Table_1_Long-Term Effects of Climate and Competition on Radial Growth, Recovery, and Resistance in Mongolian Pines.docx

Understanding the response of tree growth and drought vulnerability to climate and competition is critical for managing plantation forests. We analyzed the growth of Mongolian pines in six forests planted by the Three-North Shelter Forest Program with tree-ring data and stand structures. A retroactive reconstruction method was used to depict the growth-competition relationships of Mongolian pines during the growth period and their climatic responses under different competition levels. Drought vulnerability was analyzed by measuring the basal area increment (BAI) of different competition indices (CIs). In young trees, differences in BAIs in stands with different CIs were not statistically significant. After 15–20 years, medium- and high-CI stands had significantly lower tree-ring widths (TWs) and BAIs than the low-CI stands (p < 0.05). The standardized precipitation evapotranspiration index (SPEI), precipitation, relative humidity, and vapor pressure deficit were major factors affecting tree growth. On a regional scale, climate outweighed competition in determining radial growth. The relative contribution of climatic factors increased with the gap in SPEI between plantation sites and the native range, while the reverse pattern of the competition-growth relationship was observed. Drought reduced TWs and BAIs at all sites. Stands of different CIs exhibited similar resistance, but, compared with low-CI stands, high- and medium-CI stands had significantly lower recovery, resilience, and relative resilience, indicating they were more susceptible to drought stresses. Modeled CI was significantly negatively related to resistance, resilience, and relative resilience, indicating a density-dependence of tree response to drought. After exposure to multiple sequential drought events, the relative resilience of high-CI stands decreased to almost zero; this failure to fully recover to pre-drought growth rates suggests increased mortality in the future. In contrast, low-CI stands are more likely to survive in hotter, more arid climates. These results provide a better understanding of the roles of competition and climate on the growth of Mongolian pines and offer a new perspective for investigating the density-dependent recovery and resilience of these forests.

深入理解树木生长及干旱脆弱性对气候与竞争的响应，对于管理人工林至关重要。本研究通过树轮数据与林分结构，分析了由三北防护林工程种植的六处林分中蒙古栎的生长状况。采用逆向重建法，描绘了蒙古栎在生长期间的生长-竞争关系及其在不同竞争水平下的气候响应。通过测量不同竞争指数（CI）的基面积增量（BAI），分析了干旱脆弱性。在幼树阶段，不同CI林分的BAI差异并无统计学意义。经过15至20年后，中高CI林分的树轮宽度（TWs）和BAI显著低于低CI林分（p < 0.05）。标准化降水量蒸散指数（SPEI）、降水量、相对湿度和水汽压亏缺是影响树木生长的主要因素。在区域尺度上，气候因素在决定径向生长方面超越了竞争作用。随着种植地与原生地之间SPEI差异的增加，气候因素的相对贡献也随之提高，而竞争-生长关系的模式则呈现相反趋势。干旱导致所有地点的TWs和BAI均下降。不同CI林分的抗性表现相似，但与低CI林分相比，高CI和中CI林分的恢复力、弹性和相对弹性显著降低，表明它们对干旱胁迫更为敏感。模型化的CI与抗性、弹性和相对弹性呈显著负相关，表明树木对干旱的反应存在密度依赖性。在经历多次连续干旱事件后，高CI林分的相对弹性降至几乎为零；这种未能完全恢复至干旱前生长速率的现象暗示未来死亡率可能增加。相反，低CI林分更有可能在更热、更干旱的气候中生存。这些研究结果深化了对竞争与气候对蒙古栎生长作用的理解，并为调查这些森林的密度依赖性恢复和弹性提供了新的视角。