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

与世界上许多半干旱生态系统一样，南加州也遭受着日益频繁和严重的野火和干旱。这些极端事件可能导致生态系统的长期变化，使入侵的杂草取代原生灌木，破坏土壤的物理结构和微生物（微生物）群落，如生活在土壤中的细菌和真菌。植物、微生物和土壤的这些变化会影响整个生态系统的运作方式，包括生态系统如何有效地从大气中吸收二氧化碳（CO2），并将碳作为有机物质长期储存。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，目前还没有足够的信息来预测野火和干旱将如何改变像南加州灌木丛这样的半干旱生态系统。研究野火和干旱的综合影响需要进行实验，否则类似的景观部分要么被烧毁，要么未被烧毁，并且可以控制落在实验地块上的降雨量。出于安全原因，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。这使得很难计划一个实验，需要可比的烧毁和未烧毁的景观，进入位置，有关火灾前的区域的初步信息，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，其中野火烧毁了一个经过充分研究的自然保护区的一部分，并且可以在主雨季之前建立降雨实验，立即采取行动。这个项目利用了一个罕见的和时间-一个敏感的机会，联合收割机结合自然野火与控制降雨实验的析因设计，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气候变化的偶发性和渐进性的反应。这一RAPID奖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。该项目测试的假设，火灾和干旱将相互作用，以促进草和其他草本植物物种的入侵，并导致1）更早的物候生态系统碳增益以及更短的生长季节，和2）增加生态系统的脆弱性，通过土壤结构和微生物群落的变化损失的C和N。因此，火灾和干旱将协同降低生态系统的碳储量。该项目将（1）建立因子干旱×野火实验，（2）测量生态系统对因子处理的响应（包括植物和土壤成分），（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），以便对美国西南部和世界各地的其他半干旱生态系统进行预测。C和N将在土壤物理组分中测量（各种大小等级的团聚体、颗粒有机物、微生物生物量等），并且微生物群落结构将使用鸟枪法宏基因组测序来表征。植物测量将包括群落组成、物候和树冠和叶片一级的生理生态测量，以及景观一级的遥感图像。生态系统建模活动将把这些数据放入生态系统级框架。这个为期一年的项目将为更长期的研究奠定基础。这个奖项反映了NSF的法定使命，并被认为是值得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的支持。

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.