古树被认为是本树种潜在的遗传多样性资源和较高繁殖适合度的“储藏仓库”，但在这方面缺乏相关的实验论据支持。在北京这座有着悠久历史的城市，有许多的皇家园林和寺院古刹；在这些地方分布着大量的古树资源，侧柏（Platycladus orientalis）古树数量最多，很多园林中都分布着其古树种群；而古银杏（Ginkgo biloba）虽然数量不多，却是分布最规律的，基本都在寺庙里。对于这两种起源于中国的古老物种，目前尚无从不同的年龄角度对其繁殖适合度和遗传多样性变化情况的研究；而300年甚至1000年生以上的树木是否有较好的繁殖适合度，是否蕴藏着特殊的基因型和较高的遗传多样性则不得而知。本研究针对以上这些问题进行了较为深入的探讨。首先对这两种树种不同年龄段（> 1000a，300 – 999a 和 < 100a）的种群中具有繁殖特征的样株数目在取样种群中所占的比例进行了调查分析，其次对侧柏种群的种子的大小和重量进行了测量和比较，对不同年龄段的种群的繁殖能力进行初步的评价；然后用本研究新开发出来的9个侧柏微卫星位点和8个以往已有的银杏微卫星位点对这两个物种的遗传多样性和遗传变异分别进行了分析和评价。主要得到如下结果：（1）两个树种在3个年龄段的繁殖能力之间存在着差异。对3个年龄阶段的侧柏种群中具有球果的个体数占种群个体总数的比率统计结果经检验300a的古柏种群（54.18%）与100a年轻种群有极其显著的差异（27.71%）（p < 0.001）；对3个年龄段银杏具球果或者雄花序的个体数比率检验得到两个年龄组的古银杏具有球果的比率均与年轻种群有极其显著的差异（p < 0.01），具有球果或者雄花序的样株数目之和与种群平均胸径之间有一定的正相关（p = 0.076）。（2）对4个300a以上侧柏栽种古树种群和4个100a生以下年轻栽种种群的球果及种子大小和重量测量比较结果显示，虽然2个年龄段的球果种子特征之间没有显著性的差异，但种子的重量与种群平均胸径大小呈显著正相关（p < 0.05），表明年龄较大的古树种群相对于年轻种群会产出质量较高的种子，拥有较高的繁殖适合度。（3）用本研究新开发的9个高度多态的侧柏微卫星位点（POA5，POA9，POA10，POA12，POA14，POC3，POD1，PO3F12和PO4G10）对29个种群的侧柏进行遗传多样性分析，共得到252个等位基因，平均每个位点有9.310个等位基因（NA），其中有效等位基因（NE）为5.870个；多态信息指数（I）为1.722，观测杂合度（Ho）为0.695，期望杂合度（He） 为0.732，等位基因丰富度（AR）为1.807，等位基因长度变化范围为37.533。在这6个遗传多样性参数中，栽种古树种群与自然种群间均无显著性差异（p > 0.05），有的古树种群甚至高于自然种群。在侧柏栽种种群中，NE、AR、I和等位基因长度变化范围与种群的平均胸径之间有着显著的正相关关系（p < 0.05），观测杂合度与平均胸径之间存在着极其显著的正相关关系（p < 0.01）。（4）用9个微卫星位点比较了侧柏栽种古树种群、年轻种群和自然古树种群、年轻种群的遗传结构，4个群组的Fst 在0.025 ~ 0.092之间；表明这些侧柏种群之间都是中度的遗传分化。AMOVA分析得出遗传变异主要来自于种群内部（93.383%）。对侧柏29个种群的UPGMA聚类分析结果显示一些地理位置相近的种群能够聚类，但也有相反的情况。用8个微卫星位点对古银杏遗传结构分析表明，观测杂合度和期望杂合度达0.745和0.892；AMOVA分析得出16.6%的遗传变异来自于银杏单株之间，属较高程度的遗传分化；对42个银杏单株进行UPGMA聚类分析得到与侧柏类似的结果，说明这些古银杏来源广泛。基于聚类结果，并结合种群遗传多样性和繁殖能力，可做为不同种群的保护重要程度进行排序的依据。 本研究的这些结果表明古银杏相对与年轻银杏种群具有更强的结实能力；年龄较大的栽种侧柏古树种群不但有更高的繁殖适合度，且包含着丰富的遗传信息，拥有高度的遗传多样性。这些栽种的古柏种群生长状态优于自然古柏且基本能够代表侧柏物种的遗传多样性，而承载这些古树的园林和寺庙在维持两物种较高的繁殖适合度和遗传多样性水平方面意义重大。

Old-growth tree populations are assumed to be potential resources of genetic diversity and higher reproductive fitness of the tree species, however, there is little relevant research and empirical evidence for this assumption. A great number of imperial parks and ancient temples were established in Beijing, an old city with quite a long history as Capital of China, and these places are rich in reserves of old-growth populations of Platycladus orientalis and quite a number of ancient Ginkgo biloba, two tree species which have both originated in China and can live for centuries. There are few studies about the possibility of changes in genetic diversity along with age scales of these two species; and whether the aged tree populations have a higher reproductive fitness, more private and rare alleles or can serve as resources of genetic diversity is not investigated. In this study, we recorded and compared the proportion of individuals with reproductive traits (cones or male inflorescence) in each population of different age groups (exceeds 1000 years, 300 – 999 years and younger than 100 years) of both the tree species; we measured cone and seed traits including length, width thickness and weight of Platycladus orientalis to assess the reproduction ability of different age groups; and after the microsatellite marker development of Platycladus orientalis, we investigated the genetic diversity and variation of both the tree species using microsatellite markers.(1) There were differences in reproductive ability between the three age groups. In 300 – 999a age group of Platycladus orientalis, the proportion of individuals with cones (54.18%) were significantly higher than 100a age group (27.71%) (p < 0.001). Percentage of individuals of Ginkgo biloba with cones in both aged groups were obviously higher than that in young populations (p < 0.01); and the relationship between the average value of DBH (diameter at breast height) and proportion of individuals with reproductive traits (with cones or male inflorescence) was positive (p = 0.076, marginally significant).(2) The comparison of cone and seed traits between two age groups, four artificial populations of age 300 - 999a and four artificial populations of young Platycladus orientalis, showed that the seed weight was significantly positively correlated to the average value of DBH of the population (p < 0.05). This result indicated that older population of Platycladus orientalis could produce seeds of better quality than young populations.(3) Using the nine highy polymorphic microsatellite loci (POA5，POA9，POA10，POA12，POA14，POC3，POD1，PO3F12 and PO4G10) of Platycladus orientalis developed here, we investigated the genetic diversity and variation of 29 populations of this species. There were 252 alleles in total, and the average number of alleles (NA), effective number of alleles (NE), Shannon’s information index (I), Observed heterozygotes (Ho), expected heterozygotes (He), allele richness (AR) and allele range were 9.310, 5.870, 1.722, 0.695, 0.732, 1.087 and 37.533, respectively. There was no significant difference between artificial ancient tree populations and natural populations in all these six genetic diversity parameters (p > 0.05), and some of the artificial old-growth tree populations were even higher than natural populations in these parameters. In artificial populations, genetic diversity parameters such as NE, AR, I and allele range was obviously increased with the average value of DBH (p < 0.05). In addition, Ho was strongly correlated to DBH (p < 0.01).(4) Using these 9 microsatellite loci we examined the genetic variation of four types of groups of Platycladus orientalis populations, including artificial old-growth population and young population, and the natural old-growth population and young population. Genetic differentiation of these populations in the four groups were moderate (Fst ranged from 0.025 – 0.092), and result of AMOVA (analysis of molecular variance) revealed that most of the genetic variation was within population. UPGMA cluster analysis showed that some populations that were sampled closer in distance were clustered together, but there were also some reverse results. With eight microsatellite loci we investigated genetic variation of the 42 sampled individuals of ancient Ginkgo biloba, which was rich in genetic diversity (Ho = 0.745; He = 0.892). AMOVA showed that 16.6% of the variation was between the individuals, indicating that ancient Ginkgo biloba in Beijing was relatively high in genetic composition. UPGMA cluster analysis of the 42 individuals was similar to the result of Platycladus orientalis. Using the cluster results and the information of genetic diversity and reproductive ability of these populations or individuals, we could point out which was more important and must be adequately protected. The results of this study showed that: the ancient Ginkgo biloba was better at reproduction ability than young populations; Old-growth artificial populations of Platycladus orientalis had high reproduction fitness and genetic diversity, and the individual growth condition of artificial old-growth populations was better than that of natural old-growth populations. They could mostly represent the whole genetic diversity and variation of this species; thus the imperial parks and ancient temples, which protected these old-growth trees for centuries, played an important role in maintaining the genetic diversity and reproductive fitness of the tree species.