Stem turnover strategy of multiple-stemmed woody plants

We have investigated stem turnover strategy for Lindera umbellata, an understory shrub that sprouts from its rootstock under natural conditions to replace constituent stems, on the basis of the hypothesis that the multiple-stemmed form of woody species is an adaptation enabling efficient reproduction in high-stress environments. We tested the hypothesis that the timing of stem replacement maximizes sexual reproduction for the shrub. We developed a model for the time of optimum replacement of a stem by a daughter stem which maximizes the sexual reproduction of a shrub and tested the model using L. umbellata growing in the field. From the model, the optimum time of replacement of a stem with a daughter stem is when cumulative sexual reproduction per unit time for the stem is maximum. In practice, this will be the last age (t opt) at which annual sexual reproduction in a stem can potentially exceed cumulative sexual reproduction per unit time for the stem. Half of the stems died at almost t opt and had sexually mature daughter stems at that time. Other stems, however, died at times more remote from t opt when daughter stems were sexually immature. It is thought that normal replacement of the latter stems was prevented by accidents such as breakage. We conclude that clumps of L. umbellata achieve efficient sexual reproduction by stem replacement at the optimum time, although accidents can, to some extent, determine when the stem actually dies.