RAPID: Interactive effects of wildfire and severe drought on plants, soil microbes and C storage in a semiarid shrubland ecosystem

像世界上许多半干旱的生态系统一样，南加州受到越来越频繁和强烈的野火和干旱的影响。这些极端事件可能导致生态系统的长期变化，因为它们允许入侵杂草取代原生灌木，破坏土壤的物理结构和生活在土壤中的细菌和真菌等微生物（微生物）群落。这些对植物、微生物和土壤的变化会影响整个生态系统的运作方式，包括生态系统从大气中吸收二氧化碳（CO2）的效率，以及碳作为有机物长期储存的效率。土地吸收和储存碳的能力非常重要，因为它可以减缓大气中二氧化碳的积累，减缓其对气候的影响。计算机模型预测未来生态系统的变化将如何影响大气中的二氧化碳。然而，没有足够的信息来自信地预测野火和干旱将如何改变南加州灌木丛等半干旱生态系统。研究野火和干旱的综合影响需要进行实验，在其他类似的景观部分被烧毁或未被烧毁，并且可以控制实验地块的降雨量。出于安全考虑，规定的燃烧是在与自然野火非常不同的条件下进行的，并且可能代表真实的野火将如何影响生态系统。另一方面，野火是不可预测的。这使得计划一个实验变得困难，因为实验需要可比较的燃烧和未燃烧的景观、进入地点、火灾前区域的初步信息以及在季节性降雨开始之前建立避雨棚的时间。这个NSF RAPID项目利用了这样一个机会：一场野火烧毁了一个被充分研究过的自然保护区的一部分，如果立即采取行动，就可以在主要雨季之前建立降雨实验。本项目利用了一个难得的、时间敏感的机会，在因子设计中将自然野火与控制降雨实验结合起来，并将数据纳入一个改进的模型，用于预测半干旱生态系统对偶发性和渐进性气候变化的响应。该RAPID项目允许在季节性降雨开始之前，在被烧毁和未被烧毁的地区建造降雨避难所。该项目验证了以下假设：火灾和干旱将相互作用，促进草和其他草本物种的入侵，并导致1)生态系统碳积累的物候提前和生长季节缩短；2)生态系统通过土壤结构和微生物群落的变化，增加了对碳和氮损失的脆弱性。因此，火灾和干旱将共同促进生态系统C储量的减少。该项目将(1)建立因子干旱×野火实验，(2)测量生态系统（包括植物和土壤成分）对因子处理的响应，(3)组织这些数据，将其纳入千禧年模型和社区地球系统模型（CESM），从而对美国西南部和世界其他半干旱生态系统进行预测。C和N将在土壤物理组分（不同大小类别的团聚体，颗粒有机质，微生物生物量等）中进行测量，微生物群落结构将使用霰弹枪宏基因组测序进行表征。植物测量将包括在冠层和叶片水平的群落组成、物候和生态生理测量，以及在景观水平的遥感图像。生态系统建模活动将把这些数据放入生态系统级框架中。这个为期一年的项目将为长期研究奠定基础。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。

Like many semiarid ecosystems around the world, southern California is subject to increasingly frequent and intense wildfires and droughts. These extreme events can cause long-term changes in the ecosystem by allowing invasive weeds to replace native shrubs and disrupting the physical structure of soil and the communities of microscopic organisms (microbes) like bacteria and fungi that live in the soil. These changes to plants, microbes and soils can affect how the whole ecosystem works, including how effectively the ecosystem can absorb carbon dioxide (CO2) from the atmosphere and store carbon for a long time as organic matter. The ability of land to absorb and store carbon is very important because it can slow down the accumulation of CO2 in the atmosphere, slowing down its effects on the climate. Computer models predict how future changes in ecosystems will influence atmospheric CO2. However, there is not enough information to make confident predictions of how wildfire and drought will change semiarid ecosystems like southern California shrublands. Studying the combined effects of wildfire and drought requires experiments where otherwise similar parts of the landscape are either burned or unburned, and the amount of rain falling on an experimental plot can be controlled. For safety reasons, prescribed burns are carried out under conditions that are very different from natural wildfires, and might represent how real wildfires will affect ecosystems. On the other hand, wildfires are unpredictable. This makes it hard to plan an experiment that requires comparable burned and unburned landscape, access to the location, preliminary information about the area before the fire, and the time to establish rain shelters before the onset of seasonal rains. This NSF RAPID project takes advantage of such an opportunity, in which a wildfire burned part of a well-studied nature preserve, and a rainfall experiment could be established before the main rainy season, given immediate action.This project takes advantage of a rare and time-sensitive opportunity to combine a natural wildfire with a controlled rainfall experiment in a factorial design and incorporate the data into an improved model for predicting the response of semiarid ecosystems to episodic and gradual aspects of climate change. This RAPID award allows rainfall shelters to be constructed in burned and unburned areas prior to the onset of seasonal rains. The project tests the hypotheses that fire and drought will interact to promote invasion by grasses and other herbaceous species, and cause 1) earlier phenology of ecosystem C gain as well as a shorter growing season, and 2) an increase in the vulnerability of the ecosystem to loss of C and N via changes in soil structure and microbial communities. Thus, together fire and drought will synergistically contribute to lower ecosystem C stocks. This project will (1) establish a factorial drought × wild fire experiment, (2) measure ecosystem responses (including plant and soil components) to the factorial treatments, and (3) organize these data for incorporation into the Millennial Model and the Community Earth System Model (CESM), enabling predictions for both the U.S. Southwest, and other semiarid ecosystems around the world. C and N will be measured in soil physical fractions (various size classes of aggregates, particulate organic matter, microbial biomass, etc.), and the microbial community structure will be characterized using shotgun metagenomic sequencing. Plant measurements will include community composition, phenology, and ecophysiological measurements at the canopy and leaf level, and remotely sensed imagery at the landscape level. The ecosystem modeling activities will place these data into an ecosystem-level framework. This one-year project will lay the groundwork for longer-term studies.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.