Lösch (2001) concluded that information on the ecophysiology of tropical mountain trees — key components for the understanding of the functional role of vegetation in these ecosystems — is entirely missing; therefore we concentrate on transpiration. Available knowledge on stands is almost exclusively from catchment studies (e.g. Bruijnzeel and Proctor 1995; see Chapter 12 in this volume), but types of “gully”, “slope” and “stunted” forests are found everywhere. To evaluate their contribution to water consumption and buffering in the landscape it is essential to characterize these forest types individually, here via up-scaling from sap flow measurements on individual trees to plot-scale. In this context it is important to know whether sap flow varies species-specifically (then many species have to be studied) or whether the size of individual plants, their life-forms, and associations with functional groups are more important (then only a few representative individuals need to be investigated).