Effects of density on monospecific stands of marine algae

Abstract

Current ideas on intraspecific competition between plants in monospecific stands stem from work on terrestrial floras. The central tenet is that higher plant density exacts a price from the individuals in a stand: (1) total yield (biomass) per area reaches a constant value at high density (the law of constant final yield^1–5); (2) the weight distribution of individuals becomes skewed, with few large and many small plants^1,5–8; (3) survival rates may be reduced when plants are sown at higher densities^1,9. The effects of density seem to be remarkably similar for primitive as well as higher plants^1,2,9–11 and are often summarized by the 3/2 thinning law^{1,2,5–7,9–11}, which indicates that if a stand of plants is sampled through time, the relationship between log weight and log density is a line of −1.5 slope. The basis of this is that mortality is density dependent, so that density is related to time through the thinning process due to crowding. We report here that these effects do not obtain for two species of subtidal marine algae. For the characters which we examined—total yield, plant length, plant dry weight and dry weight of reproductive parts—algae fared better at higher densities. We conclude that there is a difference between marine and terrestrial plants in their responses to density, and we suggest a difference in herbivory due to arthropod–plant associations in each system.