This citizen science project aims to measure how tree growth responds to weather and climate throughout the world. Classrooms and neighborhoods on different continents track growth with a simple but sensitive technique that can be used by anyone, anywhere. The tree banding network covers several thousand trees in more than 500 institutions, 48 U.S. states and 42 countries. The accumulating database has allowed us to track how the growth of trees responds to both the range of global climates and to local weather changes.
How we measure tree growth
Until very recently, most forest researchers have measured the diameters of trees at breast height (DBH) with a special tape measure called a diameter tape at a standard height on the trunk. This approach is subject to a number of errors. And the margin of error large enough that scientists only measure the same tree once every few years. This long interval means that it is hard to understand the influences of weather or climate.
The tree banding project used a different technology, metal bands called manual band dendrometers. Dendrometers latch around a tree and expand or contract with it. Students, teachers or researchers only needed to fasten a dendrometer around a tree once. Afterward, they could return and track how much the tree had swollen or shrunk whenever they liked—on a yearly, weekly or even daily basis.
With dendrometers, we discovered trees are far more sensitive than previously imagined. We expected to detect seasonal changes when the growing season began and ended. But a tree stem’s diameter fluctuates even on a daily schedule—usually driven by daily changes in water content. Access to light also seems to impact the timing and amount of growth, while short-term water availability can lead to short-term changes in diameter.
Trees care about their neighborhood, weather and climate
Weather (temperature and rainfall) can influence the health and growth of trees. In fact, we’ve learned that trees can respond to these influences over periods of days and weeks—they are not as slow and unresponsive as one might suspect. And, with regular measurements of the change in a tree’s diameter using a dendrometer band, we can track these changes very closely.
However, because of the many factors that can affect growth, it is difficult to know exactly what the cause of any change we measure was. Ideally, we would like to find many different tree species in places with a wide range of climates. If we track the growth of all those trees using the same method, we can learn something about how weather and climate affect trees in general. If we are lucky enough to find trees all over the world, we may able to unravel which factors are at work most strongly.
Weather vs. Climate
Weather changes at all scales. Conditions in space can vary from the large, continental scale climate all the way to microclimates which can have a very small range. And weather conditions can change in response to daily and seasonal changes, the multiyear phenomena such as the El Niño Southern Oscillation, and the progressive or abrupt changes anticipated for global climate change.
The effects of weather, especially the temperature and precipitation components, can be studied in any one place and time. The impacts of climate (the long-term trend in weather conditions) on trees can also be studied in one place with a sustained, long-term study. But the effect of climate change, which happens over very large regions, is not readily investigated in one location. To understand the influence of changing climate on the status of trees, we need to track their growth in many places with a wide range of climates. The Global Tree Banding Project has enabled us to make a start. In the future, we hope to expand it to cover more regions and more climates, to create a complete picture of how the world’s forests are adapting to climate change.