Seed dispersal is an important stage in life style of plant population, the diffusion Pattern of seed directly affect the seedling colonization, distribution and the main resource of propagule. The production of seed is mainly controlled by the seed yield characteristic of tree species, but the seed dispersal not only connected with the tree species, but also with the various environmental disturbance factors. There are difference of seed dispersal pattern among species even, even within population.The characteristic of seed rain directly affect the size, quality, pattern and the reproductive of young seedling. Elm woodland-steppe is spread in the typical temperate steppe zone. Sand elm is the top community which adapt well to the semiarid and sub-humid sandy. Sand elm in Otindag Sand Land can protect the local environment effectively. But with the decrease of population of Sand elm recently, the stability and sustainable development will be hindered. Therefore, there has an imperative require for the population renewal strategies and vegetation restoration when considering the seed rain spatial and temporal distribution features.
Through observing the seed rain's quantity, composition and dissemination process for different age isolation elm wood in Otindag Sand Land, we try to illuminate the diffusion rule of seed rain and the relationship between the diffusion of seed rain and the external environment by continuous observation in original position. By selected the different ages of sand elm wood (23 years, 40 years and 56 years) located in Otindag Sandy Land, the seed rain's quantity, composition, distribution process and the relationship between the disperse rule of seed rain and the environmental factors. The study area is located in the middle of Otindag Sandy Land in Sang Genda town attached to Zhenglan Banner(N42°41'-43°43',E115°57'-115°59'). The experiment was conduced continuously from 18th May to the end of 2th July 2010. We selected three kind of wood isolated with different ages (23 years, 40 years, 56 years) which in the uniform habitat such as elevation, slope, aspect and other conditions. At the same time, no sand elm was found around 100 m in the upper direction and around 50 m in the other three direction of isolated wood. The Seed rain collector was laid as the local main direction, surrounded with the isolate wood and separated for 5 meters from one collector to another. 120 collectors were distributed in 10 sub-direction. The Seed rain was collected every 2 day from the seed rain began to drop, until the seed falling completed. The collected sand elm seeds (with Samara) were divided into 5 levels according the intact and mature degree of seed. The level 1 of seed means the intact Samara and mature seeds, the level 2 means the winged Samara and the incomplete mature seeds, the level 3 means mature seed without Samar, the level 4 means seeds rotted or destroyed by insects and the level 5 means immature or empty seeds. One way AVOVA (SPSS 13.0, SPSS Inc. Chicago, Illinois, USA) was used to assess the significance of the treatment impacts, the least significant difference (LSD) and correlation analysis of different data sets, compare the difference between.
The results showed that: (1) the quantities of seed rain increase with the age of elm, the order is old elm > medium age elm > small age elm, the quantities of seed rain for the 56, 40, 23 years old elm is(7744±16.3), (5236±18.9) and (2456±12.7) per square meter; respectively. (2)The quality of middle age Isolation seeds is better than that for the young trees and old trees which shows in the lower proportion of immature, empty grain and seed pests. The quality of 23 years old isolation seeds is worst. The seeds of old trees and the middle age trees are the main sources of natural regeneration seed. (3) Elm seed begins to mature in late May 2010, begins to fall in early June, reaches the peak of fall in mid-June and complete until late June. (4) Elm seeds diffuse within 50m of the main wind direction along the downwind and the range of 20m around crown in the course of the pre-dispersal, then scattered to the 50m of the downwind areas or more distant areas in the course of the. (5) The process of post-dispersal shows the "cut peak" effect to the pre-dispersal of seed rain.