Evapotranspiration (E_T) from tropical rainforests has an important role in the regional and global water cycling. However, the component fluxes of E_T remain unquantified. Quantifying contribution of individual trees to stand transpiration (E_sta) will advance our understanding of the sources of spatial variation of E_T. The aim of this study is to answer a basic question: Do few large trees contribute more to E_sta than the numerous small individuals?

Tree transpiration was measured in three plots in Lambir Hills National Park. Depending on tree size, two or four fixed sensors were installed in the stems to account for azimuthal variation of sapflux density (F_d). Also, a moving probe was used to evaluate radial variation of F_d.

No clear azimuthal pattern of F_d was identified, but F_d decreased with increasing sensor depth. Because inter-tree variation of hydroactive xylem area was greater than that of their F_d, tree transpiration increased linearly with basal area. The largest 15% of samle trees accounted for 61% of E_sta. Therefore, in stands of similar diameter-distribution, a few large individuals disproportionately contribute to E_sta.