在宁夏六盘山区龙潭林区华北落叶松(Larix principis-rupprechtii )人工林的不同样带, 按6个密度梯度(3.24、2.56、1.96、1.44、1.00和0.64株·m^-2)移栽萌发生根的辽东栎(Quercus wutaishanica)幼苗, 研究了啮齿动物对幼苗子叶取食的密度效应及其对幼苗存活与生长的影响。结果表明, 辽东栎幼苗的子叶留存率随着幼苗密度的增大而减小。2个高密度处理(3.24和2.56株·m^-2)的幼苗子叶留存率分别在移栽6周和5周后稳定在最低水平, 分别为8.64%和7.81%; 而2个较低密度处理(1.44和0.64株·m^-2)的幼苗在最后一次观测时子叶留存率仍很高, 分别为44.44%和31.25%, 且二者均显著高于其他密度处理(p < 0.05)。受啮齿动物取食的影响, 在从高到低的不同密度处理下, 幼苗的主根留存率分别为64.16%、80.46%、87.23%、80.25%、69.84%和69.84%; 在1.44株·m^-2处理下, 顶芽留存率最高(25.23%), 显著大于3.24株·m^-2处理(4.19%) (p < 0.05), 其他密度处理的顶芽留存率均不足20%。子叶被动物取食的幼苗和子叶完整幼苗的最终留存率基本一致, 仅在个别密度处理的某一观测时期差异显著; 顶芽被动物取食的幼苗留存率略低于顶芽完整幼苗的留存率, 但个别密度处理完全相反; 不同类型幼苗的留存率随时间推移而上下波动, 可能与幼苗顶芽被动物取食后萌生形成新芽有关。幼苗的株高、基径、叶片数和单株叶面积均随密度的减小而略有增大, 但除株高外, 其他3个生长参数均在1.44株·m^-2处理时最大, 与子叶是否被动物取食有一定的对应关系, 即高密度下更多的幼苗丢失了供应其生长所需营养物质的子叶, 从而影响了幼苗的早期生长。研究结果不仅丰富了“负密度效应”理论和种群更新的“增补限制”理论, 也可为六盘山区退化辽东栎灌丛和辽东栎次生林的恢复及生态系统管理提供 参考。

Aims Our objective are to explore cotyledon predation by rodents and its effects on survival and growth of Quercus wutaishanica seedlings under different densities, and to disclose the potential bottlenecks limiting seedling recruitment and natural regeneration of populations.
Methods We transplanted Q. wutaishanica seedlings at six densities of 3.24, 2.56, 1.96, 1.44, 1.00 and 0.64 individuals·m^-2 in a Larix principis-rupprechtii plantation of China’s Dadaogou forest region, Liupan Mountains. We surveyed retention rate of seedling cotyledon (RRC), retention rates of taproot (RRT) and apical bud (RRA) after cotyledon predation by rodents and retention rates of cotyledon- and apical bud-predated (RRCP, RRAP) and cotyledon- and apical bud-remained seedlings (RRCR, RRAR). At the end of the experiments, we harvested all surviving seedlings and determined seedling height (SH), basal stem diameter (BSD), leaf number (LN), taproot length, leaf area per seedling (LAPS), total dry mass, root shoot ratio and specific leaf area.
Important findings Density effect of cotyledon predation on Q. wutaishanica seedlings was observed and the RRC decreased with increase of seedling density. Six and five weeks after transplanting, the RRCs of two higher densities (3.24 and 2.56 individual·m^-2) stabilized at their minimums (8.64% and 7.81%), while those of two lower densities (1.44 and 0.64 individual·m^-2) were still higher at the last cotyledon survey and were signifycantly higher than those of all other density treatments (p < 0.05). The RRA was the highest (25.23%) in 1.44 individual·m^-2density and was higher (4.19%) than that in 3.24 individual·m^-2 density (p < 0.05), and all other densities resulted in < 20% of RRA. Significant difference between RRCP and RRCR was detected only on certain survey dates of individual density. RRAP was slightly lower than RRAR, and individual density exhibited the inverse. The fluctuation of retention rate of different seedling types may have be resulted from new germinated sprouts after apical buds were gnawed. SH, BSD, LN and LAPS all increased slightly with density decrease. The maxima of all parameters except SH were detected under the density of 1.44 individual·m^-2and, to some extent, were related to whether cotyledon was predated by rodents or not, indicating that more seedlings grown at higher density lost their cotyledons and thus exerted influence on seedling growth.