Phenology in a nutshell

by Carina Kübert

What is phenology? The word phenology originates from the ancient greek words phaíno (φαίνω – “to show up”, “to occur”, “to appear”) and logos (λόγος – “study”, “reasoning”). It is the study of phenomena that occur regularly in plant and animal life cycles. A short definition was giving within the International Biosphere Program back in 1974:

Phenology is the study of the timing of recurring biological events, the causes of their timing with regard to biotic and abiotic forces, and the interrelation among phases of the same or different species.

Lieth 1974

Let’s have a closer look at this definition:
There are a lot of examples for life cycle events: In spring, plants are sprouting, trees foliate. Nature wakes up showing flowers and blossoms in different colors, like the strong yellow on meadows during the flowering of dendelion or the dark purple of Provence’s lavender fields. Some are accompanied with traditional customs like picnicking under blooming cherry trees (called hanami in Japan). Migratory birds such as the White Storks coming back from Africa to Europe, finding good nesting and feeding conditions to secure the breeding of the young. During the vegetation period, fauna and flora are developing, building complex relationships  between species. One example of those interactions are allergic reactions (e.g. hay fever) to certain pollen while grasses are flowering – if you are affected, this is a yearly recurring event for you. Bees, other insects and some bird species are busy pollinating plants (this is called zoophily) securing the ripening of crops. Among the 107 most often cultivated plants, 91 are dependent on pollinators: strawberries, cherries, rape, and even coffee! In late summer and fall, lush greens turn into yellow and brown colors – not without showing another color explosion when leaves turn orange and red – famous as Indian Summer in the US but also well-known in other temperate latitudes (commonly referred to as old women’s summer). The end of the vegetation period is again marked by regional feasts such as Thanksgiving or traditional mountain-meadow homecoming after fruits and crops are harvested. Migratory birds and other species already are on their journey to warmer climates and some species like squirrels prepare for winter sleep. Nature slows down, taking a deep breath while most plants are photosynthetically inactive  and animals are hibernating. In general, this period called dormancy is linked to daily temperatures lower than 5° C (for some species 10° C). As soon as those thresholds are exceeded, the cycle starts again.
So far, we gave some examples of life cycle events that are recurring every year in temperate climates. But phenology is not only about the timing of these events, although recordings reach back more than 3000 years (Chen 2003). Another topic is finding out about the drivers and reasons for the timing. Daily temperatures were already mentioned as a trigger for vegetation development. Other abiotic drivers are photo period, precipitation or soil moisture, all varying for different species, plant functional types or biomes (Richardson et al. 2013). Seasonal changes in temperature and photo period are traced back to the Earth orbiting the Sun resulting in a variable radiation supply during one year (the reason is the obliquity of the ecliptic (axial tilt), meaning that the line connecting the Earth’s poles is tilt by 23° perpendicular to the Earth’s orbit around the sun). This leads to the intra-annual formation of seasons with characteristic phenomena such as snow melt, monsoon rains, freezing of waters or glacier expansion, as well as reactions of flora and fauna. Besides those abiotic drivers, biotic triggers such as genetic predisposition (genotype), invasive species or species competition have an influence on phenological development.
While Lieth is restricting the term phenology to plants and animals, newer definitions generalize the scope of phenological research:

Phenology deals with patterns and evolution of seasonality in the biotic and abiotic environment […].

Jeanneret and Rutishauser 2010

As a consequence, phenology is not longer seen as a topic limited to botany or zoology but it integrates various research fields and technological innovations against the background of global change.

What’s the role of phenology in global change?

Mankind increasingly influences the shape of the our planet: as the population is growing, our cities are growing. We are increasing the demand for food, fiber, and water, we are sealing the ground with parking lots, emitting carbon and particulate matter. We are altering the composition of the atmosphere, changing land cover and degrading ecosystems. Keywords like Anthropocene (Crutzen 2002) and Global Change are frequently discussed. The term global change encompasses

biophysical and socio-economic changes that alter the structure and function of the Earth system” (Steffen et al. 2015)

and includes numerous changes in land cover and land use, urbanization, atmospheric composition, material and energy cycles or biodiversity on a global scale. Human activities have changed the Earth extensively and to an unprecedented extent (Millennium Ecosystem Assessment 2005).
On the other hand, we can also recognize increasing awareness and efforts to protect our planet: We studied this change more than ever in the past decades and the technological advances in data acquisition and the further development of computer-based algorithms made a significant contribution to documenting and compiling our findings and recommendations summarized in global assessments concerning

Not only the changes themselves but also the complex interactions between these changes play an important role. An outstanding review can be found in Richardson et al. (2013). The authors give us an overview of the environmental drivers of phenology in different biomes and examine the feedback of vegetation and climate.

Since I found their conceptual model very useful to show those complex relations (especially as an introduction to talks for students and colleagues), I redraw the figure to point up the important role of phenology as an Essential Biodiversity Variable (EBV, Pereira et al. 2013, Skidmore et al. 2015, Pettorelli et al. 2016) having feedback on Essential Climate Variables (ECV, GCOS 2006; Noormets 2009).
In the context of global change, the research field of phenology has become more and more the focus of interdisciplinary issues in the last two decades. Since the 1980s, the number of journal articles has increased tenfold (Tang et al. 2016). This is partly due to the numerous publications related to anthropogenic climate change: the Fourth Assessment Report of the IPCC states as

Phenology is perhaps the simplest process for changing the ecology of species as a response to climate change.

Rosenzweig et al. 2007

In addition, phenology as an integrative science is part of research in numerous disciplines such as biology, meteorology, agriculture and forestry or environmental medicine (Schwartz 2003, Hudson and Keatley 2010, Schwartz 2013).

We can conclude, that due to the central role of phenology in the context of global change, routine monitoring of vegetation phenology is of vital importance.