Seedling growth and morphology of three oak species along field resource gradients and seed mass variation: a seedling age-dependent response

Question What is the relative importance of seed mass and abiotic factors in species-specific seedling growth and morphology during the first and the second growing season? How do oak species respond along gradients of these factors? Location Mediterranean oak forest in southern Spain. Methods We analysed seedling growth components and morphology of three co-occurring Quercus species (two deciduous and one evergreen). Oak seeds with a wide variety of sizes were sown along broad gradients of abiotic conditions. Intra- and inter-specific differences were evaluated by calibrating maximum likelihood estimators of seedling growth during the first two years of life. Results We found multiple resources and conditions affecting seedling morphology and biomass allocation. However, the integrative variables of seedling growth - total aboveground biomass and relative growth rate (RGR) - were affected by two main factors: seed mass and light conditions. The relative contribution of these two factors depended strongly on seedling age. Seed mass explained most of the growth and morphological variables during the first year, while light conditions were the best predictor in the second growing season. In contrast, soil factors did not play an important role in seedling growth. We found some evidence of regeneration niche partitioning between oak species along the light gradient, a reflection of their distribution patterns as adults at the study site. Conclusions We conclude that inter-specific differences in seedling growth, arising from seed size variability and microsite heterogeneity, could be of paramount importance in oak species niche segregation, driving stand dynamics and composition along environmental gradients.