Axial xylem architecture ofLarix deciduaexposed to CO₂enrichment and soil warming at the tree line

Abstract

Trees continuously adjust their axial xylem structure to meet changing needs imposed by ontogenetic and environmental changes. These axial structure–function responses need to be coordinated among competing biophysical constraints to avoid failure of the xylem system. Here, we investigated if ontogeny or experimental manipulation of CO2 and soil temperature influence these structure–function responses. We performed detailed xylem cell anatomical quantification along the axis of 40‐year‐old Larix decidua trees planted at the Swiss tree line and exposed to a combination of elevated CO2 (+200 ppm) and soil warming (+4°C) between 2001 and 2012. We assessed how mean hydraulic tracheid diameter (Dh), the cell wall reinforcement ((t/b)2), tracheid wall thickness (CWT) and the percent area of ray parenchyma (PERPAR)—proxies for hydraulic efficiency, hydraulic safety, biomechanical support and metabolic xylem functions, respectively—covary along the tree axis. Dh increased from the stem apex to base, strictly following a power function (R2=0.81), independent from ontogeny and experimental treatments. In contrast, axial trends of (t/b)2 and CWT were either influenced by treatment and/or ontogeny, or showed no axial trend (PERPAR). Additionally, we found that a larger Dh only at the stem apex promoted primary and secondary growth. Our approach of analysing xylem anatomical traits along the tree axis and across tree rings provides novel insights into xylem functional architecture and allows reconstructing xylem function over time. We conclude that the maintenance of hydraulic efficiency during ontogeny is very robust, that the tracheid diameter undergoes a strong apical control, and plays a fundamental role for assimilation and tree growth. Instead, the other functional traits more plastically vary with ontogeny and environmental changes. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12986/suppinfo is available for this article.

Weak Apical Control of Swiss Stone Pine (Pinus cembra L.) May Serve as a Protection against Environmental Stress above Treeline in the Central European Alps

“…Hence, findings of this study suggest that upward bending of basal branches, i.e., a weak apical control in the sapling stage, might be an adaptation to extreme environmental conditions prevailing in the alpine treeline ecotone in cool temperate and boreal mountains, especially during winter. Our interpretation is corroborated by strong apical control found in co-occurring European larch (L. decidua [33]), which, due to its deciduous behavior, is less prone to winter drought and snow mold.…”

Long-Term Impacts of Defoliator Outbreaks on Larch Xylem Structure and Tree-Ring Biomass

“…The theoretical tree-ring hydraulic conductivity (Kh r ) was assessed as the sum of Kh c of the average radial file (Figure 1). The ratio between Kh r and RWA provided the hydraulic carbon use efficiency (HCUE; Prendin et al, 2018).…”

“…Summing the cell wall area (Fonti et al, 2013) of all tracheids provides an estimate of wood material in the ring (tree-ring biomass) (Björklund et al, 2019). Finally, the 'hydraulic carbon use efficiency' expresses the (yearly) theoretical tree-ring hydraulic conductivity for a given carbon investment (Prendin et al, 2018).…”

Age-Effect on Intra-Annual δ13C-Variability within Scots Pine Tree-Rings from Central Siberia

“…Moreover, measurements performed on trees representing different levels of intrinsic growth rates in several conifers in Alpine, Temperate, and Boreal Europe indicated that the effect of summer drought in fast-growing trees does not occur in slow-growing ones [20]. In contrast, there are some characteristics of the wood structure, such as those related to hydraulic efficiency (via the tracheid lumen diameter), that are much more constrained over age and size changes, since they play a fundamental role in assimilation and growth [29].…”