ON THE ANALBIS OF SELF‐THINNING AMONG SEAWEEDS¹

Abstract

Plant self-thinning is the process that involves density-dependent mortality of plants that are actively growing in crowded conditions in a monospecific, even-aged stand (Weller 1987). This process has mainly been studied in terrestrial plants, but evidence that it occurs among seaweeds is also available (Black 1974, Dean et al. 1989, Reed 1990, Ang and De Wreede 1992, Creed 1995. A recent note presented an analysis of self-thinning as it occurs for two seaweeds: the kelp Phylluriopsis puqurascens ( C . Agardh) Henry et South and the red species Asparagopsis armata Harvey. The analytical approach that was followed in that study was unfortunately inadequate in part, but it could be improved upon. The problems, which will be outlined below, have already been discussed in the ecological literature, yet awareness among ecologists, including those acting as either authors or manuscript reviewers, does not seem to be widespread. The objective of this comment is to identify and discuss these problems and to suggest a better analysis of self-thinning among seaweeds that could help improve future efforts.The first problem relates to the appropriate choice of variables that are used to detect and describe self-thinning through correlation and regression analyses. Originally, Yoda et al. (1963) examined the temporal variation of the relationship between the logarithm of mean plant biomass and the logarithm of plant density for crowded stands during active growth, and subsequent authors followed that choice (see a list of relevant papers in Weller 1987). However, Weller (1987) later showed that this approach presents problems for the interpretation of data, suggesting that total stand biomass (expressed per unit area) be used instead of mean plant biomass.For example, if 1) small plants die and the growth of large plants just barely compensates for that loss of biomass (i.e. at the end of the growth season, when growth rates slow down), stand biomass will not vary while plant density decreases. If 2) later the growth of large plants stops and their biomass remains constant for a while, but some small plants are still dying, stand biomass will be negatively related to plant density. conditions 1 and 2 do not represent active growth of plants. Active growth occurs during self-thinning and involves the highest possible growth rates of plants, which depend on plant size and can be constrained by intraspecific competition, but do not include senescence. If stand biomass and plant density are involuntarily measured during those two stages, a posteriovi data editing should allow one to identify and eliminate those points from statistical analyses of correlation and regression between stand biomass and plant density (Weller 1987). However, for conditions 1 and 2, the variable "mean plant biomass" (calculated by dividing total biomass by plant density, regardless of whether one measures them for the entire stand or for a random sample with fixed size), will actually increase while plant density decreases. This will lead one to belie...

SEASONAL CHANGES IN SIZE STRUCTURE OFSARGASSUMANDTURBINARIAPOPULATIONS (PHAEOPHYCEAE) ON TROPICAL REEF FLATS IN THE SOUTHERN RED SEA1

“…and there is no decrease in the slope of the relationship as the growth season advances and total biomass increases. Self-thinning has been reported in artificial and natural stands of some large canopy algae (Creed 1995, Flores-Moya et al 1996, 1997, Creed et al 1998, Arenas and Fernández 2000, Steen and Scrosati 2004) but appears to be usually absent among the fronds of clonal algae (Santos 1995, Scrosati and De Wreede 1997, Scrosati and Servière-Zaragoza 2000. Rivera and Scrosati (2008) recently hypothesized that the occurrence of self-thinning in clonal species may depend on thallus architecture, self-thinning being more likely in species with small holdfasts and large upright fronds than in species with large holdfasts and relatively small fronds.…”

“…Differences in growth rate between recruits and regenerating plants may also play a role in deciduous species (e.g., Ang 1985, Arenas and Fernández 2000). Subsequently, size inequality decreases as self‐thinning sets in, due to higher mortality rates of small plants, although self‐thinning may be absent in most clonal species (reviewed by Scrosati 1997, 2000, 2005, Rivera and Scrosati 2008). In dense populations of higher plants, plastic growth responses may counteract the development of size hierarchies.…”

“…PCA was chosen for reasons of consistency. This technique has been recommended for analyzing log‐total biomass versus log‐density relationships, which were also determined in this paper (see below) (Weller 1987, Scrosati 1997). Confidence limits (95%) for the slope of the relationship were calculated, and slopes for different months were compared according to Sokal and Rohlf (1995).…”

RAMET DYNAMICS FOR THE CLONAL SEAWEED PTEROCLADIELLA CAPILLACEA (RHODOPHYTA): A COMPARISON WITH CHONDRUS CRISPUS AND WITH MAZZAELLA CORNUCOPIAE (GIGARTINALES)

“…Negative relationships were observed, however, between mean ramet biomass and ramet density for a few herbs for some time intervals (Hutchings 1979), but not all of the existing ramets were considered in his analyses. Additionally, for negative relationships, increases of mean ramet biomass may erroneously indicate that ramets are growing when they may not, the use of stand biomass being a better alternative (Weller 1987a, Scrosati 1997). The negative relationships between mean ramet biomass and ramet density reported for ramets of an additional herb for some time intervals (Kays and Harper 1974) were accompanied by the mortality of genets.…”

“…Testing for self‐thinning may be done by analyzing the temporal variation of the relationship between plant (or ramet) density and stand biomass (Weller 1987a), which can be referred to as the dynamic biomass–density relationship. Using mean plant (or ramet) biomass instead of stand biomass, as done frequently in the past, has some potential problems that may compromise the conclusions of analyses (Weller 1987a, Scrosati 1997). The magnitude and the sign of the slope of dynamic biomass–density relationships give information on the type and on the intensity of interactions among individuals (or ramets) during growth, suggesting ecological differences when slopes differ (Zeide 1985, Weller 1987a, b).…”