An Experimental Evaluation of Density and Plant Size in Two Large Brown Seaweeds

Abstract

The effects of density on the growth rate and survival of individual plants as well as changes in population structure (hierarchy) and biomass accumulation (self-thinning) were experimentally evaluated in two brown macroalgae. Laminaria digitata (Hudson) Lamouroux and Fucus serratus Linnaeus populations were constructed at five (650-5156 plants·m Ϫ2 ) and four densities (650-2668 plants·m Ϫ2 ), respectively, and were cultivated in tanks. The relative growth rates and survivorship of individuals and the populations' biomass and density (estimated dry mass) were periodically measured. To investigate how plant population size hierarchies influence conspecifics, single density populations of L. digitata were constructed of up to three sizes of plants in equal proportions, and these parts of the populations were compared through time with plants of the three sizes grown singly. At higher density, L. digitata plants grew more slowly, while F. serratus populations showed a similar trend that was never statistically significant. Survival of plants of both species was lower at high densities, and mortality selectively removed smaller plants. Plants of both species exhibited zero growth rates before death, when parts of the fronds were lost, but meristems (apical in F. serratus, at the base of the frond in L. digitata) were preserved until the death of the plant. All singly grown L. digitata plants survived, but survivorship was low in the fractions of small plants in mixedsize populations compared with that of the largest size plant fractions. Small L. digitata plants grew relatively faster than did large ones singly, but in mixed-size populations, small plants grew relatively slowly. Plant sizes became progressively more unequal (Gini and skewness coefficients) until self-thinning started reducing the size variability. The seaweeds followed self-thinning (density-biomass) trajectories predicted by the self-thinning ''rule'', and self-thinning appeared to be seasonal-rather than species-dependent, as it occurred at a time of year when ambient light levels start to fall in the Isle of Man. Culture studies of this kind, despite their considerable potential, are a tool as yet underexploited by marine ecologists as a means of assessing intraspecific competitive interaction among seaweeds.

Effects of density on the vital rates of a modular seaweed

“…This phenomenon of dominance and suppression has been observed in other seaweed species (Reed 1990, Ang & De Wreede 1992, Creed et al 1998), but our results did not support the hypothesis. It may be argued that the temporal scale of the experiment was too short to show any differential growth among plants.…”

“…Our results conform to those of many terrestrial and marine studies, in which plants growing at higher densities experience reductions in the net growth (e.g., Black 1974, Harper 1977, Antonovics & Levin 1980 Reed 1990, Ang & De Wreede 1992, Creed et al 1996, Creed et al 1998. Marine algae are frequently subject to erosion from waves, ice or herbivores.…”

“…Asymmetrical competition between modules was also observed in terrestrial clonal plants when genet density was manipulated (de Kroon et al 1992). Light is often cited as a key factor influencing growth and survival in both terrestrial and marine systems (Schmitt et al 1986, Dean et al 1989, Reed 1990, Jurik 1991, Creed et al 1998, Sjøtun et al 1998. In Ascophyllum nodosum stands, it can actually be an important factor determining the suppression of short shoots and the regulation of shoot inception, as the irradiance levels decrease abruptly under a canopy of A. nodosum (Cousens 1985).…”

“…Considerable progress has recently been made with such experiments (Reed 1990, Ang & De Wreede 1992, Kendrik 1994, Creed et al 1996, Creed et al 1998, Lazo & Chapman 1998. However, observational studies are also valuable especially as a first attempt to investigate processes correlated with density.…”

“…Self-thinning or density-dependent mortality is common in macroalgal stands (Schiel & Choat 1980, Dean et al 1989, Kendrick 1994, Creed et al 1996 as it is in terrestrial systems (Harper 1977, Westoby & Howell 1986, Schmitt et al 1987. Algal individuals often have lower growth rates in crowded stands, and the effects are more dramatic in small plants (Reed 1990, Ang & De Wreede 1992, Creed et al 1998. Changes in the morphology of algae have likewise been reported, with plants at high density being more elongated or less ramified than at low density (Reed 1990, Holbrook et al 1991, Sjøtun & Frediksen 1995, Andrew & Viejo 1998, a response similar to that observed in terrestrial plants (Harper 1977, Silvertown & Lovett Doust 1993, Hutchings & de Kroon 1994.…”