Abstract: We investigated three communities of 35 year-old Populus×xiaozuanica and two communities of 37 year-old Ulmus pumila in Ningxia Habahu National Nature Reserve, where the annual rainfall was 300 mm. The results showed that, in the habitats of growing grass, the average plant height of P.×xiaozuanica was 6.23 m, its average circumference was 15.68 cm, there was no seedlings under the canopy, and the plants belonged to senility status, while in the habitats of earth surface covered by sands, the average plant height of P.×xiaozuanica was 0.85-1.71 m, its average canopy diameter was 0.50-0.74 m, the seedlings that plant height was less than 0.6 m occupied 43.04%-47.77% under the canopy, and the plants existed vigorous status. The average plant height of U.×pumila was 5.42-5.73 m, its average circumference was 20.08-61.20 cm, there was no seedlings under the canopy, and the plants belonged to senility status. The niche width of P.×xiaozuanica and U.×pumila was 1.06-1.87 and 1.12-1.17, respectively. Wholly, here U.×pumila could not naturally breed, majority of P.×xiaozuanica could not naturally breed, but they had played an important role in improving living conditions and maintaining ecological security.

Key words: semi-arid area, arbor stand, Populus×xiaozuanica;, Ulmus pumila, Ningxia Habahu National Nature Reserve