Tree-ring data is due to its accurate dating, high climate sensitivity and wide distribution (Fritts 1976; Hughes et al. 2011) the most widely used high-resolution proxy for the reconstruction of past-millennium climate variables, such as air temperature (Neukom et al. 2019; PAGES 2k Consortium 2013; Stocker et al. 2013; Zhang et al. 2020b), hydroclimate (Cook et al. 2015; Fang et al. 2018; Ljungqvist et al. 2016) and internal climate variability (Fang et al. 2019; Wang et al. 2017). These reconstructions provide a long-term context to evaluate current climate change. Thus, it is vital that the tree-ring data used faithfully reflects the annual growth responses of trees to climate.
When building a chronology, which is the base for further reconstructions, all ring widths from trees growing in a particularly year are averaged with the aim of reducing any growth disturbances from individual trees and strengthening climate signals (Cook and Kairiukstis 1990; Hughes et al. 2011). This method is based on the assumption that the annual increments (the tree rings) of a tree are concentric, and that any point around the circumference represents these equally well. In fact, trees often show asymmetric growth in different parts of the trunk (Harmon 1942). For example, in response to force of gravity, tree rings in coniferous (broadleaf) species tend to be wider at the downslope (upslope) side than the opposite in order to keep the trees as vertical as possible, known as compression (tension) wood (Fritts 1976). Similarly, persistent, intense wind stress can also cause asymmetric growth of the trees, resulting in compression wood on the leeward side for coniferous trees (Wade and Hewson 1979). Accordingly, previous studies have employed the asymmetric variations of tree-ring widths to reconstruct wind speed changes (Fang et al. 2021; Tomczak et al. 2020; Wade and Hewson 1979). However, existing studies have lacked an accurate mathematical description of the asymmetric feature of tree derived from tree-ring cores (Bakker 2005; Biondi 2020). Here we provide a novel method to quantify the asymmetric growth of tree rings and investigate its relationships with wind speed changes.
Trees strive to maintain a circular shape even under gravity or wind stress, because a circle maximizes the cross area for a given perimeter according to the Isoperimetric Inequality (Osserman 1978). In addition, a cylinder-shaped trunk has the largest capacity, in comparison with other shapes, to maintain the upright position of a tree (Dugas 2011). Thus, we hypothesize that trees tend to shift their tree-ring geometric center (TRGC) to have asymmetric growth in response to mechanical forcings. That is, the radial growth of trees can, in addition to ring-widths, be expressed by shifts in the TRGC. To assess this feature, we developed a method to detect the shifts in the TRGC, which was employed on wind stressed trees in Huanggangliang, northeast China. We assume that TRGC shift is sensitive better proxy for wind stress than previously used growth differences at the windward and leeward sides of trees (Fang et al. 2021; Tomczak et al. 2020; Wade and Hewson 1979). In addition, we used to the TRGC shifts to make improved calculation of the tree basal area increment (BAI), which were compared with the traditional tree-ring-based BAI method (Biondi and Qeadan 2008; Monserud and Sterba 1996).