Transpiration reduction and root distribution functions for non-crop species Schefflera heptaphylla

Quantifying soil suction induced by plant transpiration is vital for engineers to analyse the performance of geotechnical infrastructure such as landfill covers. Transpiration reduction function (T_rf) and root distribution function (Rdf) are the two plant properties that govern root-water uptake ability. These two functions have been quantified for various crop species, but they are sometimes used to study the behaviour non-crop species, even though they are known to be plant-specific. In this study, specific T_rf and R_df were measured for six replicates of S. heptaphylla that have a range of leaf area index (LAI) from 1.0 to 3.5 in silty sand. S. heptaphylla is a non-crop tree species that has been commonly used for ecological restoration in many subtropical regions. T_rf of each replicate was obtained by relating normalised transpiration rate with suction. After testing, the root system of each tree individual was imaged to determine normalised root area index (RAI) profile (i.e., R_df). Normalised transpiration rate for S. heptaphylla with higher LAI (3 and 3.5) has lower tolerance of water stress as their normalised transpiration rate reduced at much lower suctions, as compared to those with lower LAI (i.e., 1 – 2.5). It is found that only when suction is lower than 50 kPa, the measured T_rf of S. heptaphylla is similar to some of those presumed in the literature. The measurement of R_df shows that the maximum amount of roots for S. heptaphylla was at depths of 70-80% of the root depth, in contrast to crops species whose root distribution is typically uniform or linearly decreasing.