As the last leaf falls

This was my first year doing the Green-Down Protocol with GLOBE. I am trained as an atmospheric scientist, so I have taken many atmospheric measurements over the course of my career. I had not ventured into the world of phenology until I joined GLOBE. More so, I am intrigued by this field of Earth Science, since it is closely connected to climate and can be a very good indicator of a climate change.

This year, as part of the GLOBE Phenology and Climate Project, myself and other scientists at The GLOBE Program Office established a phenology site near our office building. Our colleague and GLOBE Master Trainer, Gary Randolph, trained us in how to follow the Green-Down Protocol. We found a non-irrigated tree that faced toward the equator (south since we are in the Northern Hemisphere), identified the tree as a Boxelder (scientific name of Acer negundo) using our Rocky Mountain tree finder guide, and documented the site location using a GPS.

Using the Rocky Mountain Tree Finder to identify our tree

Then, we selected four leaves on an accessible branch, including a terminal leaf and marked them by tying bright orange tape around the stems. The marking tape we utilized was actually used construction caution tape that we were “recycling” for our measurements, and we had labeled each piece with a number for each leaf.

Marking the leaves with recycled tape at the phenology site

We each took turns comparing the leaves with the GLOBE Plant Color Guide. It was not always easy to tell exactly which color the leaf was, but we matched the predominant color of the leaf and came to a consensus among our group before recording the observations. Most of the time, we were all in agreement, but when we weren’t we discussed it and came to a decision as a group. We also established a soil temperature site near the base of our tree, in which we took soil temperature measurements at 5 cm and 10 cm and the air temperature every time we did green-down observations.

Determining the color of our leaves with the GLOBE Plant Color Guide

Over the past month and a half, we went out to collect data about twice per week. At first the leaves’ color didn’t change. While this made taking the measurements less exciting at first, it was very important that we had begun early in order to make sure we captured the onset of the green-down process. Nonetheless, in the past couple weeks we realized the leaves were turning color quickly, so we started going out three times per week to capture the color changes in our data. Early this week, we arrived at our green-down site to find three of the four leaves had fallen to the ground. The one leaf that remained intact was very fragile, curled up, and lacking in color compared to what it had once been.

GLOBE Director Dr. Tony Murphy makes one of the final green-down measurements

As I looked around at the trees near our site, I was amazed at the variety of stages of green-down that the nearby trees were in. Some trees had already lost almost all of their leaves, while some still had vibrant greens and yellows on display. I wondered what causes such variation in the timing of green-down from tree-to-tree? Could it be dependent on the species of tree or the amount of sunlight it gets? Perhaps it relates more to the soil conditions and soil moisture? Or maybe it is a combination of factors together, such that maybe each species needs certain amounts of sunlight and moisture to prevent green-down from beginning? When I asked myself these questions I was exercising my curiosity about the world around me. For me, this is the essence of science: to inquire about things I don’t understand, create hypotheses while I consider the problem, and then design a process to find the answers.

As the last leaf falls and I reflect on my experience doing the Green-Down Protocol, I realize I’ve learned much more than I could have by reading about it in a book and the experience has had a much bigger impact than a typical desk assignment. The data we collected will be archived in the GLOBE database where it can be used for research. The observations of nature that I made while being outside have left a lasting impression on me, as well as energized me to keep learning more about my environment. And this was all because four special leaves beckoned us to get outside, breath some fresh air, and do science.

Suggested activity: As you take measurements with GLOBE protocols, pay attention to what is happening around your measurement sites. Do you understand all of the changes and processes that you observe? If not, design a research project to help you answer the questions that make you curious about the environment and share it with us at science@globe.gov.

-Sarah Tessendorf