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

世界各地的许多山脉都有一种独特的植被。这包括一个以橡树或相关物种为主的森林树冠，以及一个被茂密的竹子灌木丛填满的林下层。这些竹子灌木丛投下的深荫，加上竹子对水分和养分的竞争，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同寻常的特征--它们一生中开过一次花，然后就死了。根据物种的不同，每隔20到120年，一个地点的整个竹子种群就会一起开花。在这段大量开花和大量死亡的插曲之后，光线穿透到林地，刺激树苗生长几年，然后竹子才回来。然而，枯死的竹子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获得性，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，橡树林是一种具有关键保护价值的濒危生态系统。具体地说，它将检验这样一个假设，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定类型的联系，称为外生菌根。外生菌根真菌使橡树受益，因为它提供了一种其他竞争物种无法获得的氮源。了解山地橡树林的功能对于它们的管理和预测海拔较低的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。该项目将利用现有的地块网络，这些地块建立在沃尔坎·巴鲁国家公园山坡上的橡树-竹林中。在这些地块中绘制了竹子、橡树和其他树种的地图，每年都在监测树木的生长速度，并已经测量了土壤氮素的有效性。除了确定竹子的死亡如何影响这些现有林地的苗木补充，以及幼树和成年树的生长，还将建立一个氮素添加实验，以测试橡树的补充是否由于它们的外生菌根联系而选择性地受到青睐。重复的森林斑块将位于具有类似乔木群落组成的位置，但有三类林下竹子：活竹子、死竹子和没有竹子。两个当地优势的外生菌根栎树种Quercus salicifolia和Q.costaricens的种子和幼苗，以及四个类似的非外生菌根树种将被移植到这些斑块的地块中。此外，还将在每个地块设置氮肥和对照处理，并对苗木生长进行为期一年的监测。如果外生菌根氮供应有利于竹子腐烂期间的橡树，那么在有枯死的竹子和没有额外氮素的地块中，橡树相对于非橡树的苗木生长优势最大，在没有施氮肥的竹子地块中最小。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.