Studies On Water Use Patterns Of Strangler Fig Ficus Tinctoria In Tropical Rainforest In Xishuangbanna, SW China

The Xishuangbanna forest reserve is situated on the northern edge of the tropical zone in Southwest China. There are a rainy season (from May to October) and a dry season (from November to April). And the dry season can be divided into a foggy season (from November to February) and a hot-dry season (from February to April). Strangler figs play important roles in the renovation of the tree species and the progress of the regional vegetation restoration, so seasonal variation in the patterns of water utilization of strangler figs becomes concerned. Strangler figs undergo a tri-phasic development in growth form:epiphytic, hemiepiphytic and arborescent life stages. And the hemiepiphytic life stage can be divided into the early stage and the late stage. Each life stage was hypothesized to obtain its water from a different location because of differences in terms of where each stage placed its roots. The objectives and methods in this study were:(1) to determine the water source, water use efficiency and survival mechanism of plants by measuring the stable hydrogen (8D), oxygen isotope composition (δ18O) of fog drip, rainwater, humus water, soil water at different depths, stem xylem water and carbon isotope of plants'leaves. (2) to determine seasonal variation of soil water content at different depths (5 cm, 15 cm, 20 cm, 30 cm, 40 cm, 50 cm, 60 cm, 80 cm, 100 cm, 120 cm). (3) to determine the seasonal variation of soil water potential by UIT plug-in tensiometer (Germany). (4) to determine the seasonal variation of leaf water potential by pump-up pressure chamber(USA). (5) to confirm the plants'water use efficiency by collecting leaves of F. tinctoria at different life stages and Elaeis guineensis for stable carbon isotope analysis. We tried to reveal the seasonal variation of water use pattern of F. tinctoria at different stages by determining the stable isotope ratio of stem xylem water and its available water sources.The following is the main results of this study.1. The minimum soil water potential can reach to-0.0305 MPa at 30 cm depth in hot-dry season while the maximum can reach to-0.0145 MPa at 30 cm depth in foggy season. There was no variation of soil water potential at 70 cm depth and the water condition of the deep soil was well through all seasons.2. The soil water content was significantly higher in the rainy season than in the hot-dry season and foggy season (P< 0.001), but the difference of the soil water content between the foggy season and the hot-dry season was not significant (P= 0.64). The soil water condition was fine in rainy season while it was not well in foggy season and hot-dry season.3. Predawn leaf water potential and midday leaf water potential of F. tinctoria varied with the life stages. Predawn leaf water potential in rainy season was higher than that in foggy season and hot-dry season. The ability of water absorption of the arborescent stage of F. tinctoria in dry season was strongest. Because of the high precipitation, midday leaf water potential of F. tinctoria in rainy season was high among all life stages. Plants suffered from a light water stress in rainy season.4. The data of leaf water potential, soil water potential and 813C indicated that plants suffered from a severe water stress in dry season while not in rainy season. The data of carbon isotope showed the WUE of different life stages varied in the whole dry season, while in rainy season the WUE varied little.5. With the data of the stable isotope ratio and other parameters, we found that a seasonal variation of water use pattern of F. tinctoria did exist at different life stages.(1) In rainy season, rain water was the main water source. Humus water was the complementary source of the epiphytic and hemiepiphytic life stages. And the water in the shallow soil was one of the main sources of the hemiepiphytic and arborescent life stages.(2) In foggy season, the main sources of the epiphytic stage were rain, fog and humus water. At the early stage of the hemiepiphytic, the shallow soil water was the main source, while at the late stage of the hemiepiphytic the soil water (including the shallow and the deep soil water) was the main source. The fog drip water and the humus water were the complementary sources.(3) In hot-dry season, the main source of the epiphytic stage was rain water, and' the main source of the hemiepiphytic and arborescent stage was the shallow water. Plants did not use the water in deep soil through all the life stages.6. We analyzed the water use sources of the hemiepiphytic stage by three compartment linear mixing model. The results indicated that 7% of the water input of F. tinctoria at the early stage of the hemiepiphytic in foggy season was from the fog drip water, and 90% of that was from the shallow water, while F. tinctoria at the late stage of the hemiepiphytic did not use the fog drip water.These results were related to the protection of the rainforest, especially the strangler figs. The generality of this finding for other tropical hemiepiphytes needs to be investigated. Further research can provide more bases for the protection of rainforest.