The hydraulic acclimation of old and dwarf Populus simonii trees growing on sandy soil in northern shaanxi province, China

Abstract

One of the most serious problems occurring during the rehabilitation of vegetation in the Loess Plateau in China is the existence of large areas of old and dwarf trees as a result of irrational afforestation measures. These old and dwarf trees not only are not effective in soil and water conservation, as windbreaks and in stabilizing blowing sand, but also do not provide direct economic benefits to local farmers. Populus simonii trees make up the largest area of old and dwarf tree plantations on the Loess Plateau, so information related to how this species adapts to habitats with differing levels of water availability is critical for predicting their survival, growth and water use, for preventing the planting of additional low productivity forest plantations, and for providing theoretical guidance for the future transformation of these old and dwarf trees. Hence, the growth, photosynthesis and hydraulic traits of planted Populus simonii trees were studied with the aim of elucidating the hydraulic acclimation mechanisms of old and dwarf trees growing in two habitats, wet gully-channels (habitat and dry gully-slope sandy soils (habitat B). The results indicated that tree height, basal diameter and diameter at 1 m of tree height for poplar trees in habitat B were significantly lower and branch dieback length on the major trunk was longer than those for trees in habitat A. Poplar trees in habitat B had a lower leaf photosynthetic rate (Pn) and stomatal conductance (gs) than trees in habitat A, while no difference in leaf-level intrinsic water use efficiency was found. Predawn and midday leaf water potential were the same for trees in both habitats. Field midday specific hydraulic conductivity (Ks) of branches of trees in habitat B was lower than that of those in habitat A, but leaf-level specific conductivity (Kl) was almost the same; the Huber value of poplar trees in habitat B was higher than that for trees in habitat A. Stem xylem water potential at 50% loss of conductivity (P50) for poplar trees growing in habitat A was -1.85 MPa, and for two plots in habitat B was -2.54 and -2.68 MPa respectively, indicating that poplar trees on dry gully-slope sandy soils had stronger cavitation resistance. Stomata of poplar trees in habitat B showed lower sensitivity to leaf water potential and closed 0.2 MPa later than trees in habitat A. Poplar trees growing in habitat B had a greater safety margin than trees in habitat A. These results showed that a larger Huber value and a lower stomatal conductance in dry habitats help this species to maintain hydraulic homeostasis in their leaves; meanwhile, lower carbon assimilation rates and a larger carbon investment of the trees for maintaining a greater safety margin in dry habitat are probably the important reasons of formation of areas with old and dwarf trees in the Loess region.