The mysterious inner workings of our trees and forests are a subject of fascination for nature enthusiasts and researchers alike. Perhaps now more so than ever, as it is widely appreciated that trees across the globe are a silent yet powerful force in the regulation of major greenhouse gasses.
It all starts when trees draw carbon from the atmosphere through stomata in the leaves and use it in the process of photosynthesis. Within the trees’ leaves, the sun’s energy, carbon dioxide and water are converted to oxygen and glucose. The resulting glucose is manufactured further into starches and cellulose that are used as the building blocks for cell creation, growth, and repair. The carbon absorbed from the atmosphere by trees is incorporated into the structure of the tree – into the wood, leaves, and roots that make up its unique form. Over time, leaves, branches, and eventually whole trees tumble to the ground and a portion of the carbon contained in their structure is incorporated and stored as part of the soil layer and some portion returns to the atmosphere through decomposition. The process of absorption and storage of carbon in trees and soils is considered carbon sequestration.
While trees in other parts of the globe are actively photosynthesizing and participating in the sequestration of carbon year-round, in most of the northern hemisphere, trees enter a period of dormancy outside of the growing season. Dormancy is triggered by environmental conditions such as changes in day length and temperature and results in a slow down or full cessation of energy use and growth (1). As with hibernating animals, dormancy for trees is a period of rest and an important defense against harsh winter conditions.
In Canada, the length of the growing season and therefore the period of dormancy varies regionally based on geographic and environmental conditions. For example, the growing season in the temperate Pacific Coast (141-160 days) is weeks to months longer than in the Northwestern forest of the Prairies (101-120 days) (2).
Interestingly, researchers have discovered that roots can continue to grow when underground temperatures are above 0°C. It is thought that this ability to continue to grow below ground in winter helps to protect conifers from drying out and prepares trees to surge into the warmer growing season come spring (3).
So even though the rest of the tree is dormant there is likely some minimal growth within root systems during winter, likely using glucose derived from the previous spring and summer. For a tree to truly absorb CO2 it requires photosynthesis in the leaves, and as we observe in our parks and streets, branches are more likely to be laden with snow during the period of dormancy. While there may be some storage of carbon through conversion of existing glucose reserves, new sequestration of carbon requires photosynthesis which is focused almost completely during the growing season.
The effect of this seasonal variability in carbon sequestration is not to be underestimated! It actually impacts the global atmospheric concentrations of CO2 as shown in the classic Keeling Curve. The curve clearly shows the seasonal fluctuation of the respiration of northern forests as CO2 levels increase during the northern hemisphere winter (hemisphere with the majority of land and forest cover) and decreases in spring and summer as respiration peaks owing to the photosynthesis of plants (4).
While the answer to the question of carbon sequestration in winter is not straightforward, it is indisputable that our northern trees surge into action at the first blush of spring, with immense global impact!
- Singh et al. (2016) https://www.researchgate.net/publication/311247583_Photoperiod-_and_temperature-mediated_control_of_phenology_in_trees_-_a_molecular_perspective
- Natural Resources Canada https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/forest/length_growing_season_1951_2010_3000.jpg