The most severe global warming is predicted to occur in the Amazon region of Brazil, with an estimated temperature increase of 4 to 8 degrees C between 2071 and 2100. Our study evaluated the effects of increased temperatures on germination success of seeds in the soil seed bank of secondary forests in Central Amazonia. Soil samples (3cm depth) were collected in six secondary forests with different land-use histories and ages of regrowth. Soils from all sites were sieved and mixed, and germination and seedling development were assessed during 4 months of incubation in germination chambers at (1) a constant temperature of 25 degrees C and a 12:12 h alternating temperature regime of 20:30 degrees C, considered as controls; (2) a simulated 5 degrees C temperature increase to 30 degrees C and 25:35 degrees C and (3) a 10 degrees C temperature increase to 35 degrees C and 30:40 degrees C. To determine the effect of light vs. dark on germination, germination was compared for samples in light (12 h daily photoperiod) vs. continuously dark for 1 month, followed by 3 months in light. Six temperature conditions were tested with 30 subsamples each. In the mixed soil seed bank, 3640 seeds germinated, of which 18 families, 28 genera and 41 species (grouped in 20 trees, 11 shrubs, eight herbs and two lianas) could be identified. A light requirement for germination was detected for 94-97% of the species. Alternating temperatures did not stimulate germination in the dark. Seedling density m- 2 was 1.7 to 2.3 times higher at alternating (20:30 degrees C and 25:35 degrees C) than at constant temperatures (25 degrees C and 30 degrees C); but an increase of 10 degrees C decreased seedling density to 47% at 35 degrees C and 26% at 30:40 degrees C compared to control (100% at 25 degrees C and 20:30 degrees C); and furthermore increased seedling mortality. Thus, the secondary forest at the study area may maintain its integrity with a 5 degrees C but not with a 10 degrees C temperature increase. Species-specific sensitivities to temperature were detected. Temperature may function to fine-tune germination at a specific microsite, and the variety of germination requirements may enhance coexistence and maintenance of species diversity within the pioneer species subset. In general, seeds of tree species were more sensitive to increased temperatures than those of shrubs and herbs, supporting the hypothesis of replacement of the tropical forest by savanna-like vegetation in the Amazon region. Our results show that natural regeneration may be significantly affected by global warming and with potential to alter floristic composition.