Wood Formation Modeling – A Research Review and Future Perspectives

Abstract

Wood formation has received considerable attention across various research fields as a key process to model. Historical and contemporary models of wood formation from various disciplines have encapsulated hypotheses such as the influence of external (e.g., climatic) or internal (e.g., hormonal) factors on the successive stages of wood cell differentiation. This review covers 17 wood formation models from three different disciplines, the earliest from 1968 and the latest from 2020. The described processes, as well as their external and internal drivers and their level of complexity, are discussed. This work is the first systematic cataloging, characterization, and process-focused review of wood formation models. Remaining open questions concerning wood formation processes are identified, and relate to: (1) the extent of hormonal influence on the final tree ring structure; (2) the mechanism underlying the transition from earlywood to latewood in extratropical regions; and (3) the extent to which carbon plays a role as “active” driver or “passive” substrate for growth. We conclude by arguing that wood formation models remain to be fully exploited, with the potential to contribute to studies concerning individual tree carbon sequestration-storage dynamics and regional to global carbon sequestration dynamics in terrestrial vegetation models.

Ecological and methodological drivers of non‐stationarity in tree growth response to climate

“…From a methodological point of view, we endorse the use of monthly non‐linear process‐based models as we identified systematically lower non‐stationarity of these models compared with linear approaches with daily temporal resolution. In doing so, we strongly support earlier calls of other scientists for using more complex but widely applicable climate–growth models that might better reflect ecological reality (Eckes‐Shephard et al, 2022).…”

“…Non‐linear process‐based models are capable of reducing non‐stationarity by reflecting abrupt thresholds in the response of wood formation to environmental variability (Rossi et al, 2008). The higher stationarity we found for non‐linear models argues for the use of non‐linear process‐based models as the preferred tool for improving forecasts of forest ecosystem dynamics under global change (Eckes‐Shephard et al, 2022).…”

“…Both the VS‐Orig and VS‐Lite non‐linear models belong to the category of climate‐driven models (Eckes‐Shephard et al, 2022), that is, they only require climatic data and observed tree‐ring width site chronologies for calibration. Although we showed their significant stationarity, we are aware of the limitations that restrict their application to large data sets.…”

“…Consequently, some studies reported that non‐stationarity might be amplified by using unrealistic linear models to approximate the non‐linear response of tree‐ring width to climate (Ljungqvist, Thejll, et al, 2020; Wilmking et al, 2020; Wolkovich et al, 2021). In contrast to linear climate–growth correlations, non‐linear process‐based models of wood formation represent an approximation of current knowledge of the mechanisms driving wood formation at fine temporal scales (Eckes‐Shephard et al, 2022; Guiot et al, 2014). Many process‐based models provide highly reliable simulations exhibiting high coherence with observational data sets (Cabon et al, 2020; Drew et al, 2010; Gennaretti et al, 2017; Vaganov et al, 2006).…”

Epigenetics at the crossroads of secondary growth regulation

“…Secondary xylem differentiation (xylogenesis) follows four steps: (i) cambium cell division with inward daughter cells enlargement and shape modification, (ii) secondary cell wall formation with deposition of cellulose, hemicellulose, and lignin, and (iii) developmentally induced programmed cell death (dPCD) whereby mature xylem cells turn into tracheary elements (Rathgeber et al, 2016). Over time, different molecular models of wood formation from the vascular cambium were proposed and have been extensively reviewed (Eckes-Shephard et al, 2022), but its epigenetic regulation is yet to be explored.…”