Notes from the Field

Illuminating a Boreal Forest’s Spring Wake-Up

May 18th, 2023 by Fred Huemmrich and Petya Campbell, University of Maryland Baltimore County

Spruce trees in the boreal forest are evergreen and have green needles all year long.

The boreal forest is Earth’s northernmost forest. It circles the Earth at high latitudes, covering large parts of Russia, Canada, Scandinavia, and Alaska. These forests are some of the largest intact forests on Earth. They store a large amount of carbon—as much as (if not more than) is stored in tropical forests. The trees use photosynthesis to grow, and in the process, they take in carbon dioxide from the atmosphere. Carbon dioxide is an important greenhouse gas and a driver of global climate change, which is why it is vital to understand the rates of the transfer and storage of carbon between the atmosphere and the boreal forest.

Due to their vast area and remoteness, boreal forests are difficult to monitor from the ground. Data from satellites provide the means for observing these forests’ condition and detecting change across wide areas. In our project, “Clarifying Linkages Between Canopy Solar Induced Fluorescence (SIF) and Physiological Function for High Latitude Vegetation,” our team from University of Maryland Baltimore County, NASA’s Goddard Space Flight Center, and the University of Texas El Paso is working to develop advanced approaches to using satellite data to describe boreal forest productivity and detect stress responses. Our project is part of the NASA Terrestrial Ecology program’s Arctic-Boreal Vulnerability Experiment (ABoVE), a large-scale field study in Alaska and western Canada, whose overall goals are to make use of NASA technology to gain a better understanding of ecosystems at high latitudes, their responses to environmental change, and the effects of those changes.

A large proportion of the trees in the boreal forest are conifers, such as spruce trees (images above). These trees keep their green needles all year long. This makes it hard to determine when they start photosynthesizing in the spring, compared to deciduous trees, like oaks, where we can clearly see the growth of new green leaves in the spring.

The timing of the start of the growing season is key to determining the overall productivity of the forest and can be a useful predictor of possible stress events later in the summer. So, one of the goals of our project is to find ways to use light to detect when these evergreens “turn on” photosynthesis in the spring and actively start taking up carbon from the atmosphere.

This brought team members to Fairbanks, Alaska, right in the heart of the boreal forest. We arrived with our instruments in April 2023 to observe the very start of the boreal growing season. Our instruments use different methods to detect the onset and rate of photosynthetic activity in plants. One method we are using to identify photosynthetic activity in the evergreens is based on light that is actually emitted from the trees. Plants absorb light to power photosynthesis, but in the process of photosynthesis some of that light energy is radiated out from the plant; this is called chlorophyll fluorescence. This fluoresced light is very dim, which is why we don’t see plants glowing, but we can use sensitive instruments to measure fluorescence at leaf and canopy scales, which can even be done with instruments on satellites from space.

A second method is to detect very subtle changes in the color of the needles that are related to changes in the pigments in the leaves. Pigments such as chlorophyll, which makes leaves green, and carotenoids, which cause the yellow color of leaves in the fall, control potential rates of photosynthesis as well as provide protection to the leaves when stressed. These color changes are also subtle enough that we don’t see them with our naked eyes, but our instruments can measure and detect these pigment changes.

Riding on the snowmobile out to the flux tower site.

Petya Campbell wades through the snow to measure a spruce branch using STELLA, a do-it-yourself multiband radiometer. The snow was deep and soft, so you would sink up to your thighs if you stepped off the packed trail.

Instruments that are already on the flux tower measure the transfer of heat, moisture, and carbon dioxide between the atmosphere and the forest (images below).

The Caribou Creek NEON flux tower showing the tower and the hut that houses some of the equipment supporting the tower measurements. In the right picture, the boom for the FLoX can be seen extending to the right off of the top of the tower.

With the help of Jeb Timm, a NEON tower lead technician, we mounted our FLoX (Fluorescence Box) on the top of the tower. The FLoX looks down on the forest and measures the reflected light and solar induced chlorophyll fluorescence every few minutes continuously through the growing season (images below). FLoX measurements are similar to the data satellites provide, but with far more detail.

From the flux data we can determine photosynthesis rates and compare them with our fluorescence and reflectance measurements for relating the remotely sensed optical measurements to forest productivity. The continuous measurements allow us to examine the effects of varying light levels, moisture, and temperatures on the forest.

Installing the FLoX on the tower. Upper left, Jeb Timm hauls the FloX boom to the top of the tower. Upper right, the FLoX instrument (brown case) and the boom that holds the fiber optics that look down on the forest. Lower left, Petya Campbell wearing warm clothes and tower climbing safety equipment at the top of the tower. Lower right, the view for the FLoX looking down from the tower.

Near the flux tower we also put our MONITORING-PAM (MoniPAM) instruments whose probes actively illuminate individual spruce shoots with controlled pulses of light to measure fluorescence and photosynthetic processes at the leaf level (images below).

MoniPAMs measure fluorescence of individual spruce shoots to describe photosynthetic processes. Left and middle photo: MoniPAMs deployed in spruce trees near the Caribou Creek flux tower. Right photo: the MoniPAM probes illuminatied spruce shoots as part of their measurement cycle.

Besides setting up our instruments to catch the start of the growing season, we were hoping to be around when the spruce started to photosynthesize. This would allow us to test if we could detect the onset of photosynthesis through changes in needle reflectance due to changing pigment pools and/or fluorescence measured using a special leaf clip. To get consistent measurements using the same amount of needles, we had to pull off the individual tiny needles then line them up to make a solid mat to measure. And because photosynthesis and fluorescence are temperature sensitive, we had to make our measurements at the temperature the needles experience, so we worked on the deck outside in the cold (images below). The deck looked out on a big white spruce that was full of busy red squirrels who chattered and scolded us while we made our measurements.

Measuring spruce needles. Upper left, a solid mat of spruce needles for measurement. Lower left, pulling the individual needles off of the shoots and aligning them into the mat. This has to be done outside so the needles do not warm up. Upper right, measuring the fluorescence and reflectance spectra using the FluoWat leaf clip. Lower right, measuring the chlorophyll content of the needles.

Unfortunately, the temperatures mostly stayed below freezing the entire time we were there, so we didn’t get a chance to measure needles as they became photosynthetically active.

While we were there, there was also a NASA funded study of snow called SnowEx. In this part of the SnowEx study, researchers were studying changes in snow characteristics during the thaw period. The SnowEx field team made measurements of the snow on the ground, and NASA flew the Airborne Visible-Infrared Imaging Spectrometer – Next Generation  (AVIRIS-NG) imaging system on an airplane, collecting high resolution canopy spectroscopy measurements. We plan to make use of the airplane imagery in our study to see if we can identify changes in tree reflectance (which is noise to the snow scientists) indicating the start of photosynthesis.

We will return in late July to collect measurements during the period of peak summer forest productivity.

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One Response to “Illuminating a Boreal Forest’s Spring Wake-Up”

  1. RJ crackers says:

    This article offers a captivating glimpse into the vital research being conducted on boreal forests. The use of innovative satellite data and advanced methodologies to understand the forests’ productivity and responses to environmental changes is truly inspiring. The dedication and collaborative efforts of the research team are commendable as they work towards unraveling the mysteries of these remarkable ecosystems. well written.

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