Modelling stem height and diameter growth in plants

A model of stem height and diameter growth in plants is developed. This is formulated and implemented within the framework of an existing tree plantat ion growth model: the ITE Edinburgh Forest Model. It is proposed that the h eight:diameter growth rate ratio is a function of a within-plant allocation ratio determined by the transport-resistance model of partitioning, multip lied by a foliage turgor pressure modifier. First it is demonstrated that t he method leads to a stable long-term growth trajectory. Diurnal and season al 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 a tmospheric carbon dioxide concentration, increased planting density, increa sed temperature and decreased rainfall. High nitrogen and increased tempera ture give initially higher stem height:diameter ratios, whereas high CO2 gi ves an initially lower stem height:diameter ratio. However, the responses a re complex, reflecting interactions between factors which often have opposi ng influences on height:diameter ratios, for example: stem density, competi tion for light and for nitrogen; carbon dioxide and decreased water stress; rainfall, leaching and nitrogen nutrition. The approach relates stem heigh t and diameter growth variables via internal plant variables to environment al and management variables. Potentially, a coherent view of many observati ons 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 w hich have usually been employed. (C) 1999 Annals of Botany Company.