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

San Diego State University Foundation, San Diego CA

Investigators

Abstract

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.