文峪河上游华北落叶松林的种子雨、种子库与幼苗更新

华北落叶松林下更新不良,为探究其制约因素,开展了山西省文峪河上游5个华北落叶松林分的种子雨、土壤种子库和幼苗更新的研究。结果表明:(1)华北落叶松种子主要集中于9—10月散落。2011年为华北落叶松种子丰年:种子产量高,种子雨密度达(961.93±377.40)粒/m2;种子质量高,完整种子占(89.31±16.13)%。2012年为种子平年,种子产量低,种子雨密度为(252.73±115.12)粒/m2。华北落叶松种子雨主要源于毗邻树木,华北落叶松纯林和落叶松云杉林的种子雨密度显著高于其他3个针阔混交林。(2)土壤种子库主要由上年种子雨组成,2012年4月的土壤种子库密度为(695.18±297.23)粒/m2,完整种子占(59.73±9.56)%。种子自然萌发前,约(78.98±24.76)粒/m2具发芽力,基本可满足更新需要。但种子活力保持期少于2 a,只能形成短期持久土壤种子库。(3)华北落叶松更新不良,种子年后仍难以实现幼苗建成,当年生幼苗的出现频度平均为1.6%,且林下难以存活。幼苗发生与种子储量关联性不强,种源条件不是制约华北落叶松更新的主要因素。

Seed rain, soil seed bank and regeneration of Larix principis-rupprechtii stands in the Upper Reaches of Wenyuhe

Larix principis-rupprechtii is one of dominant deciduous coniferous tree species in subalpine temperate forests of North China. It plays significant roles in water conservation, ecotourism and biodiversity maintenance in its distributional areas. It is also one of major economic tree species in this region due to its fast growth and good-quality timber. Previous studies suggest that L. principis-rupprechtii experiences natural regeneration failure, as indicated by low proportion of saplings and seedlings in natural conditions. The failure of regeneration is a threat to persistence of L. principis-rupprechtii forests in its distributional areas. However, the causes of natural regeneration failure are still poorly understood. This study was to examine whether the natural regeneration failure of this species was driven by seed limitation. We investigated seed rain, soil seed bank and seedling regeneration in five typical larch stand types, i.e., pure L. principis-rupprechtii stand, L. rincipis-rupprechtii + Betula platyphylla stand, L. principis-rupprechtii + Populus davidiana stand, L. principis-rupprechtii + P. davidiana + B. platyphylla stand, L. principis-rupprechtii + Picea wilsonii stand. Fifteen 50 cm × 50 cm seed traps were deployed in each plot for seed rain collecting. For each plot, 15 quadrats of 20 cm×20 cm with 10cm depth and 50 quadrats of 1 m × 1 m were examined nearby seed traps for soil seed bank and recruitment survey, respectively. We found that most of seeds were dispersed from September to October. In 2011, which was a seed year of L. principis-rupprechtii, seed production was amounted to (961.93±377.40) seeds/m², (89.31±16.13)% of which was full seeds. In 2012, a common seed year, both seed production and full seeds proportion were lower than those in previous year, with (252.73±115.12) seeds/m² and (36.04±4.26)%, respectively. Furthermore, seed rain of L. principis-rupprechtii was mainly from the surrounding mother trees. Seed rain densities of pure L. principis-rupprechtii stand and L. principis-rupprechtii + P. wilsonii stand were higher than those of the other three stand types, which were mixed with broad-leave trees. Thirdly, soil seed bank of L. principis-rupprechtii was mainly from the seed rain that produced in previous year. Seed bank in April of 2012 was (695.18±297.23) seeds/m², accounting for (76.05±13.79)% of seed rain in autumn of 2011. More than 85% of seeds were found in the litter layer. About (59.73±9.56)% of the seed bank was full seeds. There were (78.98±24.76) seeds/m² remained viable to germinate before natural regeneration, accounting for (19.02±5.00)% of full seeds in soil seed bank. The germination capacity decreased with time being in forest floor, hence, forming a short-term persistent soil seed bank for L. principis-rupprechtii. Lastly, the emergence of seedlings uncoupled with the seed storage. The frequency of seedling emergence was 1.6%. Seedlings could hardly survive understory. Collectively, this study demonstrates that seed source, especially in seed year, is not the limiting factor for the poor natural regeneration of L. principis-rupprechtii. Our study suggests that further studies should be carried out to examine the detrimental or limiting factors, such as effects of soil moisture and litter, for the natural regeneration failure of the species.