Acorn characteristics, seed germination, seedling development, and leaf traits of three oak species from Talamanca, Costa Rica

Three oak species dominate the mature montane cloud forests in the Talamanca mountain range, Costa Rica. These oak species exist at different elevations and encounter increasing soil and atmospheric stress conditions as elevation increases along a gradient (2,000-3,200 m). We compared acorn characteristics, seed germination, seedling morphology, and leaf morphology of these three oak species to determine if these traits varied along this elevation gradient. Seeds were measured, weighed, and examined for viability and germination. Seeds were planted in a common garden at 2,700 m to evaluate seedling development, morphology, and growth. We found that seeds had high water content and germination patterns typical of the recalcitrant syndrome. Quercus costaricensis acorns were bigger and had higher viability and seed germination than acorns and seeds of Quercus salicifolia and Quercus bumelioides. However, seeds from the smaller acorns of Q. salicifolia and Q. bumelioides were not less prone to germinate than Q. costaricensis. Germination was of the phanerocotylar-semihypogeal-reserve type, consistent with the temperate origin of the genus. The developing seedlings produced a leafy crown and a vigorous adventitious root system soon after germination. Together, these new organs allow seedlings to substitute or complement the resources provided by the fleshy cotyledons. Leaves of Q. costaricensis seedlings (from the highest elevation) were smaller, darker, and thicker than those of Q. bumelioides and Q. salicifolia. However, there were no differences in leaf mass per area among seedlings of different species. We discuss the importance of interspecific differences in seedling growth resulting from seed size differences and environmental heterogeneity for niche segregation and the observed distribution of Q. salicifolia, Q. bumelioides, and Q. costaricensis along the elevation gradient.