This guest post is contributed by Josh Gray, Earth & Environment Research Assistant Professor at Boston University.
Natural ecosystems are the subject of the bulk of phenological research. This makes sense because we are interested in understanding the natural cues that trigger the changing of the seasons, if seasonality will change in the future, and how seasonality affects things like the carbon cycle and migration patterns. But, nearly 35% of Earth’s land surface is occupied by agricultural (croplands and pastures), and the phenology of these ecosystems is as interesting and important as their natural counterparts. I hope to convince you of this by way of three examples in which understanding cropland phenology:
- helps us understand how crops influence the global carbon cycle
- improves maps of land-cover and land-use
- better reveals the influence of weather on crop yields
The fact that carbon dioxide concentrations are increasing in the atmosphere – largely in response to human-caused emissions – is commonly known. Less widely appreciated, however, is the fact that the difference between the summer low and winter high concentration (carbon dioxide seasonality) has also been increasing. Carbon dioxide changes each season because plants pull carbon dioxide from the air (“assimilation”) mostly in the summer, while carbon dioxide is returned to the atmosphere by plant respiration throughout the year. Thus, atmospheric carbon dioxide concentrations decrease as plants “turn on” in the spring, and begin to rise again as respiration overtakes assimilation in autumn.
In a very literal way, this seasonality can be thought of as the “breath” of the biosphere. The increase between the high (winter) and low (summer) concentration levels over time indicates that more carbon is being exchanged between the biosphere and atmosphere. Since this phenomenon was first observed more than three decades ago, explanations have largely centered on the role of climate warming in increasing photosynthetic activity near the north and south poles.
While that is certainly true, it turns out that those explanations have been missing a big component of this change: increased production of crops. Nearly three times as much corn is produced now compared to fifty years ago, and it is sometimes easy to forget that all of this biomass was created, quite literally, “out of thin air”. Furthermore, because most crops are grown during relatively short periods in the summer, and the majority of their assimilated carbon is returned to the atmosphere each year (unlike forests), increased crop production can have a larger impact on atmospheric carbon dioxide seasonality than natural ecosystems. We were able to measure this impact using satellite data, devices that measure carbon dioxide at crop sites, and global crop production data. We found that just four crops (corn, wheat, rice, and soybeans) account for up to a quarter of the observed increase in carbon dioxide seasonality.
The shorter active growing seasons of croplands compared to natural ecosystems allows crops to punch above their weight when it comes to the global carbon cycle, but it also is a key feature that allows for the discrimination of croplands from other land cover types and land uses. I am working with other remote sensing scientists at Boston University to create a global land cover dataset from satellite images. While some land cover types are fairly easy to distinguish based on the fact they always “look” different, many land cover types are just varying shades of green! For example: if given a single summer time image, it might be hard to separate a field of wheat from a grassland, or even certain types of forest. We can do better by assembling time series of satellite imagery which may be used to characterize the phenology of a particular place on Earth. This information is useful for all ecosystems, but it turns out to be most useful for separating croplands from other cover types. Specifically, we have found that growing season length alone can capture most of the spatial variability in croplands around Earth.
This figure shows the growing season length for part of South America near the Pampas. Red indicates shorter growing season length and is, by itself, a pretty good map of croplands in this region. Finally, the phenology of croplands can improve our understanding of how weather determines crop yields. Improved understanding of that relationship is critical in order to forecast how future climate changes will influence the food production system. It is fairly well appreciated that excessively hot and dry conditions can reduce crop yields. (For example, hot and dry weather substantially reduced US corn production in 2012, “Cornageddon”.) However, it is also known that crops are particularly sensitive to excess heat and aridity in specific portions of their growth cycle. Corn, for instance, is most sensitive to heat during the week-long “tasseling” period when the “silks” are established. Hot and dry conditions after this period may actually improve yields as they allow the crop to mature with reduced risk of fungus development. Thus, when attempting to construct models that forecast crop yields based on weather, it is critical that weather exposure is considered with respect to the crops’ developmental stage. Failing to do so reduces the skill of these models in forecasting crop yields and can distort our understanding of the impact of future weather.
These are just three examples of the many ways in which cropland phenology is critical to scientific understanding. The PhenoCam network is currently expanding to include more agricultural ecosystems, and those data will be invaluable for answering the sorts of scientific questions outlined above, and many more that haven’t been asked yet.