COMPETITIVE EFFECTS AND COMPETITIVENESS IN ANNUAL PLANT STANDS .2. MEASUREMENTS OF PLANT-GROWTH AS INFLUENCED BY DENSITY AND RELATIVE-TIME OF EMERGENCE

This second paper reviewing the author's basic research on competitive effects in plant stands presents studies of the biomass production ov er time. Plant responses to density and relative time of emergence in different stands are investigated in pot and box experiments. Principl es shown have relevance to annual crop and crop-weed stands. Measureme nt methods are in focus as a continuation of discussions in ''Paper 1' ' (Hakansson, 1997). Growth is described by graphs representing regres sion equations based on dry weights of aerial shoots measured at diffe rent times. From these equations, growth rate (GR) and relative growth rate (RGR) and other biomass ratios, such as the unit production rati o (UPR), are calculated as functions of time (in days or temperature s ums). Rather soon after emergence, aerial shoot growth is affected by competition. With an increased intra- and/or interspecific competition resulting from increased plant densities, GR reaches its maximum earl ier and RGR decreases more rapidly with the progress of time in early periods. In many respects, a delayed emergence of ''B plants'' in a st and of ''A plants'' affects growth similarly to an increased density o f A. Such effects of competition should be considered when plants are to be characterized regarding their growth rhythm. The growth reductio n rate (RR), i.e. the reduction of the biomass growth per unit of time , reaches its maximum rather early, near the times of GR maxima, i.e. at times depending on densities. Biomass ratios such as ''Y(rel.)'' an d UPR show that the regulation of biomass proportions between differen t plants in a stand caused by competition mainly occurs in a rather co ncentrated period before and up to GR maxima. This phenomenon is of gr eat interest for the choice of suitable times of measuring competitive effects and ought to be studied in more detail. Problems in judging t he ''need for weed control'' in a particular crop by predicting the yi eld loss on the basis of early assessments of the crop's and weeds' le af cover, or the like - reflected here by measurements of biomasses - are discussed. It is argued that interest should be increasingly focus ed on knowledge allowing long-term predictions of the occurrence and i mpact of different weeds in different cropping and management systems.