Impact Of Sites And Tree Age On Leaf Hydraulic Traits Of Robinia Pseudoacacia And Populus Simonii

Site and tree age are major factors of forming small dwarf tree on the Loess Plateau. Thus, taking two typical tree species of small dwarf trees on the Loess Plateau, Robinia pseudoacacia and Populus simonii as objects,, this paper studied the impact of sites (gully tableland with good water supply and slopeland with low water supply) and tree age on leaf hydraulic traits, leaf morpho-physiological traits and drought resistance aimed to elucidate the related hydraulic mechanism of small dwarf tree-forming. The main results obtained from this study were as follows:(1) For both trees growing on tableland had larger leaf maximum hydraulic conductance than those trees on slopeland, leaf maximum hydraulic conductance decreased with the increase of tree age. Tableland-growing trees had higher leaf epidermal conductance and PV curve parameters (including relative water content at turgor pressure loss point RWCtip, water potential at turgor pressure loss point ψtlp, and osmotic potential at saturated water content ψsat) than those on slopeland; leaf epidermal conductance decreased with the increase of tree age, and the PV curve parameters decreased to a different extent for the two species. Leaf maximum hydraulic conductance was positively related with ψtlp, suggesting there existed a tradeoff relationship between leaf flux capacity and drought-resistance.(2) The results indicated that for both species, trees growing on tableland had higher hydraulic vulnerability (P50) and safety margin of water transport than those trees on slopeland, but the vulnerability was almost the same with the increase of tree age. Safety margin of water transport decreased with the increase of tree age. P50was positively related with some PV curve parameters, indicating for both trees at different age P50was an important drought-resistant trait.(3) Black locust growing on the slopeland had smaller leaf area, higher specific leaf mass and leaf thickness, lower whole-tree hydraulic conductance, decreased net photosynthetic rate and stomatal conductance, increased water use efficiency and N,P nutrient content. Changes of leaf morpho-physiological traits of two species with tree age showed the same trend as in drought habitat. Worse leaf water status and decreased proportion of N in photosynthetic apparatus contributed to the decrease of photosynthetic rate in old-aged trees. Site and tree age showed significant interaction for some important leaf traits, indicating decreased photosynthetic area, raised leaf construction cost and lower net photosynthetic rate are major reasons of forming small dwarf tree on the Loess Plateau.