This project will map root and soil carbon in three dimensions at sites across a regional climate gradient in the Kalahari Desert of Botswana. The Kalahari is one of the only places on Earth where this type of natural experiment can be conducted without having to worry about problems of variations in soil characteristics. This research seeks to understand 1) how the distribution of roots in the soil varies among different plant species and across large distances, and 2) how carbon found in soils changes in response to climate change at multiple timescales.
Arid and semiarid regions, such as the Kalahari, cover over 40% of the Earth's land surface. Roots are an important component of the productivity and composition of these ecosystems, and play an important role in their response to climate variability. Thus, understanding the root structure of water-limited vegetation is vital to the study of the global biosphere?which includes its human inhabitant?and global carbon budgets amidst a changing climate. This research will address how belowground plant processes in water-limited ecosystems control belowground carbon dynamics under different climate conditions. The interdisciplinary nature of this research will include mentoring of graduate and undergraduate students from Botswana and the U.S., and collaborations with colleagues in Botswana. A short course on the research and related fields will be offered to high-school science teachers to encourage the discussion of plant-water interactions and active research in high school classrooms.
The purpose of this research was to determine the distribution of belowground carbon - primarily roots - in savanna ecosystems to help understand how the combined impacts of climate and fire impact soil carbon sequestration in these important ecosystems. The research was conducted in the Kalahari Desert in Botswana. The Kalahari is an important place for understanding dryland ecosystems, which cover about 1/3 of the Earth’s land surface, because it has significantly more precipitation in the north (around 600 mm/yr) than in the south (around 150 mm/yr), all on consistent sandy soils. Thus the effect of climate on carbon storage in belowground biomass can be studied without the confounding effects of soil variability. The major activities that took place during the research were 1) excavating and mapping the root system of individual trees along the transect and 2) random sampling of 1 m x 1 m soil pits to determine the distribution of roots to 120 cm. Soil samples were also taken to determine how roots and soil organic carbon are related. In the root excavation and mapping, we found that the relationship between the radius of the tree and the fraction of coarse root biomass within that radius followed an exponential distribution for all individuals excavated. The cumulative distribution of root biomass with depth, which generally follows an exponential distribution at the landscape scale, displayed two distinct forms. Individuals of three species followed an exponential depth profile with the majority of coarse roots near the surface. However, one species exhibited a depth distribution more closely followed by a gamma distribution, with very little biomass in the upper-most layers. These results suggest that interspecies differences exist in the water-use strategies employed by woody savanna plants. We determined that root systems are the result of a stochastic (random) space-filling process. Determining the rules that govern this process would lead to a better understanding of plant water-use strategies and the ability to simulate belowground biomass distributions, and thus belowground carbon storage. In the soil pits, we found that the ratio of below to above ground biomass decreases with decreasing precipitation. At the dry end of the transect trees 'invest' less in their root system, while wetter areas most of the tree biomass is found below ground, possibly due to the impact of fire in these wetter areas. Roots are not necessarily the 'hidden half' of tree biomass (as it is often believed): they are about four times the above ground biomass in the wetter area and only one fourth of the above ground biomass in the driest area. The root depth overall increases with decreasing precipitation, but this decrease is not monotonic: at a site with intermediate precipitation, we found much deeper roots than at the driest site, suggesting that, as expected, changes in plant community composition may add complexity to the dependence of root depth on precipitation. These results are proving important for the understanding of carbon sequestration in dryland ecosystems, and have caused us to rethink how plants control through their root systems their access to water and their storage of carbon for regrowth after fires. Changes in climate, particularly reduced effective precipitation, would lead to the effective drying out of wetter sites, potentially leading to less carbon sequestration once the ecosystems are in equilibrium with the new climate. This project has served as an excellent training ground for the next generation of scientists at all levels. We have worked with several K-12 teachers in Colorado and Virginia to develop curricula for the classroom to help students understand the Earth’s diverse ecosystems and how they are studied. At least eight undergraduate students, half of them women, traveled to Botswana to participate directly in the field research. Seven graduate students participated in the research, including one from the University of Botswana. The participation of the University of Botswana and the Botswana Department of Forestry and Range Resources provided crucial in-region outreach and we have now established stable, ongoing partnerships with these organizations.
