Phenology is the study of recurring life-cycle events and their environmental drivers. Classic examples include plant flowering and animal migration. Since phenological events respond to, as well as interact with changes in climate, the topic of phenology is increasingly relevant to various environmental, ecological, and land management issues.
The Miracle of Fall:
Certain phenological events capture public attention. Many people are excited to view the dramatic autumn color change in deciduous forests, anticipate blooming flowers in the spring, or watch for migratory birds as indicators of a long winter’s end.
On the other hand, those with allergies may dread the start of the pollen season, and every year many homeowners are faced with the daunting task of dealing with a yard full of fallen leaves. These examples show how phenology can impact our daily lives. With its connection to climate and its tangible signs all around us, phenological events provide evidence of how the natural cycle of climate impacts our world.
Phenology and Climate
It is important to note the distinction between phenology and climate. Phenology pertains to repeated events in the lives of all (biotic) organisms; whereas climate and weather are the non-living (abiotic) events driving these lives (mainly temperature, precipitation, and winds). While weather captures the short term status of the atmosphere, climate reflects its behavior over relatively long periods of time It then is fairly obvious to appreciate how climate can impact phenology. For example, Alaska’s growing season is shorter than that of California. Similarly, the impact of weather (especially extreme weather) can influence phenology. An early frost can decimate a fruit crop. Less obvious, but important for both climate and phenology research, is to understand how phenology in turn impacts weather and climate. This is an area of emerging research, but it is generally accepted that plant phenology can indeed affect climate and ecosystem processes through biogeochemical and biophysical feedback effects (Schwartz, 1996, 2003; Peñuelas et al., 2009).
The “phenology – climate connection” presents a challenge to ecologists and climatologists alike. It is crucial for the ecological community to better quantify phenological responses to climatic drivers. Likewise, it is crucial for the climate community to better quantify the control exerted by phenology on the climate system (e.g., through surface-atmosphere exchanges).
The animation below shows the phenological differences between native cottonwood trees in the background and the invasive species tamarisk (or Salt Cedar) in the foreground. As the animation plays, note how the cottonwoods green up before the tamarisk. Monitoring phenology can help in the control of invasive plants: the differences in phenological patterns between the natives and invasive can help locate the invasives using airborne or satellite imagery. Furthermore, chemical and biological control efforts often require treatment while a given plant is photosynthetically active. So, it is critical to know when this is happening to maximize the effectiveness of control.
Animation provided by NASA Goddard Space Flight Center’s Scientific Visualization Studio with photos courtesy of Paul Evangelista, Colorado State University. The photos were taken at Grand Staircase Escalante National Monument in Cottonwood Canyon Creek over the course of 2004-2005
Land Management issues
Many of the ecosystem services with direct social or economic implications depend on climatic patterns and follow seasonal cycles. The ability to understand and predict the annual and inter-annual variation in phenological events has significant management implications for grazing, forestry, agriculture, pest management, vector-borne diseases and allergens, energy consumption, water availability and conservation, and tourism and hunting, amongst other applications. In each of these disciplines, improved understanding and forecasting of phenologies help improve management techniques and, in some cases, reduce the risk of undesirable outcomes (e.g., disease outbreaks, crop failure, or forest fires). In terms of public health, better phenological forecasting would be relevant to improved preparation and treatment of asthma and allergies.
There are several methods to measure or monitor phenology, but in general they can be broken down as in situ, or in close contact with the living thing being observed, or remotely, as with a digital camera or earth observing satellite.
In situ monitoring of plants and animals has traditionally been conducted at the ground level. Traditionally, records of plant phenology have been restricted to few species at dozens to hundreds of locations (Morisette et al, 2009, table 1). Often this in situ monitoring is carried out by a large number of observers noting such events as blooming dates of flowers or bird observations in their area. The past decade has seen new phenology monitoring networks springing up around the globe; for example the USA National Phenology Network and the United Kingdom’s Nature’s Calendar. Using the power of the Internet, monitoring networks like these hold promise for greatly expanded phenological monitoring across many taxa at thousands of locations.
In the last 25 years monitoring of the vegetated land surface by space-borne sensors has introduced multiple new scales of observation, which now extend from ecosystems to regions and continents. These remote sensing phenology products are produced by collecting multiple satellite-based observations of vegetation throughout the year and summarizing those observations through phenology metrics. Some example metrics include the start of the growing season, end of the growing season, or length of the growing season. There are now multiple satellite sensors from which to derive phenology metrics as well as various ways to manipulate those data to produce remote sensing phenology products (Reed et al. 2009). Comparing results from different sensor and different methods and rectifying potential discrepancies is an area of active research (White et al., 2009).
Within the last decade, novel approaches to measuring phenology have come from less distant remote sensing measurements. An exciting new technique for near-surface phenology has been developed by using web-cams. These cameras are mounted on towers strategically located over or adjacent to vegetation targets of interest and give a very local estimate of remote sensing phenology metrics.
Whether the new information on phenology is derived from tower-based web cams, satellite date, or greatly expanded networks of in situ observations, the scientific community can use this information to learn more about the timing of life cycle events all around us.
References and Further Reading
- Morisette, J.. et al., 2009. Learning the rhythm of the seasons in the face of global change: phenological research provides the key. Frontiers in Ecology and the Environment 7: 253-260.
- Peñuelas, J., Rutishauser, T., and I. Filella. 2009. Phenology Feedbacks on Climate Change. Science 324: 887-888.
- Reed, at el., 2009. Remote Sensing Phenology: Status and the Way Forward, in Noormets, A. Phenology of Ecosystem Processes: Application in Global Change Research. Springer, Dordrecht, pp. 275.
- Richardson, A. D., et al. 2009, Near-surface remote sensing of spatial and temporal variation in canopy phenology, Ecol. Appl., 19(6), 1417--?1428.
- Schwartz, M.D. 2003. Phenology: An Integrative Environmental Science. Kluwer Academic Publishers, Dordrecht, pp. 592.
- Schwartz, M. D. 1996. Examining the Spring Discontinuity in Daily Temperature Ranges. Journal of Climate 9: 803-808.
- Bin Tan, et al., in review, An Enhanced Timesat Algorithm for Estimating Vegetation Phenology Metrics from MODIS Data.
- White, M. A., et al. (2009), Intercomparison, interpretation, and assessment of spring phenology in North America estimated from remote sensing for 1982-2006, Glob. Change Biol., 15(10), 2335-2359.
- For a non-scientific, but very entertaining demonstration of phenology see the YouTube video “One Fine Day” by Brian Eno and David Byrne