Nitrogen nutrition during ontogeny of hemiepiphytic Clusia species

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. We investigated the nitrogen (N) nutrition of Clusia osaensis, C. peninsulae and C. valerii during the seedling, epiphytic and hemiepiphytic phase in a lowland tropical forest in Costa Rica in order to elucidate nutritional adaptations of different plant growth stages to their habitat. Although all Clusia individuals were non-mycorrhizal, excised roots of seedlings, but also of epiphytic and hemiepiphytic stages, showed a distinct preference for glycine uptake. The shift in main rooting site from canopy soil to terrestrial soil was not reflected by changes in uptake rate or preference, although N availability and the composition of the available N pool changed significantly. High foliar N concentrations indicated that epiphytic seedlings seemed to be sufficiently supplied with N by maternal seed resources and canopy soils. With development, the epiphytic plants of Clusia may face N limitation due to higher N demands with increasing growth and restricted resources in the tree crowns. ¹⁵N natural abundance data indicate that epiphytes mainly accessed atmospheric and, to a lesser extent, canopy soil N sources and, after becoming terrestrially anchored, in the hemiepiphytic life stage exploited the larger nutrient reservoir of the ground soil. In consequence, Clusia species did not show an adaptation to the different N availability situations that they experienced whether canopy or ground-rooted.