Abstract:

The valleys along the rivers in southwestern China are hot and dry because of the rain shadow effect. The primary vegetation on the slopes in these valleys is savanna. However, due to heavy human disturbance, the vegetation on these slopes has been devastated or severely degraded, resulting in severe water and soil erosion. Rehabilitation or revegetation of these slopes is urgently needed. Some eucalypt and legume trees have been introduced for the purpose of revegetation, and these have shown fast growth during the first seven years. However, it is not known how these trees cope with the hot and dry stresses during the dry period. In the present study, we monitored the water status, stomatal conductance and osmotic regulation, and their seasonal changes of 9 tree species in a hot and dry valley. The height and diameter of these trees attained over 5 years of growth also were compared. The mean height of the 9 species ranged from 0.7 to 7.6 m, the diameter from 2.6 to 6.7 cm. Four species (Eucalyptus dulebsis, Acacia leptocarpa, A. auriculiformis and A. cunninghamii) showed fast growth, two species (E. citriodora, Leucaena leucocephala) had intermediate growth and the remaining species (A. mangium and Robinia pseudoacacia) had slow growth. The physiological measurements showed that as drought conditions progressed, all species decreased their leaf water potentials and stomatal conductance. However, the decrease was more rapid in A. leptocarpa, A. auriculiformis, A. cunninghamii and Eucalyptus dulebis. During the peak drought period (March and April), these species also reached the lowest water potentials (predawn water potential lower than -1.0 MPa) and nearly closed their stomata (A. leptocarpa, A. auriculiformis, A. Cunninghamii) or remained at low stomatal conductance (Eucalyptus dulebsis, A. holosericea and Dodonaea wiscosa) while the remaining species maintained relatively high water potentials and a low stomatal conductance (Eucalyptus citriodora and Leucaena leucocephala) or nearly closed stomata (A. mangium). Nevertheless, during the drought period, no species showed leaf wilting, except R. pseudoacacia and the native species Vitex negundo and Terminalia franchetii, which were deciduous. All species increased osmoregulation capacity, as indicated by the decrease of osmotic potentials at full saturation, and the increase of osmoregulation substances. Osmoregulatory substances increased over time in the following sequence: K+, free Pro, other free amino acids and soluble sugars. It appeared that K+ was important in osmoregulation at the beginning of the drought, while soluble sugars were important in the late drought period. E. dulebsis, A. auriculiformis and A. cunninghamii exhibited the highest osmoregulation capacities. Our results show that most of the introduced species are suitable to grow in these hot and dry habitats because of, among other things, they have strong capacities of water-absorption and water conservation.