Hi again! In my past few posts, I’ve talked about why leaf phenology is different in different places, including places further from, or closer to, urban areas, and different places within a single forest. Continuing on this theme, today I’d like to talk about how phenology is different across latitudes.
If you live in what is called the temperate zone of the northern hemisphere, generally defined as above the Tropic of Cancer at 23.5º latitude, but below the Arctic Circle at 66.5º latitude, you are probably aware that things are warmer down south. And, since we know that leaf phenology is largely determined by temperature, you may suspect that phenology would be different in these places. You would be correct!
In the northern hemisphere, leaves in more southerly climates appear earlier on trees in the spring, and stay on longer in autumn. This is illustrated in the following figure from this scientific paper:
We see in panels (a) and (b), that in North America, vegetation green-up, an indicator of the start of spring, begins earlier in the south (blue color), while in autumn the leaves hang on longer (red color). A similar trend is seen across Eurasia in panels (c) and (d). You may wonder where information at such a vast spatial scale comes from, to make images such as this. The answer is remote sensing, a technique where satellites orbit the earth at regular time intervals. As they do this, the satellites gather images of earth’s surface, and how it changes over time, for example as leaves emerge and senesce. An earlier blog post by Eli Melaas can tell you a little more about the fascinating practice of remote sensing.
The link between phenology and latitude is important because it allows scientists to study how different climates can affect the same kinds of plants. This does not happen without some effort, though: the plants that are native to a certain place are said to be adapted to their local climate. So the temperature effects on phenology of an oak tree in Florida might be quite different from those for an oak tree in Massachusetts, due to local adaptation.
Scientists overcome this issue by making what are called common gardens. In common gardens, plants that were originally grown in the same place are transplanted to new locations, to see how they fare in different climates. This is more easily done with small plants, and one of the largest common garden networks consists of cloned, or genetically identical, lilacs and honeysuckles:
With information from common garden networks like these, scientists are able to see how identical plants are affected by warmer temperatures, which are likely to represent the climate of the future.