Germination Responses to Water Potential in Neotropical Pioneers Suggest Large-seeded Species Take More Risks

• Published in: Annals of botany Vol. 102; no. 6; pp. 945 - 951
• Main Authors: Daws, Matthew I, Crabtree, Lora M, Dalling, James W, Mullins, Christopher E, Burslem, David F.R.P
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01.12.2008; Oxford Publishing Limited (England)
• Subjects: Biological taxonomies; Biomass; Canopies; canopy gaps; Clidemia; Desiccation; Drought resistance; drought tolerance; dry environmental conditions; Environmental risk; Forest ecology; forests; Germination; Germination - physiology; light intensity; Moisture content; mortality; neotropical; Ochroma pyramidale; Original; Panamá; physiology; pioneer; Pioneer species; Plants; Risk; Risk reduction; Seed germination; Seed size; seedling growth; Seedlings; Seeds; Seeds - physiology; Soil water; soil water content; Time Factors; trees; Trees - physiology; Tropical Climate; Water; Water - physiology; Water availability; Water potential; neotropical; water potential; pioneer; Germination; Panamá; seed size
• ISSN: 0305-7364; 1095-8290; 1095-8290
• DOI: 10.1093/aob/mcn186

BACKGROUND AND AIMS: In neotropical forests, very small-seeded pioneer species (<0·1 mg seed mass) recruit preferentially in small tree fall gaps and at gap edges, but large-seeded pioneers do not. Since water availability is related to gap size, these differences in microsite preference may reflect in part species-specific differences in germination at reduced water potentials. METHODS: For 14 neotropical pioneer species, the hypothesis is tested that small-seeded species, with shallow initial rooting depths, reduce the risks associated with desiccation by germinating more slowly and at higher water potentials than large-seeded species. KEY RESULTS: Germination occurred both more quickly and at lower water potentials with increasing seed mass. For example, Ochroma pyramidale (seed mass 5·5 mg) had a time to 50 % germination (T₅₀) of 2·8 d and a median base potential for germination (ψb₅₀) of -1·8 MPa while Clidemia quinquenervia (seed mass 0·017 mg) had a T₅₀ of 17·6 d and ψb₅₀ of -1·1 MPa. CONCLUSIONS: These data suggest that small-seeded species germinate only in comparatively moist microsites, such as small canopy gaps, which may reduce the risk of drought-induced mortality. Conversely, large-seeded species are able to germinate in the drier environment of large gaps, where they benefit by enhanced seedling growth in a high irradiance environment. The positive association of seed size and canopy gap size for optimal seedling establishment is maintained by differential germination responses to soil water availability coupled with the scaling of radicle growth rate and seed size, which collectively confer greater drought tolerance on large-seeded species.