As humans, we've presumably been aware of changes and patterns in our surrounding from the dawn of time. Ancient legends from various areas of the world, as well as Roman records dating back to around 200 BCE, relate particular events linked with the passage of time and its influence on nature. Linnaeus (the same person who developed the taxonomy system) began creating phenological calendars in the 18th century, and other scholars fell in line, combining the different stages of a crop's cycle into scales based on macro and micro indicators.
What is phenology?
Strictly speaking, it is the study of plant and animal life cycles as they are regulated by weather and environment (how much something changes in a specific timeframe).
Why is it so important for us?
It's the universal language of agronomists, farmers, researchers, and anybody who works with plants. Do we just have one scale? No. Do we use the same scale for all crops? Neither. Is this a problem? Sometimes, but we've learned how to deal with it.
Each scale provides relevant information about the plant, and the absence of a unique scale for each crop aids in describing the specific stages and key times that are not present in all species. As a result, the scale we use for wheat is vastly different from those used for maize or soybeans. Each crop has unique features, and the macro and micro indicators we look at change depending on the specie. Monitoring the crop's current stage allows us to know if it will require additional fertilizers or water in the coming days. Knowing when the yield will be determined, when to harvest, and so forth, and making the necessary arrangements.
How do we study phenology?
Observational techniques rely on in-field visual evaluation of the plant, which works well if we can visit the field nearly every day. This takes time and effort, as well as a highly-trained individual to gather the data (we will discuss about protocols and the importance of ground truth data soon in another post).
When these resources are unavailable, or when we need to cover a large area, such as districts, entire states, or extensive regions, additional tools and approaches come in handy. Models based on growing degree days ("My kind of Math" by Rachel Veenstra) perform quite well and are widely used to approximate the phases, although their accuracy depends on the quality of the temperature readings. Remote sensing is another tool, utilized for many years, particularly to examine large areas at once. For now, (soon we will have another post about this topic) we will describe it as the art and science of learning about an object (in our instance, plants) without touching it, using different sensors. In most situations, these sensors will gather spectral data (the result of the interaction between the electromagnetic energy from the Sun with the cells in the plant).
We can create spectral signatures for a given species using the spectral information acquired over time. Learning how the signal's strength varies and as the crop grows. These signatures, for the most part, differ between species; for example, the spectral signature of a corn crop differs from that of soybeans or wheat. Do you notice a pattern here? We can combine both the earth observation technologies and our understanding of crop phenology to forecast the future phenological events.
In the following posts we will dive deeper on remote sensing, sensors, tools, statistical methods, (buzzwords), ground truth data, and many more topics related to the beautiful world of agronomy looked from the sky.
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