The rise and fall of carbohydrates in the trees of the north

Thorhild County lies north of Edmonton, Canada, at the southern edge of Alberta’s boreal dry mixedwood forest region. It’s a place that swings in daylight, from seven and a half hours at the darkest to seventeen hours in the summer. The temperature varies too. The leaves that power a tree in the summer either have to be protected or lost in the winter, when the temperature can fall below -20 C. After the snows of winter depart, it’s time for the tree to grow again. But where does it find the energy? Carbon reserves are critical sources of energy that allow trees to cope with high carbon demands. Coral Fermaniuk and colleagues set out to track the seasonal changes in whole-tree and organ-level carbon reserves in mature boreal Betula papyrifera trees.

B. papyrifera, or paper birch, is a fast-growing deciduous tree found in the northernmost states of the USA and Canada. It’s an important winter food for moose, but it provides food for many other animals in other seasons. In the autumn, white-tailed deer eat the leave. Grouse eat the buds. Chickadees eat the seeds. Other animals go deeper, with the beaver eating the inner bark and the yellow-bellied sapsucker drilling holes to get at the sap.

The sap and bark are of particular interest, as they store non-structural carbohydrates, usually abbreviated to NSCs. When times are good, sugars formed through photosynthesis are transported through the sap and deposited as starch in roots or stems. It’s in these NSCs that a tree can store energy for regrowth. 

To find out how trees manage their stores of non-structural carbohydrates, Fermaniuk and colleagues measured the concentrations of soluble sugars and starch in ten trees for a year. The team measured the concentrations of non-structural carbohydrates, taking samples from fine roots, coarse roots, inner bark, stemwood, branches, twigs and leaves & buds.

The team also measured fine root and stem growth to see how the timing of growth correlated, or not, with changes in the carbon reserves.

As expected, the trees hit a minimum of reserves in spring. The maximum occurred at the end of summer with bud set and the end of long shoot growth. What surprised the authors was the scale of the difference. “[T]he magnitude of the fluctuation in NSC pool size over the growing season was truly remarkable. From their minima, the mass of non-structural carbohydrates in the study’s trees increased by over 72% throughout the growing season, greatly exceeding the seasonal NSC fluctuation observed in B. papyrifera from a temperate environment (ca. 28% increase; Furze et al., 2019),” write Fermaniuk and colleagues.

The biggest changes in carbon stores were found in the branches of the trees, note the authors. “Considering that the minimum branch NSC pool size occurred shortly after leaf out, we can, at least in part, attribute this fluctuation in NSC reserves to be associated with the remobilization of branch reserves to fuel the flushing and expansion of new leaves and shoots.”

As winter came, the trees converted sugars to starches, with branches holding around half of the carbohydrate stores. The botanists argue that the reason for this may be to build protection against the cold. The cells in the branch phloem need to be kept alive, but the bark is thinner in the branches, giving less insulation. In addition, the branches are exposed to the winds, giving more cooling.

Quite how B. papyrifera in this study has developed these larger swings of carbon accumulation is not certain. The trees in Alberta may be adapted to the harsher climate or honed by natural selection at a local scale.

The authors state this is the first study that comprehensively estimates whole-tree and organ level non-structural pool size dynamics in relation to the phenology of mature boreal B. papyrifera. Their research indicates that comparing more trees across different climates may produce some interesting results.