| Karst landscape distributes continuously and extensively in SW China. Limestone is the main karst type consisted mainly of carbonas rocks. The soil layer is thin and the chemical leaching in limestone is intense. Consequently, soil is very infertile. Limestone tropical seasonal moist forest developed in this kind of inclement habitat. Rock coverage in this forest type is more than 80% and humus is accumulated in rock cracks. Root system of trees develops in these rock cracks to absorb moisture and nutrients. The objectives and methods in this study were: (1) to determine the water source, water use efficiency and survival mechanism of plants, we collected intercepted fog drip and throughfall and measured the stable hydrogen (δD), oxygen isotope composition (δ18O) and carbon isotope of fog drip, throughfall, rainwater, soil water, groundwater, stem xylem water and plants leaves using an isotope ratio mass spectrometer; (2) to ascertain whether HR(hydraulic redistribution)occur, we observed the soil volumetric water content of Litsea glutinosa and Garuga pinnata hourly on three days using TDR(Time-Domain Reflectrometry) (6050X1,TRASE-2100,USA); 3) to determine seasonal variation of soil volumetric water content at different depths (10 cm, 30 cm, 50 cm, 75 cm) using TDR too; (4) to determine the seasonal variation of soil water potential, we used soil water potential Tensiometer (DLS-Ⅱ, IGSNRR); (5) to confirm the plants'water use efficiency, we collected different plants'leaves for stable carbon isotope analysis.The following is the main results of this study.1) Theδ18O range of plant xylem water is from -13‰~-7.3‰, theδD range is -108‰~-54‰; theδ18O range of soil water is -12.3‰~-4‰, theδD range is -105‰~-40‰; theδ18O range of ground water is -10.1‰~-5.9‰, theδD range is -73‰~-50‰. This result indicates that water source of plants are soil water and ground water.2) The difference of soil volumetric water content of Litsea glutinosa and Garuga pinnata between diurnal values and nocturnal values is not significant (P>0.05), which indicate that there is no obvious soil volumetric water content fluctuation in deferent soil depths. Namely, there is no hydraulic redistribution in limestone tropical seasonal moist rainforest.3) The highest soil volumetric water content is (30.2±4.3)% at 10 cm in rainny season (May to October), and the lowest is (12.4±0.2)% at 75 cm in dry-hot season (February to April).4) The lowest water potential is -0.055 MPa at 75 cm in dry-hot season (from February to April), and the lowest water potential in rainny season (from May to October) is -0.039 MPa at 30 cm.5) Litsea glutinosa, Cleistanthus saichikii and Pistacia weinmannifolia have different water use efficiency. In foggy season (November to February), the value ofδ13C of L.glutinosa was significantly lower than C.saichikii(P=0.19, P<0.05) and P.weinmannifolia (P=0.004, P<0.05), and there is no significant difference between C.saichikii and P.weinmannifolia (P=0.314, P<0.05)These results are related to the natural environment, generally, limestone tropical seasonal moist rainforest is very vulnerable, so, protect this pattern forest is necessary, further research will be able to provide a basis for this purpose.
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