RAPID:Effects of mass-flowering and die-back of a large cane forming bamboo on nutrient cycling, seedling recruitment, and oak dominance in a tropical montane forest

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Many mountains around the world share a characteristic vegetation. This consists of a forest canopy dominated by oaks or related species, and an understory layer that is filled by dense thickets of bamboo. The deep shade cast by these bamboo thickets, coupled with competition from bamboo for water and nutrients, greatly limits the regeneration success of tree seedlings. However, most bamboos also share an unusual trait – they flower once in their lives and then die. At intervals of between 20 and 120 years, depending on species, an entire bamboo population at a site flower together. Following this episode of mass-flowering and mass mortality, light penetrates to the forest floor stimulating tree seedling growth for a few years before the bamboo returns. The decay of dead bamboo, however, also stimulates the growth of soil bacteria and fungi which compete with plants for nutrients in the soil. This competition reduces the availability of nitrogen, a key resource for plant growth. This project will examine how a bamboo die-off that began in 2020 in western Panama affects the regeneration of oak forest, an endangered ecosystem of critical conservation value. Specifically, it will test the hypothesis that bamboo mortality preferentially favors the regeneration of oaks, because these tree species form a specific type of association with fungi in the soil called ectomycorrhizas. Ectomycorrhizal fungi benefit oaks by providing access to a source of nitrogen that is unavailable to other competing plant species. Understanding how mountain oak forests function is essential to their management, and to predicting whether lower elevation tree species will be able to migrate upslope in a warming world. In addition, this project will provide training in forest ecology and plant-soil interactions for US undergraduate and graduate students.The project will take advantage of an existing network of plots established in oak-bamboo forest on the slopes of Volcan Barú National Park. Bamboo, oaks and other tree species have been mapped in these plots, tree growth rates are being monitored annually, and soil nitrogen availability has already been measured. In addition to determining how bamboo die-off affects seedling recruitment, and juvenile and adult tree growth in these existing forest plots, a nitrogen addition experiment will be established to test whether oak recruitment is selectively favored due to their ectomycorrhizal association. Replicate patches of forest will be located that have similar tree community composition, but that have three classes of understory bamboo: live bamboo, dead bamboo, and no bamboo. Seeds and seedlings of the two locally dominant ectomycorrhizal oak tree species Quercus salicifolia and Q. costaricensis, and four similar non-ectomycorrhizal tree species will be transplanted into plots in these patches. Additionally, nitrogen fertilization and control treatments will be established in each plot and seedling growth monitored for one year. If ectomycorrhizal nitrogen supply favors oaks during bamboo decay then the seedling growth advantage of oaks versus non-oaks will be greatest in plots with dead bamboo and no additional nitrogen, and smallest in the nitrogen fertilized no bamboo plots.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.