Tree canopy accession strategy changes along the latitudinal gradient of temperate Northeast Asia

Aim: Understanding how natural forest disturbances control tree regeneration is key to predict the consequences of globally accelerating forest diebacks on carbon stocks and forest biodiversity. Tropical cyclones (TCs) are important drivers of forest dynamics in Eastern Asia and it is predicted that their importance will increase. However, little is known about TC impact on forest regeneration.

Location: Latitudinal gradient from south Korea (33°N) to the Russian Far East (45°N).

Time period: Last 300 years.

Major taxa studied: Quercus mongolica, Abies nephrolepis and Pinus koraiensis.

Methods: We explore the effects of TC activity on canopy accession strategies derived from long-term tree radial growth patterns along a 1500-km latitudinal gradient of decreasing TC activity. We analyzed canopy accession strategies for more than 800 trees of three widely distributed tree species by dividing them into gap trees (GTs) that established immediately after gap formation, and released trees (RTs) that accessed the upper canopy after a period of competitive suppression.

Results: We found a substantial decrease in GTs and increase in RTs proportionally along the gradient of decreasing TC activity. P. koraiensis and A. nephrolepis exhibited high variability in the proportions of the individual canopy accession strategies along the latitudinal gradient, while it was more stable for Q. mongolica. We identified the gradient of TC activity as the main driver influencing canopy dynamics and thus changes in life history traits for P. koraiensis and Q. mongolica, while maximal growth rate was the main driver for A. nephrolepis.

Main conclusions: Flexibility in growth strategies enabled the studied species to cover extensive areas and indicates that they will be able to cope with shifts in disturbance regimes induced by the poleward migration of TCs and increasing TC intensity. Our results highlight the canopy accession strategy as an ecological indicator of past disturbance activity.

Methods
We collected core samples from all stems (>10 cm DBH) within permanent research plots at a height of 0.5-1 meter above the ground surface using a steel increment borer during the 2004-2016 period. The cores were dried and surfaces were prepared with a core microtome to increase visibility of the tree-ring structure. The widths of tree rings were measured with the TimeTable measuring device and PAST5 software, with a resolution of 0.01 mm. Cores were visually cross-dated and consequently statistically verified by the percentage of parallel variation (p < 0.05, Gleichläufigkeit) and the similarity of radial growth patterns between the individual series (Baillie-Pilcher’s t-value).

研究目标：明晰天然林干扰如何调控树木更新，是预测全球加速的森林衰退对碳储量及森林生物多样性造成影响的核心前提。热带气旋（Tropical Cyclones, TCs）是东亚森林动态的重要驱动因子，且其重要性预计将进一步提升，但目前学界对热带气旋如何影响森林更新仍知之甚少。

研究区域：沿纬度梯度分布，从韩国南部（33°N）延伸至俄罗斯远东地区（45°N）。

时间跨度：过去300年。

研究类群：蒙古栎（Quercus mongolica）、臭冷杉（Abies nephrolepis）与红松（Pinus koraiensis）。

研究方法：我们沿一条热带气旋活动强度递减的1500公里纬度梯度，基于长期树木径向生长模式，探究热带气旋活动对林冠进入策略的影响。我们针对3种广泛分布树种的800余株个体开展林冠进入策略分析，将其划分为林隙更新树（Gap Trees, GTs，即林隙形成后即刻定植的个体）与释放树（Released Trees, RTs，即经历一段竞争抑制期后才进入上层林冠的个体）两类。

研究结果：随着热带气旋活动强度梯度降低，林隙更新树的占比显著下降，而释放树的占比则相应上升。红松与臭冷杉的林冠进入策略占比沿纬度梯度呈现较高变异性，蒙古栎则相对稳定。我们明确，热带气旋活动梯度是影响红松与蒙古栎林冠动态及生活史性状变化的核心驱动因子，而最大生长速率则是调控臭冷杉的主要驱动因素。

主要结论：本研究涉及的树种凭借生长策略的灵活性得以广泛分布，这表明它们能够应对由热带气旋向极地方向迁移及强度增强所引发的干扰格局变化。我们的研究结果证实，林冠进入策略可作为反映过去干扰活动的生态学指标。

详细研究方法：
2004-2016年间，我们使用钢制树木生长锥，在永久研究样地内采集所有胸径（Diameter at Breast Height, DBH）>10 cm的植株的树芯样本，采样高度为地面以上0.5-1米。采集的树芯经干燥后，使用树芯切片机打磨表面，以提升树轮结构的辨识度。我们采用TimeTable测量装置与PAST5软件测量树轮宽度，测量分辨率达0.01 mm。树芯样本首先通过目视进行交叉定年，随后通过两项统计指标进行验证：平行变异百分比（p < 0.05，Gleichläufigkeit）与各序列间径向生长模式的相似度（贝利-皮尔彻t值，Baillie-Pilcher’s t-value）。