Study On Water Thresholds Of The Main Afforestation Species On The Loess Plateau

Fixed-position and fixed-time observations of 5 main afforestation species leaf gas change were made under natural condition, at 4 water levels, by using potted 4-year-old saplings, Li-6200 portable photosynthesis system and Wescor Psypro water potential, etc. in the semi-arid area of the Loess Plateau, Jul., 2006. Diurnal courses of physi-ecological parameters such as net photosynthetic rate (Pn), transpiration rate (Tr), leaf water potential (Wp) and their dynamic relations with soil water content (swc) were explored. The soil water thresholds on which Pn or Tr peaks or diminishes to zero and those certain swc ranges within which Pn or Tr maintains high levels were identified. The water threshold at which the dominant cause of restricted Pn changed from stomatal to non-stomatal limitations was detected by a combination analysis of the relations between swc and leaf stomatal conductance (Cs), carboxylation efficiency (CE), limiting value of stomata (Ls), intercelluar CO₂ concentration (Ci). Water use under different water conditions was studied both by instantaneous water use efficiency (wue) and by daily average water use efficiency (WUE). Swc ranges where both assimilative capacity and water use efficiency can be maintained at high levels were chosen as the most suitable soil moisture range for sapling growth. The Wp values of the saplings undergone severe water deficit were revealed. The main conclusions drawn are as follows: The response of Pn, Tr to soil moisturePn and Tr of each species shows close relations with swc, which fit to the general equation, y= ax³+bx²+cx (Tr-swc relation of Elaegnus angustifolia is logarithmic). Where, y denotes Pn or Tr; x denotes swc; a, b, c are fitting coefficients. Make dy/dx=0, the swc value, at which Pn or Tr peaks, also the swc which has the most efficiency on Pn or Tr, can be calculated. A ±2 neighborhood of this swc is considered as the swc range which maintains high Pn or Tr. Each species has higher Tr soil water range than its Pn soil water range. The starting point of this higher part, that is, the upper limit of Pn soil water range is the water threshold from which luxurious transpiration begins. Make y=0 to calculate the soil water threshold at which Pn, Tr becomes zero. Stomatal and non-stomatal factors restricting PnFor 5 studied species, the soil water threshold at which the dominant cause of restricted Pn changes from stomatal to non-stomatal limitations ranges between 8 and 10. This shows that 8-10 swc range is vital to sapling growth. Sapling leaf gas exchange is sensitive to a swc change within this range, where Cs, CE declination generally begins to accelerate, Ls starts to decrease while Ci soars, and damage occurs to the photosynthetic mechanism, mesophyll assimilative capacity is thus affected. Cs-swc, CE-swc explorationClose relation was found between Cs and swc of each species. Cs-swc relations of Ailanthus altissima,Populus bolleana and Ulmus pumila fit into quadratic curves, showing restrained Cs in high swc whileto Elaegnus angustifolia and Juglans regia, Cs continue increasing with higher swc. The relations couldbe expressed respectively by an exponential and a linear equation.Daily average leaf water use efficiency (WUE)WUE was calculated as the ratio of the irregular figure area enclosed by Pn diurnal course curve andcoordinate axes to that of the Tr diurnal course curve. Total average WUE was the sum area ratio of 4irregular figures formed by Pn or by Tr curves. It describes the integrated water use status of eachspecies under various water conditions during the observational days. According to the total averagevalues, the species could be queued as Populus bolleana > Ailanthus altissima > Elaegnus angustifolia> Ulmus pumila >Juglans regia. Total average WUE of Populus bolleana is 2 times as much as that ofJuglans regia and 1.5 times as of Ulmus pumila.The law of leaf water use efficiency change with soil moistureThe overall changing trend of Juglans regia wue to swc takes the shape of "S", while that of the other 4species can be expressed by a general equation, y = ax³ + bx² + ex + c. According to swc ranges withinwhich high wue is maintained, the order is Elaegnus angustifolia > Ulmus pumila > Populus bolleana >Juglans regia > Ailanthus altissima. This reveals that Juglans regia and Ailanthus altissima canmaintain comparatively higher level of wue under comparatively low swc.Wp-swc relationsNo proper equation fitted the Wp-swc relation of Juglans regia due to poor correlation. Wp-swcrelations of the other 4 species can be described by quadratic curves or cubic equations. Wp of these 4species raise as soil water deficit gets more and more severe, revealing a drought tolerance characteristicvia maintaining high Wp.Suitable soil moisture range for sapling growth and the wilting moistureThe suitable soil moisture ranges for subject saplings growth were found as: Juglans regia (15-17) >Elaegnus angustifolia (14.58-16.37) > Populus bolleana (13.58-15.5) > Ulmus pumila (11.86-14.41) >Ailanthus altissima (10.29-13.37). This reveals that species of economic value, like Elaegnusangustifolia and Juglans regia have high demand on soil moisture, while Ailanthus altissima and Ulmuspumila grow well in slightly dry conditions. Cite permanent wilting moisture as the standard to appraisesaplings drought resistance ability, the order is: Ulmus pumila > Elaegnus angustifolia > Ailanthusaltissima > Juglans regia > Populus bolleana.