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

世界上许多山脉都有一种特有的植被。这包括一个以橡树或相关物种为主的森林冠层，以及一个由茂密的竹林填充的下层。这些竹林投下的阴影很深，再加上竹子对水和养分的竞争，极大地限制了树苗的再生成功。然而，大多数竹子也有一个不寻常的特征——它们一生只开一次花，然后就会死去。每隔20至120年，根据不同的品种，一个地点的整个竹种群会一起开花。在竹子大量开花和大量死亡之后，光线穿透到森林地面，在竹子恢复之前的几年里刺激了树木幼苗的生长。然而，死竹的腐烂也会刺激土壤细菌和真菌的生长，这些细菌和真菌与植物争夺土壤中的养分。这种竞争减少了氮的可用性，氮是植物生长的关键资源。该项目将研究2020年在巴拿马西部开始的竹子死亡如何影响橡树林的再生，橡树林是一种具有重要保护价值的濒危生态系统。具体来说，它将检验一个假设，即竹子的死亡优先有利于橡树的再生，因为这些树种与土壤中的真菌形成了一种特殊类型的联系，这种联系被称为外生菌根。外生菌根真菌通过提供其他竞争植物无法获得的氮源而使橡树受益。了解山地橡树林的功能对它们的管理，以及预测低海拔树种是否能够在变暖的世界中向上迁移至关重要。此外，该项目将为美国本科生和研究生提供森林生态学和植物-土壤相互作用方面的培训。该项目将利用火山Barú国家公园斜坡上橡树竹林中现有的地块网络。竹子、橡树和其他树种已经在这些地块上绘制了地图，树木的生长速度每年都在进行监测，土壤氮的可用性也已经得到了测量。除了确定竹子死亡如何影响这些现有森林样地的幼苗招募以及幼树和成树生长外，还将建立一个氮添加实验，以测试橡树的外生菌根关联是否选择性地有利于招募。森林的复制斑块具有相似的树木群落组成，但有三种下层竹：活竹、死竹和无竹。将两种当地优势外生菌根栎树（Quercus salicifolia）和栎（Q. costaricensis）以及4种类似的非外生菌根栎树的种子和幼苗移植到这些斑块的地块中。此外，将在每个地块建立氮肥和对照处理，并监测幼苗生长一年。如果说在竹材腐烂过程中外生菌根氮的供给有利于栎树的生长，那么在竹材死亡且不施氮的情况下，栎树幼苗的生长优势最大，而在不施氮的情况下，栎树幼苗的生长优势最小。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.