Dendrochronological methods. For each sampled Picea koraiensis Nakai tree, cores were taken from four directions, west, east, northwest and southeast, in a horizontal cross-section at breast height. The sampling directions were designed to track the main influences of wind stress from the westerlies in the study area. Cores that did not reach the pith or with injuries or branches were excluded, and in total, we obtained 182 cores from 38 trees. The tree-ring cores were air-dried, mounted and polished25. The exact calendar year was assigned to each tree ring using the cross-dating procedure26, and the dated cores were then measured with a 0.001 mm resolution. Most raw tree-ring measurements generally showed an upward growth trends, since the trees were still in their juvenile period (herein, the mean length of the trees is 23 years) where growth is generally increasing1. Thus, we did not remove the age-related growth trends in the raw measurements by fitting negative exponential curves to generate a standard chronology27, which is a common procedure for longer-lived trees. Instead, we generated a tree-ring chronology from the robust mean of the raw measurements27.
TRGC detection and improved BAI methods. We herein propose a novel method to quantify the annual variations of the distances andorientations of the TRGCin a stem cross section with at least three cores reaching the pith with known cross-angles between them. Assuming that N cores, sampled through the method specified in Section 2.2, intersect at the tree pith O,we can define θi,j(i, j=1,2,…,N) as the cross angle between the i-th and j-th cores, and ρj(t) denoting the distance between O and the tree ring of a given year ton the j-th core. Then [ρj(t), θ1,j] is the coordinate of the tree ring in a polar coordinate system where to the first core is referred to as the polar axis and O as the pole.rj(t):=[xj(t),yj(t)]:=[ρj(t)cos(θ1,j), ρj(t)sin(θ1,j)] defines the coordinates in a Cartesian coordinate system. When N≥3, xj(t) andyj(t) can be used for determining parameters of the circle equation (xj- at)2 + (yj - bt)2 = Rt2 through a least-square fit. The fitted at and btare the TRGC coordinates in the Cartesian system and Rtis the radius. Further, we calculated the where t-1denotes the year before t.
Compared to traditional BAI estimation, which entail the assumption of concentric annual tree-ring increments22,23, our BAI estimation takes the TRGC shift into account and should therefore be more reliable. The reliable portions of the tree-ring chronologies, the mean of the series of the TRGC and the BAI series, were those with sufficient sample size, here determined when the sub-sample signal strength (SSS) statistics reached above 0.8028. The program for quantifying the TRGC is shown in the supplementary materials. Similar to ring-widths, BAI series generally show strong increasing trends in tree juvenile periods22,23. Accordingly, we removed the linearly increasing BAI trend in the juvenile period.
Study Region And Data Availability
Study region. The study site was located on the Huanggangliang Mountain (43.5 °N; 117.5 °E; 1,935 m a.s.l.), in Chifeng City jurisdiction in Inner Mongolia, northeastern China (Figure 6a). Huanggangliang Mountain is a peak in the Greater Khingan Range, which extends 1,200 kilometers from north to south in northeastern China, dividing the Inner Mongolia Plateau and the Northeast Plain of China. The mountain is located on the boundary between the Asian summer monsoon area and the forest-steppe ecotone29, with peak air temperatures and precipitation in summer16. The westerly winds dominate the study area, with an annual mean wind speed of ~3.8m s−1, making it ideal for detecting impacts of wind stress on trees. The sampling site was situated on a mountain top with flat topography on which an open Picea koraiensis Nakai forest grow (Figure 6b). Samples were taken from the western boundaries of the forests, where the trees were assumed to be strongly impacted by winds (Figure 6c).