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The Mysterious World Beneath our Feet:
Discovering how Plant Roots Respond to Environmental Change

Lisa Moore
Biological Sciences
Stanford University
June 2001

Where plant roots are located, how they function, and how they respond to the environment can affect plant performance, community interactions, and ecosystem-level processes. This information also gives scientists a better understanding of how plants interact with their neighbors and environment and is crucial for many aspects of ecosystem ecology, especially in light of ongoing climate change. The goal of my research is to understand how root structure and function respond to climate variability.

Although roots are invisible to us, they play a critical role for plants because they are responsible for extracting water and nutrients from the soil. Despite their importance, however, roots and their interactions with a fluctuating environment have received little attention from the scientific community.

In order to study root responses to environmental variation, I measure changes in plant root distribution and water uptake in controlled annual grassland communities, where I determine the amount and timing of precipitation. There are three main objectives to my research. I first seek to understand how the location of roots changes in response to year-to-year variation in rainfall: Do plants grow deeper roots in dry years than they do in wet years? My second goal is to understand the water use patterns of the plants under different conditions: Do plants rely on surface moisture or deeper water sources and are plants able to switch between water sources depending on when or where water is available? My third goal is to know how different species respond to the same conditions: What strategies do different species use for dealing with change? The results of my experiment should help scientists to not only understand more of what goes on belowground, but also to explain and predict ecological patterns and processes. This is especially important given that human activities are rapidly changing global environmental conditions.

In my research, I use California annual grasslands as a model system. The plants are small and short-lived, allowing me to study whole life cycles of several species in a reasonable amount of time. Moreover, these plants live in a highly variable environment. For example, soil is wet during fall and winter but dry through the summer. There are also wide fluctuations in the amount and timing of rainfall from year to year. In a wet, El Niño year, as much as 1,200 cm of rain may fall, whereas in other years plants may have to survive on only 200 cm of precipitation. To explore how grassland species respond to this variability, I am growing communities composed of several species of annual grassland plants in meter-deep pots to which I apply four different water treatments. Three groups (which I call "dry," "average," and "wet") receive different amounts of water, ranging from 50 to 150 percent of the long-term annual average. The timing of "rainfall" in these three groups is identical, so differences among groups can be attributed to differences in the amount of annual precipitation. The fourth group receives the same amount of precipitation as the "average" treatment, but I add the water on a different schedule. By comparing this group to the average treatment, I can determine whether plants respond to timing of rainfall.

During the growing season, I measure the root distribution and water uptake patterns of my plants. To determine root distributions, I wash roots out of the soil and measure how root mass changes with depth. For example, I test whether plants growing in the wet treatment put proportionally more roots in moist surface soils, and if plants in dry conditions have a larger proportion of their root systems in deep soils. Root harvests cannot answer my questions about plant water use, however, because root mass may not correlate with water uptake. As a result, I have to measure water use explicitly. Specifically, I am interested in how much water each species extracts from different depths in the soil. To do this, I use stable isotopes as tracers in my pots. By knowing where isotopes are in the soil, and measuring how many isotopes are in a plant, I can calculate how much water each plant takes from a particular depth in the soil. This information will allow me to distinguish water use patterns of different species, helping me to understand when and where plants compete for water.

The data from my project will be useful in many ways. Knowing how different plant species change their root structure and water uptake patterns in response to climate may help ecologists explain aboveground species dynamics. This information is also valuable for scientists who model hydrological or plant processes on regional and global scales. For example, most models assume that water uptake is proportional to root mass. If this assumption is incorrect, then the output of these models will be improved with new information on plant responses to environmental fluctuation.

Learning more about plant root processes will also lead to a richer understanding of soil ecosystem dynamics. Plant roots take water and nutrients out of the soil, but they also provide resources for millions of organisms living in the soil. The identity, location, and amount of plant roots in the soil undoubtedly affect the structure and function of soil communities, which in turn have dramatic effects on whole ecosystems through processes such as decomposition and nutrient cycling.

Finally, human-induced climate change adds one more element to an already complex system. It is important to understand how climate change will affect our world, but before we can improve our predictions of the ecological consequences of global change, we have to understand basic ecological mechanisms. By providing a window into the mysterious world beneath our feet, my project can help us to understand both the present dynamics of and future changes in ecosystem ecology.