Sugar storage is prioritized over growth in pine trees at a dry site and explains auto-correlation in tree-ring width

Georg von Arx, Alberto Arzac, Patrick Fonti, David Frank, Roman Zweifel, Andreas Rigling, Lucia Galiano, Arthur Gessler, José Miguel Olano

Carbohydrate reserves play a crucial role in tree resistance and resilience to drought. Ample reserves may buffer temporary periods when plants reduce or stop carbon uptake to prevent excessive water loss during photosynthesis. But prolonged drought might deplete carbohydrate reserves. In addition, the allocation of absorbed carbon to carbohydrate reserves competes with growth. This poses interesting questions about carbon allocation priority during periods of limited carbon uptake. For example, the availability of older carbohydrate reserves to support future growth would alleviate trade-offs between carbohydrate reserves and growth. Larger carbohydrate reserves may require larger storage capacity, which in mature conifer trees is to a large part provided by storage cells (ray parenchyma) in the stem sapwood. However, a sound understanding of how carbohydrate reserves, parenchyma abundance and growth rates are interrelated, and how they respond to changing water availability, is still missing.

In this study, we investigated these questions using 40 mature Scots pine (Pinus sylvestris L.) trees from a 10-year irrigation experiment conducted at a dry site in Switzerland. From each tree, we extracted several wood cores from bark to pith and quantified carbohydrate reserves, parenchyma abundance and ring width along radial sapwood segments. Results show that parenchyma abundance varied more than twofold among trees, but only very little from year to year within trees. In addition, parenchyma abundance slightly increased in irrigated trees, with a lag of several years, but was unrelated to the amount of carbohydrate reserves. This means we did not find evidence for parenchyma abundance limiting carbohydrate storage. While wider tree rings contained a lower concentration of stored carbohydrates, they still contained a larger absolute amount of carbohydrate reserves. We also found that rings with more carbohydrate reserves were usually followed by a wider ring.

Our results indicate a prioritization of carbon allocation to storage instead of ring growth, which we interpret as a mechanism to ensure long-term survival. In addition, the absolute amount of carbohydrate reserves proved to be a cause of the auto-correlation in tree-ring growth from one year to the next.

Image caption: The stem sapwood, visible here as the translucent part of a wood core, is an important storage container for carbohydrates. The background shows the experimental forest stand used in this study. Photo credit: Georg von Arx. 
Read the article in full here.

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