Soil Seed Banks And Seedling Recruitments Of The Dominant Tree Species Near The Alpine Treeline Ecotone On The Upper Reaches Of Minjiang River, China

Treelines on the upper region of Minjiang River differ between the north aspect and the south aspect in their appearances, altitudes and tree species. On the north aspect, trees of Abies form a sharp and abrupt treeline ranging from 3800m to 4400m, while on the south the treeline is generally lower(3 400～3 800m), more open and gradual and mostly composed of Sabina. In this study, we examined the altitudinal gradients of soil seed banks and seedling recruitments at the treeline ecotones of a N-aspect and a S-aspect by using soil sieving, germination experiment and field investigations, analyzed the characteristics of population regeneration of tree species at the transitional zone and presented a analysis of the causes to the aspect-related difference in treeline patterns in the study area. Major results of our study include:1. Soil seed bankN-aspect: Of the 50 plots investigated, the average density of soil seeds is 50.96/m2, in which well-formed seeds account for 6%, empty seeds 52%, parasitized seeds34%, and seeds damaged by animals 8%. The size of soil seed bank varies along altitude, being 1.00 seeds /m2 at the 10m above the treeline and ca.19.33 seeds/m2 at the upper limit of treeline. The highest density (136.83 seeds/m2) occurs at the treeline ecotone. By contrast, the density of soil seed for the closed forest is only 30.50 seeds/m2. In terms of vertical strata, 67.50% of the total seeds are at the surface layer, 18.84% at the middle layer (0～2cm) and 13.66% at deeper layer (2～5cm). The number of parasitized seeds is negatively correlated to soil depth.S-aspect: Of the 50 plots investigated, the average density of soil seeds is 60.16 seeds/m2, and the well-formed seeds account for 49%, empty seeds 36%, parasitized seeds10%, and seeds damaged by animals 1%. The size of soil seed bank varies along altitude, with 1.92 seeds/m2 recorded at the10m above the treeline,108.16 seeds/m2 at the upper limit of treeline, and 75.80 seeds/m2 at the treeline ecotone, while that for the closed forest is 20.00 seeds/m2. The number of seeds decreases with the depth of soil. As is on the N-aspect, the size of soil bank, from large to small, follows the order of the surface layer, the middle layer (0～4cm) and the bottom layer (4～10cm). The number of parasitized seeds is also negatively correlated to the depth of the soil.2. Seedling bankN-aspect: A mean maximum seedling abundance of 31 000 seedlings/hm2 was recorded near alpine treeline at growing season. The density of seedlings is 1 250 seedlings/ha (all being 1 or 2 years old) at the alpine meadow 10m away above treeline, 7 000 seedlings/ha at treeline ecotone...