Size-dependent reproductive pattern and short-term reproductive cost in Rumex obtusifolius L Joan Pino a,b, *, F. Xavier Sans a , Ramon M. Masalles a a Departament de biologia vegetal, Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain b Centre de recerca ecològica i aplicacions forestals (CREAF), Universitat autònoma de Barcelona, 08193 Bellaterra, Spain Received 4 January 2000; accepted 5 July 2002 Abstract This paper analyses the size-dependent reproductive pattern of Rumex obtusifolius L. growing in lucerne crops (Medicago sativa L.), and its importance in determining the existence of a short-term reproductive cost. Size effects on reproductive pattern were evaluated by deter- mining the role of plant size at the time of first reproduction, and the size-dependency of flowering probability (estimated as the proportion of flowering plants), plant fecundity, and reproductive effort (estimated as the ratio between reproductive and vegetative biomass). These param- eters were recorded over the reproductive episodes determined by crop harvesting during the reproductive period. The results showed that size was much more important than age in determining time of first reproduction. Seed output decreased progressively over the reproductive period, probably in relation to an increasing short-term reproductive cost caused by a gradual depletion of plant resources. Probability of flowering over the successive reproductive episodes increased with plant size. The allometric relationship of vegetative versus reproductive biomass indicated that reproductive biomass increased less sharply than vegetative biomass and, consequently, reproductive effort decreased as plant size increased. Assuming a direct relationship between reproductive effort and reproductive cost, the size-dependent flowering prob- ability could reflect, in turn, the existence of a size-dependent reproductive cost that would decrease as plant size increased. Ecological impli- cations of these results are discussed. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Reproductive effort; Reproductive allocation; Mowing 1. Introduction The completion of a plant life cycle and the regeneration of plant populations depends largely on the process of repro- duction. There is a great deal of variation among higher plants in the quantity, frequency, and timing of reproduction, from semelparous of variable life-spans to iteroparous that reproduce at regular or intermittent intervals. These varia- tions in life-history are influenced by both ecological and evolutionary factors, and have stimulated a great deal of re- search in plant biology in the last three decades. The evolution of these life-histories is presumed to be dictated in part by limitations in the partitioning of finite re- sources among different functions (Snow and Whigham, 1989), meaning that current reproduction may occur at the expense of future survivorship, vegetative growth, and fu- ture reproduction. Subsequent effects of reproduction, called generically reproductive costs, have been demonstrated in several iteroparous species (Snow and Whigham, 1989; Reekie and Bazzaz, 1992; Obeso, 1993). However, other studies have failed to establish any relationship between cur- rent reproduction and future costs (Horwitz and Schemske, 1988; Calvo, 1990; Karlsson et al., 1990). In some species, this relationship has only been observed after increasing re- productive success by hand-pollination (Ackerman and Montalvo, 1990; Primack and Hall, 1990; Primack et al., 1994) or after subjecting individuals to stress by defoliation (Snow and Whigham, 1989; Primack et al., 1994). The costs of reproduction may also vary with individual variation in plant traits. In the case of iteroparous plants, size appears as a key factor in determining the reproductive pattern of many species. It has been suggested that selection in these plants acts primarily on traits determining size at first reproduction (Weaver and Cavers, 1980), and size- dependent flowering probability and reproductive allocation * Corresponding author. E-mail address: joan.pino@cc.uab.es (J. Pino). Acta Oecologica 23 (2002) 321–328 www.elsevier.com/locate/actao © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 1 4 6 - 6 0 9 X ( 0 2 ) 0 1 1 6 1 - X