Angela Luisa Prendin, Giai Petit, Patrick Fonti, Christian Rixen, MelissaAutumn Dawes , Georg von Arx

 Trees are long-lived organisms and continuously increase in size during life. As tree size increases, continuous structural adjustments help trees to meet changing functional needs. Wood tissue plays a critical role in these changes by providing adequate support for trees to remain standing, but also by serving as a pathway for the transport and storage of water, sugars and nutrients. In conifers, these functions are provided by tracheids (dead, empty, water-conducting cells) and parenchyma (living storage cells) in the wood. While mechanical support is guaranteed by tracheids with thick walls and narrow interiors (lumina), water transport is greatly enhanced by tracheids with wider lumina. Wider tracheids, however, are likely more vulnerable to water transport failure. Requirements for mechanical support and water transport functions might conflict as trees grow taller, especially if the resources available to form wood are limited.

To better understand how trees manage these continuously changing requirements, we studied the structural-functional patterns of wood cells from the treetop to the main roots via the stem axis in eight 40-year old larch trees (Larix decidua Miller) growing under limited resources at the treeline in the Swiss Alps. Besides the general pattern from root to treetop, we were also interested in changes throughout life and due to exposure to elevated CO2 (+200 ppm) and soil warming (+4 °C). Results indicate that the axial pattern of tracheid lumen size was the only trait that was unaffected by tree size, elevated CO2 or soil warming. This result suggests a high priority of water transport efficiency compared to other wood functions, which were more plastic. In addition, we found that wider tracheid lumen size at the treetop was linked to increased height and ring growth. This suggests that the treetop is the location where differences in water transport capacity most strongly affect tree growth.

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Image caption: Microscope image of a cross-section of a larch tree cut close to the treetop. The central part shows the pith, and the peripheral parts show the wood tissue investigated in this study. Tracheid cells in the wood appear as polygons with white interiors and dark-red stained walls, and parenchyma cells are visible as part of dark-red lines radiating from the centre. Photo credit: Angela Luisa Prendin.

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