ALN logo; link to Arid Lands Newsletter home page No. 59, August 2007
What's so special about drylands?

Our changing biological and climate calendar, or, what is phenology and why should we care?

by Mark Losleben and Wim van Leeuwen

"...phenological events and their timing are now widely recognized as ideal indicators of how local and global climate changes are affecting ecosystems at local to global scales."



  • (Back to top) The vast yearly migrations of wildebeest and other herbivores across the savannas of East Africa as they follow rain-induced grass growth, and are in turn followed by their predators...
  • The appearance every fall of millions of monarch butterflies in the Sierra Madre mountains of Mexico, where they overwinter and produce a new generation that migrates back north the following spring...
  • The sweet scents and beautiful sight of spring-flowering citrus, apple and other fruit trees (perhaps most famously, the cherry blossoms of Japan) along with their subsequent pollination and the setting of fruit...

All of these are examples of phenology: that is, the occurrence (and the study by humans) of periodic plant and animal life cycle events that are influenced by environmental changes, especially seasonal climate-driven variations in temperature and precipitation. Important phenological events, or "phenophases," include:

  • timing of leafing, flowering, and fruiting in plants;
  • agricultural crop stages and timing of harvests;
  • emergence and first flight of insects like butterflies;
  • first appearance of migrating birds
  • dates of egg-laying in birds, amphibians and reptiles.

Variations in phenophase affect the abundance and diversity of organisms, their inter-interactions with other species, the roles they play in a given ecosystem, and their effects on fluxes in water, energy, and chemical elements at scales from local to global and from minutes to centuries.

The importance of phenology to humans

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Human beings have always noted and acted upon phenological events, from nomadic hunter-gatherers following their migrating prey to farmers interplanting crops at carefully staggered times as a pest management strategy. Historically, though, such observations and their use to develop various livelihood system strategies have been largely local in their impacts.

Over the past decades, phenology has taken on new importance as a tool for understanding and addressing a variety of complex problems, such as:

  • Agricultural production
    • Phenological information is very important for effective production forecasts (e.g wheat, soybeans) and can affect global import and export strategies (billions of $) and subsidies.
    • Many fruit crops (including tomatoes) rely on the presence of bees and/or other pollinators at flowering time to pollinate the flowers
    • Having good data about such things as plant leafing out and fruiting, and insect emergence and maturation, is crucial to the design of effective strategies for integrated pest management.
    • The timing of harvests is often determined by short and long term weather patterns (e.g. planning for harvest prior to average date of first frost).
    • Cattle stocking rates are often adjusted based on rain-influenced productivity of rangeland in arid and semi-arid climates.
  • Natural resources management and conservation
    • Understanding the phenology of invasive plant or other species can help efforts to design effective strategies to control their spread; careful timing of such efforts is often key to their success.
  • Human health
    • The timing of rainfall often goes hand in hand both with vector borne diseases like West-Nile, malaria and dengue fever, and with allergenic discomforts and diseases like asthma and hay fever. Successful control of these plant allergens and disease-bearing insects greatly depends on the phenological information of each species.
  • Recreation- and tourism-driven development
    • Tourism is increasingly seen as a tool for development and poverty alleviation. Tourists are often interested in what are essentially phenology-driven events, such as bird watching, spring wildflower displays, or autumn tree color. Being able to predict the timing of such events is thus important to the tourism industry.

Phenology and global warming

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Even more important is that phenological events and their timing are now widely recognized as ideal indicators of how local and global climate changes are affecting ecosystems at local to global scales. Furthermore, precisely because phenological events are so readily observable and trackable at local scales, phenology is potentially an effective educational tool, either inside or outside the classroom, for increasing public awareness of these important environmental issues. Finally, phenology holds great promise as a predictive tool.

That climate changes are indeed occurring, and are in large part due to human activities, is by now widely accepted. Climate variability and extreme weather events are also expected to increase, affecting not just people but also the intricate timing of development phases of all living organisms that are part of the Earth's natural systems. This latter point is crucial: if our biological and climate calendars start to run out of sync due to these global changes, there will be significant ecological and socio-economic consequences that will seriously affect humans. For example, it has been well documented that, during the 20th century, many parts of the world experienced warmer surface air temperatures that caused earlier and longer growing seasons. But what if this means that flowers start blooming before any insects have hatched, or any migrating pollinators are around, to pollinate them? It is easy to see that ultimately, such changes could filter up the food chain to have drastic effects on our food supply. Phenological data can help us document and measure the occurrence of such changes.

And phenological information is useful for more than just understanding current conditions of climate change. When spatially extensive sets of phenological data are integrated with short- and long-term climatic forecast models, powerful tools emerge to assist human adaptation to ongoing and future climate change. Given sufficient observations and understanding, these tools can be used as a predictor for other processes and variables of importance at local to global scales. Ultimately, phenological data could drive a variety of ecological forecast models with both theoretical and practical applications., enabling answers to critical questions about our changing planet and how humans can best adapt to this change through increased understanding of earth systems interactions on a scale never before possible.

At present, though, broadly distributed phenological data sets that take advantage of environmental gradients (1) are almost non-existent, because such gradients are difficult to construct without a centralized data resource. In order for phenological modeling to improve our understanding of environmental variability as a cause and/or consequence of complex problems like climate change, we need networks that can amass large numbers of phenological observations and provide tools to analyze these data at multiple scales. Fortunately, a unique convergence between rapid technological change and growing public and scientific interest is making it increasingly feasible to establish such networks.

Phenology internationally

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Phenological monitoring networks are already in place or being expanded upon in some countries and are needed in others. The European Phenology Network is a good example of how such "mega"-networks can leverage already existing networks. Many European countries have long had phenological networks in place to support their agricultural sector, but as agriculture decreased in importance towards the end of the 20th century, these networks were often reduced in size or abandoned. In addition, the data collected by each network were often not easily accessible and/or not easily integrated into other datasets due to incompatible formats.

As the importance of phenological information to climate change studies was recognized, steps were taken to address these problems, and the year 2000 saw the successful launch of a proposal to set up the European Phenology Network. Incorporating several previously established programs, the EPN aims to improve monitoring, assessment and prediction of climate-induced phenological changes and their effects in Europe.

Phenology in the United States

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In the United States, efforts over the past several years have recently led to the establishment of the United States National Phenology Network (USA-NPN), a national network that will allow gathering, integration and interpretation of phenological observations at multiple scales. The USA-NPN will build on existing observation networks and remote sensing products, emerging technologies and data management capabilities, myriad educational opportunities, and a new readiness of the public to participate in investigations of nature on a national scale.

The USA-NPN as envisioned has great potential to help safeguard and procure ecological goods and services throughout the country. With this potential in mind, the goals for the network are to:

  • facilitate a thorough understanding of phenological phenomena, including their causes and roles in the biosphere;
  • provide empirical data for ground-truthing, making the most of the large public investment in satellite platforms and remote sensing;
  • allow the detection and prediction of environmental change for myriad applications, such as assessing impacts of land use and climate variability/change;
  • develop simple and effective means to report and use phenological observations, and
  • provide the resources necessary to deliver the right information at the right time for a wide range of decisions made routinely by individual citizens and by the nation as a whole.

The framework for the USA-NPN is a four-tiered, expandable structure representing different levels of spatial coverage and quality/quantity of phenological and related environmental information:

Thumbnail link to Figure 1
Link to Figure 1, ~35K

  1. locally intensive sites focused on process studies (e.g., long-term ecological research sites);
  2. spatially extensive scientific networks (e.g., National Weather Service cooperative observer stations);
  3. volunteer and educational networks (e.g., garden clubs, plant-, bird-, and butterfly-monitoring networks, college campuses, and schools); and
  4. remote sensing products that can be ground-truthed and assimilated to extend surface phenological observations to the continental scale (Figure 1).

The USA-NPN's first official campaign was "Project Budburst," a nation-wide data-gathering campaign was held during the spring 2007 growing season (March-June 2007). This project encouraged citizen scientists to track local flowering times of a selected set of species, entering their observations into the project's web site. At the same time, work also began on linking remote sensing (that is, landscape-scale phenology) and surface phenology measurements for several sites and ecosystems across the US. In addition, the USA-NPN will search for natural history collections and historical data sets to extend the significance of current observations.

Region-specific networks in the USA-NPN

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Biological processes and phenological cahanges also occur at spatial scales smaller than the national level. Therefore, the NPN is actively encouraging development of regional phenology networks throughout the US to capture this level of information, by bringing interested parties together; functioning as a central information resource, data archive and retrieval center; and providing observation and recording guidelines.

Regional organization is currently highest in the northeastern US, but activities towards establishment of a Southwest Regional Phenology network are also underway. These include the upcoming joint conference of the American Society for Photogrammetry and Remote Sensing, US Southwest region, and the US National Phenology Network, to be held in Tucson, Arizona, on October 5, 2007. The Southwest Regional Phenology Network will be particularly well suited for arid lands studies, such as:

  • evaluation of phenological effects in the hydrological cycle (evapotranspiration, recharge, groundwater levels, and streamflow);
  • impacts of longer growing seasons on hydrology and ecology (plant moisture stress, seasonal timing, geographic range of plants, frequency and severity of fire, insect and pathogen outbreaks); and
  • new vectors for invasive species and infectious disease.


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The importance of phenological observations and phenological networks to our ultimate ability to understand and respond to the effects of global climate change cannot be overestimated. First, we need to gather and analyze massive quantities of baseline phenological data to really understand current conditions; second, a thorough understanding of current conditions is basic both to our ability to distinguish natural variations from potentially serious tipping points leading to irreversible ecological changes, and to our ability to develop appropriate strategies for coping with such changes when and if they occur.

Setting up such networks is a big job, but as the examples of the European Phenology Network and the USA-NPN show, there is also excellent potential for such networks to be established by leveraging already-existing data and networks.

For drylands in particular, where natural variations in climate have always been more extreme than in temperate regions, the need for phenological networks is perhaps most immediate. The fragility of drylands ecosystems means that they are potentially more subject to irreversible changes triggered by warming climates, and potentially less resilient in successfully adapting to such changes. Phenology networks are an emerging means of providing the knowledge and tools we need to successfully meet the challenges of dealing with climate change, in drylands and elsewhere, over the course of the 21st century.


  1. (Back to top) "Environmental gradient" means the rate of change across time and space of factors that define ecosystems: species, climate, weather, extreme events, soils, etc. As one or more of these factors change, climate or extreme events for example, the environmental gradient may change. An increased gradient suggests compression of ecosystems, increased stress on its defining species, and perhaps a reduction in species diversity or extinction if the gradient becomes too steep. An arid lands example from the US Southwest is the effect of increasing temperatures on sky-island landscapes--that is, the numerous mountain ranges in the region that are isolated from each other by broad desert valleys. The response of many species, and thus ecosystems, to warming temperatures is to move to higher elevations--but the mountains do not grow higher, thus the environmental gradient increases.(Back to text)

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Author information

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Mark Losleben ( is the Assistant Director of the USA-National Phenology Network, working out of the USA-NPN's National Coordinating Office in Tucson, Arizona. Dr. Wim van Leeuwen ( is Assistant Professor, Office of Arid Lands Studies and Dept of. Geography & Regional Development, University of Arizona.

Additional web resources

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European Phenology Network

Existing phenological networks throughout the world


Project Budburst

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