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Phenology: Changes in Ecological Lifecycles | CLIMAS


Phenology: Changes in Ecological Lifecycles

Friday, September 12, 2008

Lilac flowers bloom with cues from the weather. Caribou give birth at the peak of plant abundance so that their newborns have plenty to eat. In the Southwest, as well as all other parts of the world, variations in the climate trigger life cycle events in plants and animals. Studying these events and their relation to climate is known as phenology. The information obtained is vital for understanding the impact climate change has on humans and ecosystems.

A Gila woodpecker feeding on the flowers of the giant saguaro cactus. The timing of blooming may shift in a changing climate.
Credit: ©Frank Leung,

Phenology includes the timing of flower blooms, agricultural crop stages, insect activity, and animal migration. All of these events are changing as a result of climate change and these changes impact humans. The date flowers bloom, for example, controls the timing of allergens and infectious diseases—impacting human health—and alters when tourists visit regions to enjoy wildflowers, which impacts economies. Variations in crop phases affect agriculture by influencing the timing of planting, harvesting, and pest activity.

Quantitative assessments of the impact of phenological changes on humans in the Southwest are scant primarily because phenology is a relatively recent scientific endeavor in the Southwest. However, increasing concern about climate change has amplified efforts in the following areas:

  • Documenting observed phenological changes
  • Projecting phenological changes from climate change
  • Establishing a national phenological network

Observed phenological changes

Phenology in the Southwest is relatively young and there are only a few observational records more than 20 years old. Nonetheless, records less than 20 years are sufficient to observe trends in phenological changes, and experts believe that changes in life cycle events in the Southwest will be similar to those documented in other parts of the world where longer records exist.

Two of the more important and well-documented effects of climate change on phenology are changes in the date of flowering and food-chain disruptions.

Changes in flower blooms

Studies indicate an advance in the date that flowers bloom in the West. Important conclusions include the following:

  • Shrub specimens collected in the Sonoran Desert of the southwestern U.S. and northwestern Mexico and biological models suggest that the spring bloom of shrubs may have advanced by 20 to 41 days between 1894 and 2004
  • A study published in 2001 concluded that the average date of bloom for lilacs in the western U.S. advanced by 7.5 days between 1957 and 1994, while the average bloom date of honeysuckle advanced by 10 days between 1968 and 1994.
  • A 20-year record of the timing of flower blooms for hundreds of plant species across 4,000 vertical feet in the Santa Catalina Mountains near Tucson, Arizona, suggests more than 15 percent of the surveyed species bloom at elevations as much as 1,000 feet higher than they did in the past.
  • The same 20-year record showed the average total number of species in bloom per year increased over the 20-year period by nearly three species per year at the highest elevations—this increase was associated with increasing summer temperatures.

Food chain disruption

Important life cycle events in plants and animals are often triggered by each other. When the timing of life cycle events changes in one species, it can disrupt symbiotic relationships and affect other species. For example, in the northeastern U.S., nectar-producing trees currently bloom 25 days earlier than in the past. As a result, honey bees have switched their source of nectar from the tulip poplar tree to black locust tree, impacting the pollination of tulip poplars and causing their numbers to crash. In the Arctic, the peak in plant abundance and caribou births no longer coincide, causing a 400 percent jump in offspring mortality.

Projected phenological changes

Future phenological changes will be localized, depending on the specific plant and animal species and the magnitude of climate change. Some species may profit, while others suffer. In general, flowers will likely bloom earlier and food-chain disruptions will likely be more frequent. Several changes are likely in the Southwest:

  • Because the date and abundance of flower blooms are highly correlated with winter snowpack, projected declines in snowpack will decrease flower abundance and advance the date of flowering.
  • Global warming may have a disproportionate effect on montane plant communities. Some mountain species may not be able to respond to changes in temperature by migrating north or south. In addition, an upward shift in altitudinal range of species to encounter cooler temperatures will decrease habitat area.
  • Earlier flower blooms could have substantial impacts on plant and animal communities in the Sonoran Desert, especially on shrubs and migratory hummingbirds.

In addition, climate change will cause plant species to move in response to changes in temperature and precipitation. This may be most evident on mountains, where changes in elevation help create specific habitat zones within small areas. In the Santa Catalina Mountains near Tucson, Arizona, for example, the habitat of many species has expanded upslope, and to a lesser extent downslope.

The main message is that different plants respond differently to climate and other changes.

National phenological network

The USA National Phenology Network (NPN) is headquartered in Tucson, Arizona. Its mission is to facilitate collection and dissemination of phenological data from the United States. NPN primarily supports scientific research concerning interactions among plants, animals, and the lower atmosphere, especially the long-term impacts of climate change.

NPN encourages involvement in phenological research and provides opportunities for interested people to contribute to science. Scholars, students of all grades, and citizens record the timing of life cycle events in key plant and animal species and submit their observations on-line. In this manner, a detailed database is growing. Currently, 800 people in the U.S. participate in NPN. Among them, amateur scientists in the Southwest have provided some of the more valuable and longer observational data.


  1. Bowers, J. E. 2007. Has climatic warming altered spring flowering date of Sonoran desert shrubs? The Southwestern Naturalist, 52(3):347-355.
  2. Cayan, D. R., et al. 2001. Changes in the onset of spring in the western United States. Bulletin of the American Meteorological Society, 82(3):399-415.
  3. Personal communication with Dave Bertelsen, August 4, 2008.
  4. Crimmins, T. H., M. A. Crimmins, D. Bertelsen and J. Balmat. 2008. Relationships between alpha diversity of plant species in bloom and climatic variables across an elevation gradient. International Journal of Biometeorology, 52:353-366.
  5. Personal communication with Jake Weltzin, July 21, 2008.
  6. Inouye, D. W., M. A. Morales and G. J. Dodge. 2002. Variation in timing and abundance of flowering by Delphinium barbeyi Huth (Ranunculaceae): The roles of snowpack, frost, and La Niña, in the context of climate change. Oecologia, 130:543–550.