Abstract Gap formation and closure play important roles in forest succession. Most studies focused on regeneration within gaps, but less is known about the growth dynamics of canopy trees surrounding the gaps (i.e., gap border trees), which limits our understanding of the gap-filling process. In this study, terrestrial laser scanning (TLS) was used to quantify the crown asymmetry of six canopy tree species with different wood densities among young gaps (15a), old gaps (32a), and closed forest stands (CK). The size of expanded gaps ranged from 166.8 to 408.1 m2 with an average of 271.5 m2. Each gap border tree was separated into two parts, i.e., one part facing the gap (FG) and the other facing the forest interior (FF). The ratios (FG/FF) of crown length, crown projected area, and crown volume of these two parts were calculated to represent one-, two-, and three-dimensional crown asymmetry, respectively. We found that 90% gap border trees had asymmetric crown toward gaps with an average ratio of 1.58. For trees in the closed forest stands, although crown asymmetry occurred randomly in different directions and had a large variation, the average ratio was close to 1.00. However, the gap age and the location of gap border tree (i.e., north, east, south, and west of the gap) did not show significant influences on the crown asymmetry because differences of crown asymmetry probably disappeared after the long period of gap formation. The crown asymmetry exhibited inter-specific variations and was positively correlated with wood density, which indicated that tree species with a higher wood density such as Acer mono (0.61 g cm−3) could support a more asymmetric crown and have a higher asymmetric crown threshold than others such as Juglans mandshurica (0.45 g cm−3). Crown asymmetry had similar results among different dimensions. Our findings indicated that the general crown asymmetry of gap border trees might increase snapping and uprooting risks during wind and snow disturbances. Although gap-based silviculture is widely applied in forest management, additional practices such as thinning or selective harvesting should be considered to create more space and alleviate an asymmetric crown induced by gap formation.

中文翻译：

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使用陆地激光扫描检测温带次生林自然间隙引起的冠不对称动态和变化

摘要 林隙的形成和闭合在森林演替中起着重要作用。大多数研究侧重于间隙内的再生，但对间隙周围树冠（即间隙边界树）的生长动态知之甚少，这限制了我们对间隙填充过程的理解。在这项研究中，地面激光扫描 (TLS) 用于量化具有不同木材密度的 6 种树冠树种在幼林 (15a)、老林 (32a) 和封闭林分 (CK) 之间的树冠不对称性。扩大的缝隙大小从 166.8 到 408.1 平方米不等，平均为 271.5 平方米。每个间隙边界树被分成两部分，即一部分面向间隙（FG），另一部分面向森林内部（FF）。计算这两个部分的冠长、冠投影面积和冠体积的比值（FG/FF）代表一、二维和三维冠不对称，分别。我们发现 90% 的间隙边界树具有不对称的树冠朝向间隙的平均比率为 1.58。对于封闭林分中的树木，虽然树冠不对称在不同方向随机发生并且变化很大，但平均比例接近1.00。然而，缝隙年龄和缝隙边界树的位置（即缝隙的北、东、南、西）对冠不对称没有显着影响，因为冠不对称的差异可能在缝隙形成很长时间后消失。 . 树冠不对称表现出种间变异，并与木材密度呈正相关，表明木材密度较高的树种如 Acer mono (0. 61 g cm−3) 可以支撑更不对称的冠，并且比其他如胡桃 (0.45 g cm−3) 具有更高的不对称冠阈值。冠不对称在不同维度之间具有相似的结果。我们的研究结果表明，间隙边界树的一般树冠不对称可能会在风雪干扰期间增加折断和连根拔起的风险。尽管基于间隙的造林广泛应用于森林管理，但应考虑采取额外的措施，例如间伐或选择性采伐，以创造更多空间并缓解由间隙形成引起的不对称树冠。我们的研究结果表明，在风雪干扰期间，间隙边界树的一般树冠不对称可能会增加折断和连根拔起的风险。尽管基于间隙的造林广泛应用于森林管理，但应考虑采取额外的措施，例如间伐或选择性采伐，以创造更多空间并缓解由间隙形成引起的不对称树冠。我们的研究结果表明，在风雪干扰期间，间隙边界树的一般树冠不对称可能会增加折断和连根拔起的风险。尽管基于间隙的造林广泛应用于森林管理，但应考虑采取额外的措施，例如间伐或选择性采伐，以创造更多空间并缓解由间隙形成引起的不对称树冠。