ABSTRACT
Leaf-cutter ants (LCAs) are widely distributed and alter the physical and biotic architecture above and below ground. In neotropical rainforests, they create aboveground and belowground disturbance gaps that facilitate oxygen and carbon dioxide exchange. Within the hyperdiverse neotropical rainforests, arbuscular mycorrhizal (AM) fungi occupy nearly all of the forest floor. Nearly every cubic centimeter of soil contains a network of hyphae of Glomeromycotina, fungi that form arbuscular mycorrhizae. Our broad question is as follows: how can alternative mycorrhizae, which are-especially ectomycorrhizae-essential for the survival of some plant species, become established? Specifically, is there an ant-mycorrhizal fungus interaction that facilitates their establishment in these hyperdiverse ecosystems? In one lowland Costa Rican rainforest, nests of the LCA Atta cephalotes cover approximately 1.2% of the land surface that is broadly scattered throughout the forest. On sequencing the DNA from soil organisms, we found the inocula of many AM fungi in their nests, but the nests also contained the inocula of ectomycorrhizal, orchid mycorrhizal, and ericoid mycorrhizal fungi, including Scleroderma sinnamariense, a fungus critical to Gnetum leyboldii, an obligate ectomycorrhizal plant. When the nests were abandoned, new root growth into the nest offered opportunities for new mycorrhizal associations to develop. Thus, the patches created by LCAs appear to be crucial sites for the establishment and survival of shifting mycorrhizal plant-fungal associations, in turn facilitating the high diversity of these communities. A better understanding of the interactions of organisms, including cross-kingdom and ant-mycorrhizal fungal interactions, would improve our understanding of how these ecosystems might tolerate environmental change.
ABSTRACT
The neotropical liana Gnetum leyboldii Gnetaceae is a gymnosperm that resembles angiosperms in wood anatomy, overall morphology, and seed dispersal mechanism. Like other woody lianas, seedlings germinate in the shaded forest understory and start climbing towards the canopy, being eposed to sites with etreme differences in light conditions. However, the etent of physiological and structural adjustment to contrasting light conditions in the early regeneration stages of Gnetum is unknown. To answer this question, we analyzed seedling growth and photosynthetic responses using a common garden eperiment with two light regimes: full sun and low light 20 of full sun at La Selva Biological Station, Costa Rica. We also characterized the germination pattern of this species. We monitored one and half-month old seedlings for four months. Leaf structure finely adapted to light treatments, but gas echange properties were buffered by large seed reserves, which dominated biomass distribution about 50 of the total biomass, followed by stem 27, leaf 16 and root biomass 6 across light conditions. The presence of large seeds and the low photosynthetic rates of seedlings in both environments show that G. leyboldii is specialized to eploit deep shade. More research is needed to determine if the patterns found in G. leyboldii are typical of similar lianas that initially eploit deep-shaded understories in their ascension to the canopy.
La liana neotropical Gnetum leyboldii Gnetaceae es una gimnosperma que se asemeja a las angiospermas en la anatomía de la madera, morfología general de la planta y mecanismo de dispersión de semillas. Al igual que otras lianas leñosas, las plántulas se regeneran en el sotobosque bajo dosel cerrado y eventualmente ascienden hacia el dosel, eplotando sitios con diferencias etremas en condiciones lumínicas. Se desconoce el grado de ajuste fisiológico a condiciones lumínicas contrastantes en las primeras fases de regeneración de Gnetum. Para contestar esta pregunta, analizamos las respuestas de crecimiento de las plántulas a ambientes contrastantes de luz de sol y sombra en un jardín común con condiciones de alta cielo abierto y baja luminosidad 20 del ambiente de sol en la Estación Biológica La Selva, Costa Rica. También caracterizamos su patrón de germinación. Monitoreamos plántulas de 1.5 meses de edad por 4 meses. La estructura foliar mostró una fina adaptación a los tratamientos de luz, pero las propiedades de intercambio gaseoso no cambiaron sino que fueron amortiguadas por las reservas de las semillas grandes, las cuales dominaron la distribución de biomasa aproimadamente 50 de la biomasa total seguidas por el tallo 27, la hoja 16 y raíces 6. El tener semillas grandes y plántulas con bajas tasas fotosintéticas muestra que G. leyboldii en su etapa de plántula está adaptado para eplotar la sombra profunda. Se requiere más investigación para determinar si los patrones encontrados en G. leyboldii son típicos de otras lianas que inicialmente eplotan la sombra profunda en su ascensión al dosel.