| Ergis River Watershed is one of the important regions of natural forest in China. The paper studied the natural forests and its conservation in the Xiaodonggou area and Beitun area, which located in Ergis River Watershed and represent the mountain and riparian vegetation respectively. The main conclusions of the study are as follows:There are 551 vascular plants that belong to 65 families and 259 genera in Ergis River riverside. The composition of families concentrate toward the preponderant which contains more than 15 species each ones on one side and scattered to smaller ones which contains less than 5 species each on the other side, the composition of genera are also dominated by small ones which contains less than 5 species each. The plant life-forms are dominated by geophytes, therophytes take the second place. All the vascular plants belong to 11 typies of areal-types, and the North Temperate distribution is in predominant, meanwhile, the Temperate distribution and Mediterranean, West Asia to Central Asia distribution also have a higher percent, reflect a complex origin of the flora.The vegetation of Xiaodonggou forest region is divided into 10 main formations that belong to 3 vegetation type groups and 4 vegetation types, the main forest formations include : Form. Larix sibiric, Form. Picea obovata, Form. Betula pendula and Form. Populus tremula. The vegetation in Ergis riverside is divided into 46 formations which belong to 7 vegetation type groups and 15 vegetation types, the 7 vegetation type groups include forest, shrub, grassland, meadow, desert, swamp and hydrophyte.The composition, structure and natural regeneration are analyzed and the most suitable conditions for natural regeneration of Larix sibiric forest are selected, meanwile, the conservation strategies of it are put forward, which include: closing the mountain for forest regeneration; artificial seeding, planting and other technology : directional cultivation.Planting experimentations indicate that the four species of Populus have differentexpression of growth rate after planting. P. alba grows slowly and often dies back at treetop at first, but have a good adaptability and fast growing rate after one year. To make planting successful, a careful choice of planting site, seedling specification, planting manner and administration measures have to done.The study of natural regeneration of riparian forest indicates that sexy propagation becomes impossible for the restoration of riparian forest and root sucker becomes a possible approach, because sexy propagation is highly dependent on the flooding mechanism. The capabilities of producing root suckers are different among the species of Populus, the roots which could produce root sucker are often distributed not more than 20cm deep and the farthest distance of root sucker could reach 2.5 times the height of the old tree in flat woodland, but 1 time the distance in slope, the highest density of root sucker appears between 1-2 times the radius of the old tree's crown, the root suckers depend mainly upon lateral roots, which are away from the old trees to absorb water and nutrition. There are 3 main reasons for the death of root suckers that includes: drought, browsing and trampling. The death rate is higher in low-level browsing woodlands than in entirely protected areas. After 5 years enclosure and irrigation, the root suckers of P. laurifolia and P. alba revealed different patterns of change following the enclosure time, the young root suckers couldn't develop into old ones even mature trees in low-level browsed woodland ,but could in entirely enclosed ones, so it's very important to take entire enclosure mearsures for the regeneration of the riparian forest. Irrigation could accelerate the amount and survive of root suckers.Ditching and root-broking tests indicate that the suitable sites for ditching and root-broking are in moderate water condition. The densities of root sucker are different in the four parts of the ditches and in both sides of it have higher ones. It's effective to increase the inclined plane of the ditches for more root sucker producing while ditching and root-broking. The root sucker numbers appeared increasing at beginning years and decreasing at the following years after ditching, while one of the mechanisms of ditching is to lessen the thickness of soil layer above the roots of old trees, it's also effective to shovel away the upper layer of greensward for theoccurring of root sucker in suitable sites.Flooding has a decisive influence not only on the occurrence and formation of riparian forest but also on its composition, structure and distribution patterns, the seed mature periods of riparian Populus are highly overlapped with that of flooding. The germination, growth of seed depend upon the intensity and duration of flooding, flooding complements for water table of riparian woodland through overflow and profile infiltration to sustain the normal growth of riparian forest. Changes to the flooding mechanisms result in the degradation of riparian forest.The existing distribution pattern of riparian forest is not only the responses of modem environment, but also of mountains, climate, soil and hydrology in geological times. The existing riparian forest originated from the remains of mountain forest conserved in riverside, which expanded and multiplied gradually. The composition of riparian tree species is decided by the breeding characteristics of tree species, riverside characteristics, climate and hydrology characteristics together. Meanwhile, all of these factors promote the riparian forest formation and disappearing continuously. In present, artificial disturbance has changed the bases of the: riparian forest's occurrence, reproduction and survival in most areas in Ergis, and this also lead to the continuously decline of riparian forest and the decrease of its area and cover. Because some of the disturbances couldn't be reversed, the continuous decline and die out of riparian forest would be inescapable if not enough protection are applied.
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