| Characteristics of plant transpiration and ecosystem water balance have been widely concerned during the ecological restoration process. Water balance in arid and semiarid transition zone is phenophase-specific, due to varing and little precipitation over seasons. Therefore, we analyzed the dynamics of stem sap flow of three desert shrub species{Hedysarum mongolicum, Artemisia ordosica and Salix psammophila) in a mixed shrubland and their responses to environmental factors in different phenophases on the basis of continuous in-situmeasurements. Water balance of the mixed shrubland plot was examined via scaling up technique. The results could provide theoretical guidance for the ecological management in arid and semiarid transition zone. The main results were:Diurnal dynamics of stem sap flow for three shrubs showed a single peak curve, with similar diurnal pattern to solar radiation. Stem sap flow started at 6:00-7:00 for three shrubs, and peaked at 10:00-13:00,10:00-12:00 and 11:00-14:00 for Hedysarum mongolicum, Artemisia ordosica and Salix psammophila, respectively. Nocturnal stem sap flow flux was observed for Hedysarum mongolicum and Salix psammophila, but not for Artemisia ordosica.Seasonal pattern of stem sap flow and their responses to environmental factors varied with phenophases for all three species. Stem sap flow flux increased rapidly in leaf expanding stage, peaked in leaf expanded stage, and decreased in leaf defoliation stage. Stem sap flow flux in Hedysarum mongolicum, Artemisia ordosica and Salix psammophila in July 2012, was 0.74,0.66 and 0.54 kg cm-2 d-1, respectively. Saturated vapor pressure deficit and air temperature were the main environmental factors affecting stem sap flow flux for three species in leaf expanding stage, soil water content was the main factor in leaf expanded stage. In leaf defoliation stage, however, solar radiation and air temperature were the main factor affecting stem sap flow for both Hedysarum mongolicum and Salix psammophila, respectively, but soil water content for Artemisia ordosica.Stem sap flow flux for Artemisia ordosica was smallest among three species, but its interannual variation was largest. Mean daily stem sap flow flux over a period 2012-2014 for Hedysarum mongolicum, Artemisia ordosica and Salix psammophila was 0.45,0.22 and 0.35 kg cm-2 d-1, respectively, and interannual variation coefficients was 0.24,0.50 and 0.17.Evapotranspiration component in the plot showed markedly interannual variation. Shrub stand transpiration was 260 mm,145 mm and 59 mm in 2012-2014, respectively, accounting for 78.7%, 66.9% and 46.5% of evapotranspiration. Soil evaporation in 2012-2014 was 65.5,62.4 and 61.6mm, respectively. About 18% of rainfall was intercepted by shrub canopy during entire growing stage. Leaf area index was the most important factor affecting stand transpiration. Drought-induced reduction in leaf area index decreased stand transpiration in 2014. Air temperature was the main factor affecting the ratio of transpiration/evapotranspiration (T/ET). Soil water moisture at 30 cm and solar radiation were the main factors affecting soil evaporation. Evapotranspiration was mostly affected by saturated vapor pressure deficit, soil temperature at 10cm and leaf area index.Stand evapotranspiration was greater than rainfall, with a water deficit of 38.3 mm in 2012. Rainfall could meet water consumption by evapotranspiration both in 2013 and 2014. Rainfall almost met the water consumtion by evapotranspiration in different phenophases, with an exception in expanded leaf stage in 2012, with a water deficit of about 63.7mm.It was concluded that Artemisia ordosica had lower water consumption and better self-regulating ability to adapt to dry conditions. Therefore, Artemisia ordosica was suggested in the early ecological restoration or in the severe water deficit areas. From the perspective of the water balance, soil water supply may not meet the water consumption by evapotranspiration as more frequent and severe drought under climate change. More attention should be paid to water resource management in ecological restoration. |
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