Despite continued studies on the ecology and physiology of strangling hemiepiphytes, there is little quantitative information about the variations in source-water acquisition by these species under different growth phases. Prof. LIU Wenjie and his colleagues of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study in XTBG (21°55'39''N, 101°15'55''E, 560 m a.s.l.) to investigate the water acquisition patterns of Ficus tinctoria, a strangler fig growing in a common garden, in relation to growth phase (epiphytic, transitional, and terrestrial) and season (foggy, hot-dry, and rainy).
The objective of their study was to determine any seasonal differences in water source among the three growth phases (epiphytic, transitional, and terrestrial) of F. tinctoria. They asked the following questions: (1) during the dry season (i.e. the foggy season and the hot-dry season), do canopy-rooted epiphytes acquire water from fog and from recent rainfall present in canopy humus soil? (2) do transitional-phase plants acquire water from both canopy humus soil and terrestrial soil because they have roots both in the canopy and on the ground? and (3) are differences in xylem isotopic 'signatures' between the three growth phases insignificant during the rainy season when water in canopy humus soil and terrestrial soil is plentiful and is recharged very frequently by current rain events?
To determine the sources of water used by F. tinctoria, all available water sources (fog water, rainfall, canopy humus water, and terrestrial soil water at several depths) were sampled and their isotopic compositions (δD and δ18O) were determined.Samples of plant xylem, terrestrial soil, and canopy humus soil were collected on seasonally distinct dates.
Hemiepiphytic F. tinctoria displayed a high degree of plasticity in source-water acquisition associated with the growth-phase transition from epiphyte to transitional plant to terrestrial tree. Each growth phase acquired different water resources. During the foggy season and the hot-dry season, epiphytic F. tinctoria utilized a combination of recently received rainwater and fog water trapped by canopy humus soil. Epiphytes utilized a combination of recently received rainwater and fog water present in canopy humus, whereas terrestrial trees exclusively depended on shallow and deep terrestrial soil water and exhibited marked flexibility in depth of soil water uptake. Transitional plants relied predominately on water from shallow soil and extracted only a small fraction of water from canopy humus soil.
The researchers regarded that plasticity of source-water uptake to cope with radical changes in rooting environment was likely the key feature enabling hemiepiphytic species to thrive and successfully establish in the tropical rainforests.