Environmental and Experimental Botany 46 (2001) 11 – 20 Heat shock, mass-dependent germination, and seed yield as related components of fitness in Cistus ladanifer Juan A. Delgado *, Jose ´ M. Serrano, Francisco Lo ´ pez, Francisco J. Acosta Departamento de Ecologı ´a, Facultad de Biologı ´a, Uniersidad Complutense de Madrid, 28040 Madrid, Spain Received 1 September 2000; received in revised form 2 January 2001; accepted 3 January 2001 Abstract The different weight-number strategies of seed production displayed by individuals of a Mediterranean fire-prone plant species (Cistus ladanifer ) were investigated in relation to seed germination responses to pre-germination heating. A control (no heating), a high temperature during a short exposure time (100°C during 5 min) and a high temperature during a long exposure time (100°C during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination treatments resemble natural germination scenarios for the studied species, absence of fire, typical Mediterranean shrub fire, and severe fire with high fuel load. Seed germination was related to heat treatments and seed mass. Seed heating increased the proportion of seeds germinating compared with the control treatment. Mean seed weight was positively correlated to the proportion of germinated seeds but only within heat treatments. These results suggest that in periods without fire, the relative contributions to the population dynamics are equal for all seeds, regardless of their mass, whereas heavier seeds would be the main contribution after wildfire events. Since lighter seeds can be produced in higher quantities than heavier ones within a given fruit, the number of seedlings produced per fruit depended strongly on the germination conditions. In the absence of wildfire, fruits producing lighter seeds gave rise to more seedlings; nevertheless, they were numerically exceeded by those producing heavy seeds after a wildfire. The implications of these results are discussed in relation to their consequences on the population dynamics of this species, considering also additional information on stand flammability and changes in seed mass with plant age. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Evergreen Mediterranean shrub; Fire; Pre-germination heating; Reproductive allocation; Seed mass www.elsevier.com/locate/envexpbot 1. Introduction Seed mass within a plant species (if measured as mean seed weight of a seed pool per individual plant) has been reported as a relatively constant feature, even when individuals differed largely in the amount of resources, as it could be inferred from their huge differences in total weight of seeds produced (Harper et al., 1970; Fenner, 1985). This result has been generally used to support the idea that seed mass has likely evolved in response to environmental pressures (Moore, * Corresponding author. Tel.: +34-1-3945085; fax: +34-1- 3945081. E-mail address: delgado@eucmos.sim.ucm.es (J.A. Delgado). S0098-8472/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0098-8472(01)00076-4