Heat shock, mass-dependent germination, and seed yield as related components of fitness in Cistus ladanifer

The different weight-number strategies of seed production displayed by indi viduals of a Mediterranean fire-prone plant species (Cistus ladanfier) 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 degreesC during 5 min) and a high temperature during a long expo sure time (100 degreesC during 15 min) were applied to seeds from different individual plants with different mean seed weight. These pre-germination t reatments resemble natural germination scenarios for the studied species, a bsence 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 th e control treatment. Mean seed weight was positively correlated to the prop ortion of germinated seeds but only within heat treatments. These results s uggest that in periods without fire, the relative contributions to the popu lation dynamics are equal for all seeds, regardless of their mass, whereas heavier seeds would be the main contribution after wildfire events. Since l ighter 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 producin g lighter seeds gave rise to more seedlings; nevertheless, they were numeri cally exceeded by those producing heavy seeds after a wildfire. The implica tions of these results are discussed in relation to their consequences on t he population dynamics of this species, considering also additional informa tion on stand flammability and changes in seed mass with plant age. (C) 200 1 Elsevier Science B.V. All rights reserved.