Competition for Light between Annual Species of Trifolium during the Vegetative Phase

The goal of the investigation was the evaluation of the relationship of leaf area and position to dry matter production by the species components of mixed (two species) communities in contrast to their production in simple (single species) communities. The species selected for study were the closely related winter-annual legumes, Trifoliur subterraneum L. (subterranean clover), T. hirtum All. (rose clover), and T. incarnatum L. (crimson clover). These three clovers are quite compatible under field conditions when sown together for pasture (Williams, Love, and Berry 1957). Their relative amounts fluctuated widely, however, during successive years under conditions of natural regeneration. Since the factors underlying these fluctuations must be of considerable ecological import, a series of investigations was initiated to study the influence of mineral nutrition, temperature, moisture, light, and other factors on the competitive ability of these three species. Species interactions were determined in relation to natural light, while all other factors were kept as favorable and as constant as possible among the communities and the species within communities. Black (1957) and Donald (1961), in recent reviews of work on competition for light in crops and pasture communities, emphasized the importance of solar radiation in determining productivity. Light is of major importance to the vegetative productivity of herbaceous legumes, which are considered to be heliotropes, i.e. plants requiring high light intensity for rapid growth (Black 1957). A large and significant part of the winter annual's life cycle, however, extends through periods of short days and a high proportion of cloud cover. The competition experiment was therefore conducted under these circumstances in the Mediterranean-type climate of Adelaide, South Australia. The experimental site was selected because of its similarity to the low elevation, intermediate rainfall range areas of Californiia.