Fitness of sexual reproduction of Toona ciliata var. pubescens natural populations and their sexual reproduction and regeneration

Abstract

To examine the reproduction fitness coefficients and individual-level fitness of Toona ciliata var. pubescens, their sexual reproduction and natural regeneration were investigated during 2006-2016, with four natural populations in Jiulianshan National Reserve as test objects. The results showed that there were only 2-10 trees for the natural populations of T. ciliata var. pubescens with a small initial number of fruiting plants (3-9 trees), which were from the initial fruiting plants or their first/second generation. The sexual reproduction of these isolated populations were significantly different, and their seed production capacities tended to decline over time. With the maturing of communities, soil seed banks and seed germinations were extremely poor, and the number of trees that could be growing to mature stage was nearly zero. The optimum maturity age of T. ciliata var. pubescens was about 40 a, and the fitness coefficients (2.0-2.8) rapidly increased in early development stage, but then was sharply reduced (0.3-0.5), and then gradually dropped to almost 0. There were significant differences in the fitness at individual level (0-14 tree·cm-2) among different populations, but their values were low (close to zero). Based on the existing reproduction rate, the actual values of sexual reproduction and regeneration fitness were much lower than the predicted ones. Due to the low level of genetic fitness, the sexual reproductive ability of different populations all showed decreasing trends. The natural sexual regeneration ability tended to decline, while the fitness of T. ciliata var. pubescens further decreased. All those factors suggested higher investment risks. Therefore, the systems of sexual reproduction became unbalanced and deteriorating. We proposed that more studies, including breeding mating, pollination, seed setting, and genetic diversity evaluation, are needed. Moreover, we should provide suitable forest environment through cleaning up litter in the fruiting stage and applying appropriate thinning during the transition period from seedling to young tree growth.以九连山国家自然保护区毛红椿4个天然种群为研究对象,于2006—2016年调查毛红椿有性繁殖和自然更新的情况,分析其繁殖适合度系数和个体水平适合度.结果表明: 九连山毛红椿各地理种群结实单株的初始数量较少(3～9株),经70余年的繁衍发育,现存结实单株的数量仅2～10株,且来源于原始单株或子1～2代.不同种群间有性繁殖差异显著,但结实能力呈逐年下降的趋势;随群落发育成熟,土壤种子库保存与种子萌发的失利,导致能正常生长发育成熟的林木数量近乎为0.毛红椿最佳性成熟年龄为40年,早期优势明显,适合度系数早期快速增长,为2.0～2.8,但急剧减少至0.3～0.5,之后较平稳减速至近乎于0;种群间个体有性繁殖适合度(0～14株·cm2)存在较显著差异,但均较低,甚至低至0;以现有繁殖率计算,有性繁殖与更新的适合度实测值均远低于预估值.总之,受有性繁殖遗传适合度低的影响,不同种群间有性繁殖与更新能力趋同衰退;个体有性繁殖适合度进一步降低且面临更高的投资风险,现有繁殖体系因此失衡并趋于恶化.建议开展繁殖交配、授粉结实及遗传多样性评价研究,同时人为干预林分环境,于结实期清理林地枯落物,在幼苗至幼树生长过渡期适当疏伐.