A model of stem height and diameter growth in plants is developed. This is formulated and implemented within the framework of an existing tree plantation growth model: the ITE Edinburgh Forest Model. It is proposed that the height:diameter growth rate ratio is a function of a within-plant allocation ratio determined by the transportresistance model of partitioning, multiplied by a foliage turgor pressure modifier. First it is demonstrated that the method leads to a stable long-term growth trajectory. Diurnal and seasonal dynamics are also examined. Predicted time courses over 20 years of stem mass, stem height, height:diameter ratio, and height:diameter growth rate ratio are presented for six treatments: control, high nitrogen, increased atmospheric carbon dioxide concentration, increased planting density, increased temperature and decreased rainfall. High nitrogen and increased temperature give initially higher stem height:diameter ratios, whereas high CO # gives an initially lower stem height:diameter ratio. However, the responses are complex, reflecting interactions between factors which often have opposing influences on height:diameter ratios, for example: stem density, competition for light and for nitrogen; carbon dioxide and decreased water stress; rainfall, leaching and nitrogen nutrition. The approach relates stem height and diameter growth variables via internal plant variables to environmental and management variables. Potentially, a coherent view of many observations which are sometimes in apparent conflict is provided. These aspects of plant growth can be considered more mechanistically than has hitherto been the case, providing an alternative to the empirical or teleonomic methods which have usually been employed. # 1999 Annals of Botany Company