Species

Genotypic variation among native plants affects their phenological responses to environmental variation. For example, when red oak (Quercus rubra) trees from across eastern North America are gathered and grown at a common location, trees from northern regions achieve spring bud-burst earlier each year than those from southern-origin ecotypes, because they have adapted to growing in a region with less solar-thermal energy available (Schlarbaum and Bagley 1981). Therefore, a continental-scale phenological monitoring network using only native (or introduced or naturalized) species would produce a “mixed signal” of genetic variability and environmental factors, not to mention genotype by environmental interactions, which are unknown for most species. This fact, and the limited range of most native species, was a main reason for using lilac clones for the historic USA lilac phenology networks. However, while cloned plant data are well-suited for general continental-scale phenological comparisons, they by definition cannot represent the local variations in response of native species. They also do not have full continental coverage because of limits to their growth in the south due to lack of chilling or lack of water. Further, native species are important for conservation purposes, or for other applied questions that are particularly relevant to native varieties (e.g., management of forest insect pests, relationships with other species in the same or other trophic levels, recreation, agriculture, etc.). So for application to the broadest set of questions, the USA National Phenology Network (USA-NPN) is being designed to include BOTH a selection of appropriate native species, and a few wide-ranging cloned species to allow for separation of environmental and genetic factors in phenological variability (Schwartz 1994, 1999). This approach is an extension of that employed in the International Phenological Garden (IPG) program in Europe, where clones of multiple native species are used at all sites (Menzel and Fabian 1999).

View references for this page