Volume growth and survival graphs: a method for evaluating process-based forest growth models

We investigated the relationships within forest stands between tree size an d (a) stem volume growth rate and (b) risk of mortality for individual tree s. Values of both x and y variables were plotted relative to the the larges t value in the stand. We refer to the resultant presentations as relative v olume growth and relative survival graphs (VGSs). A pair of VGSs can be pro duced readily from an individual-tree growth model. It can also be construc ted from consecutive sets of field measurements. Comparison of VGSs derived from model and measurement data provides a test of the validity of the com ponents of the growth model. We have analyzed VGSs based on measurement data for Scots pine (Pinus sylve stris L.) in central Finland and for beech (Fagus sylvatica L.) in southern Germany. The graphs based on measurement data varied as a consequence of d ifferences in competition, stand management, and tree species. We analyzed the relationship between VGSs and stand dynamics using a simple growth model. We found that different features of the VGSs imply character istic tree size distributions in subsequent years. Thus, we conclude that i f the VGSs generated by a model do not correspond to those based on field m easurements, the model cannot be relied on to reproduce the development of tree-size distribution correctly. Relative growth and survival graphs thus provide a tool for evaluating complicated growth models.