The use of synchronization measures in studies of plant reproductive phenology

Phenological synchronization of plant reproduction has generated a lot of interest during the last decades. Being an important character in the interactions between plants and their pollinators, dispersers, predators or herbivores, several adaptive hypotheses have addressed variation in synchronization (Rathcke and Lacey 1985, Primack 1987, Gorchov 1990; but see Ollerton and Lack 1992). Synchronized flowering or fruiting could satiate predators (Augspurger 1981) as well as attract pollinators and dispersers through its mass display effect (Rathcke and Lacey 1985). The level of within-individual and between-individual synchronization could also be related to fruit quality through the level of outcrossing (Stephenson 1982, Bawa 1983, Primack 1987). Also lack of synchrony may be interpreted as adaptive. In accordance with Gillespie's (1977) ideas on risk spreading, asynchronous reproductive phonology would be a way to hedge one's bets within the season. By opening flowers or ripening fruits asynchronously the plant could track the resources of pollinators, dispersers or any abiotic key-factor. Asynchronous flowering/fruiting has also been interpreted as a strategy to escape predators (Eriksson 1995). Willson and Thompson (1982) proposed bicoloured fruit displays as attractive to dispersers, thereby enhancing seed dispersal. For species with temporally bicoloured fruit ripening, asynchronous ripening would be a prerequisite to accomplish such an attractive display. Patterns of synchronization could be described both at the between-individual and within-individual levels. As adaptive advantages of between-individual synchronization could be confounded by within-individual patterns of synchronization we need to investigate the connection between these two levels. Earlier studies have almost always focused on between-individual synchronization and have thus become concerned with early, late and intermediate, or onand off-peak individuals (Augspurger 1981, Dieringer 1991; but see Marquis 1988, Gorchov 1990). That is, they were more interested in the mean timing of the phenophase than the distribution within the phenological phase. Some studies have used a quantification of betweenindividual synchronization based on flowering overlap (Augspurger 1983, Marquis 1988, G6mez 1993), but none of these quantitative measures has the ability to distinguish between different within-individual synchronization levels. As such patterns could be important for the reproductive success of plants we need to develop quantitative measures of within-individual synchronization. In their review of plant phenology, Rathcke and Lacey (1985) put synchronization on equal footing as variance. This straightforward approach was used by one of the three studies I have found that have quantified within-individual synchronization (Gorchov 1990). In his study of flowering and fruiting in 12 insect-pollinated and fleshy-fruited species, Gorchov (1990) used the standard deviation of onset of individual flowers/fruits (SDonset) as his synchronization measure. Any biological effect of such a dispersion could be confounded by variation in flower/fruit persistence. If flower/fruit persistence is extended, any effect of variation in the level of dispersion may be negligible. The two other studies using a quantified measure of synchronization do include flower/fruit persistence in their studies (Eriksson and Ehrlen 1991, Bolmgren 1997). Eriksson and Ehrlen (1991) compared the fruiting phenology of 34 fleshy-fruited species and they calculated the individual's level of synchronization as the quotient (fruit persistence/fruiting range). Obviously, such a measure does not differ between individuals with high and low SDonsetI Both of the parameters omitted by Gorchov (1990) and Eriksson and Ehrlen (1991), persistence and dispersion of onset, respectively, were included by Primack (1985a) in his general description of flowering phenol-