Abstract
In extratropical ecosystems, the growth of trees is cyclic, producing tree rings composed of large-lumen and thin-walled cells (earlywood) alternating with narrow-lumen and thick-walled cells (latewood). So far, the physiology behind wood formation processes and the associated kinetics has rarely been considered to explain this pattern. We developed a process-based mechanistic model that simulates the development of conifer tracheids, explicitly considering the processes of cell enlargement and the deposition and lignification of cell walls. The model assumes that (1) wall deposition gradually slows down cell enlargement and (2) the deposition of cellulose and lignin is regulated by the availability of soluble sugars. The model reliably reproduces the anatomical traits and kinetics of the tracheids of four conifer species. At the beginning of the growing season, low sugar availability in the cambium results in slow wall deposition that allows for a longer enlargement time; thus, large cells with thin walls (i.e., earlywood) are produced. In late summer and early autumn, high sugar availability produces narrower cells having thick cell walls (i.e., latewood). This modeling framework provides a mechanistic link between plant ecophysiology and wood phenology and significantly contributes to understanding the role of sugar availability during xylogenesis.
Highlights
Current research on the dynamics of tree-ring formation in conifers has provided new insights into how the rate and duration of growth processes control the size of the xylem conduits both across the tree-ring width (Cuny et al, 2013, 2014; Balducci et al, 2016) and along the stem hydraulic pathway (Anfodillo et al, 2012)
This paper presents a simulation model of tracheid development that assumes changes in tracheid anatomy from earlywood to latewood are coupled to the end of primary growth, when new shoots become sources of assimilates (Richardson and Dinwoodie, 1960; Gordon and Larson, 1968)
The simulated tree-ring patterns were in good agreement with data collected from the four conifer species (Figure 3), supporting a causal link between changes in sugar availability and wood cell traits (Figure 5A)
Summary
Current research on the dynamics of tree-ring formation in conifers has provided new insights into how the rate and duration of growth processes (i.e., xylem-cell production, enlargement and wall formation) control the size of the xylem conduits both across the tree-ring width (Cuny et al, 2013, 2014; Balducci et al, 2016) and along the stem hydraulic pathway (Anfodillo et al, 2012). These processes define the anatomical structure of the tree ring and determine the efficiency and safety of water transport, the storage capacity of water and reserves, and the mechanical resistance of the stem (Chave et al, 2009; Fonti et al, 2010). A growing cell requires water, free sugars and amino acids to build and maintain the turgor pressure for its expansion (Koch, 2004; Pantin et al, 2012; Deslauriers et al, 2014; Steppe et al, 2015), sucrose to build up secondary cell wall (Uggla et al, 2001; Simard et al, 2013; Deslauriers et al, 2016) and time to complete the maturation processes (Deslauriers et al, 2003; Schiestl-Aalto et al, 2015)
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