Interaction of seed size with light quality and temperature regimes as germination cues in 10 temperate pioneer tree species

Abstract

Pioneer species use temperature fluctuations and light level as gap detection signals, but the relative importance of these two factors may differ among species with differently sized seeds. We monitored changes in these environmental signals at different soil and litter depths in gaps and the adjacent forest understorey. We conducted a seed germination experiment in growth chambers to test the effects of darkness and red to far-red (R:FR) light ratios of 0·1, 0·4 and 1·0 under fluctuating (30/10 °C day/night) and constant (20 °C) temperatures; the experimental subjects belonged to 10 pioneer tree species with different seed sizes. In forest gaps, light is a germination cue that is detectable only at or within 2 mm below the soil surface when litter is absent, whereas temperature signals can be detected in deeper soil. The proportion of seeds germinating (percent germination) in small-seeded species (seed mass ≤1·05 mg) was strongly promoted by higher R:FR ratios, thereby ensuring germination of seeds close to the soil or litter surface. In contrast, the percent germination of large-seeded species (seed mass ≥200 mg) was strongly facilitated by temperature fluctuations, ensuring germination of seeds buried in even deep soil and litter. We detected interactive effects of the two signals in three species with medium-sized seeds (2·84–9·50 mg). Across the temperate pioneer trees tested, the relative importance of light versus temperature regimens (as germination cues for gap detection) was strongly related to seed size.