Article review: Ecosystem carbon loss with woody plant invasion of grasslands

An important aspect of using plants as agents of biological carbon sequestration is selecting the species that can capture the most carbon. While this may seem like an easy choice to make, the general assumption that woody shrubs and trees capture more carbon than grasses may not be true. The following is a review of the scientific journal article: Ecosystem Carbon Loss with Woody Plant Invasion of Grasslands, which addresses that very issue.

Purpose

The objective of this research paper was to examine the changes in soil nitrogen content, soil organic carbon content, and amount of carbon stored in woody biomass, on sites along a precipitation gradient where woody vegetation has expanded into, and taken over grasslands. The central question asked by this study is: the common assumption is that net carbon fixation increases as vegetation changes from grassland to domination by woody plants; but is that assumption correct (Jackson et al. 2002)?

Methods

Methods used in this study involved both analysis of pre-existing data and fieldwork at test sites. A global analysis of the variation of soil organic carbon was done by combining soil data from US and International soil databases and correlating it with the mean annual precipitation for each location. This was done to see if there was a relationship between precipitation and soil organic carbon, and how it differed between grassland sites and woody vegetation sites (Jackson et al. 2002).

To minimize differences due to different soil types, the fieldwork took place on a series of 6 different test sites consisting of 2 adjacent plots of land separated by a fence. One side had been maintained as grassland and the other side had allowed the invasion of woody plants 30-100 years previous. The contrast of grassland and woody plant communities adjacent to each other allowed for a “before and after” comparison of soil and biotic conditions previous to and following the invasion of woody plants (Jackson et al. 2002).

The species compositions of each of the two communities per site were recorded and four to nine soil cores of 6 cm diameter were extracted to a depth of 10 m from each of the communities at the time of approximate peak biomass. All the fine roots were extracted and quantified from the soil core samples. The soil organic nitrogen and soil organic carbon were isolated from each sample and measured by both an elemental analyzer and by mass spectrometry to determine the concentrations of carbon and nitrogen in each sample. The concentrations were then converted, based on the bulk density of the soil, to estimates of total soil organic carbon and soil organic nitrogen for each ecosystem. The proportional changes in soil organic carbon and soil organic nitrogen were then correlated with the precipitation data for each community at each site (Jackson et al. 2002).

To enhance data collected from the soil cores, roughly four 25 cm diameter 50 cm deep soil pits were dug at each of the two communities at each site and were sampled for nematode species diversity. Soil and plant material were also analyzed for the presence of a certain strontium isotope via thermal ionization mass spectrometry; which was used to indicate the various depths of nutrient uptake by different types of vegetation. Plant biomass was estimated by harvesting all the surface vegetation from sample plots and converted to biomass carbon. Fine root biomass estimates for each community were obtained from soil cores (Jackson et al. 2002).

Carbon cycling in an ecosystem can be complex, so the holistic approach used – examining not only soil organic carbon and biomass carbon, but also soil organic nitrogen, precipitation, and soil biota – seemed effective to answer such a complicated question. This study did, however, neglect to quantify the carbon of woody roots greater than 1cm in diameter, so there is an opportunity for further study on that aspect of woody plant biomass and its contribution to carbon sequestration. All in all, I think the methods used in this study adequately tracked the changes in carbon and nitrogen allocation between grassland and woody plant occupation of a site (Jackson et al. 2002).

Results

The result of the correlation of soil organic carbon with precipitation was that there was a positive relationship between soil organic carbon and precipitation that was 2.6 times greater for grassland vegetation than it was for shrubland. At 200mm mean annual precipitation, soil organic carbon values were the same for grassland and shrubland, and at 1000mm of mean annual precipitation woodlands had 43% less soil organic carbon than grasslands (Jackson et al. 2002).

The results of the samples from the test sites are as follows: Woody plant invasion increased the amount of soil organic carbon and soil organic nitrogen at drier sites and decreased them at wetter sites. The soil organic carbon and soil organic nitrogen were more deeply distributed at woody plant dominated sites than at grassland dominated sites. Grasslands had 60% of their soil organic carbon and in the top meter of soil and 40% 1-3 m below the surface; however, the reverse was true for woody plant dominated sites. Plant biomass increased at woody sites and there was a general shift of carbon from below ground (soil organic carbon) to above ground (woody biomass carbon) was observed. Nematode species diversity was reduced upon the invasion of woody plants, and the depth of nutrient uptake from the soil increased by up to 2 m as evidenced by the strontium isotope analysis (Jackson et al. 2002).

Conclusions

This study concluded that soil factors can influence soil organic carbon and the occurrence of woody encroachment. Relatively wet grasslands allocate a large proportion of carbon belowground, and have high soil organic carbon concentrations. It was also shown that sites that were originally dominated by woodlands show the greatest potential for storing carbon as managed pastures. In addition, this study concluded that recent estimates of carbon storage by woody plant invasion may be incorrect due to the observed loss in soil organic carbon upon the invasion of wetter grasslands by woody plants (Jackson et al. 2002).

I agree with the main conclusions of this study, an ecosystem’s carbon cycle is very complex, and not simply related to the above ground vegetation that we can see, but also the carbon in the soil including the carbon input by fine roots. Increasing carbon fixation on a site is not a simple as planting a species with greater carbon storage in its biomass, due to the accompanying change in soil organic carbon which often occurs. Understanding the interaction between soil types and nutrient cycling by different species, and nutrient cycling in relation to precipitation is key to selecting the best species for optimal carbon fixation on a site.

The research helped me answer the central question: should we expend resources by planting trees in order to sequester carbon, by giving me a better understanding of the different ways and locations ecosystems can fix carbon under different climatic conditions. This study shows that we could plant some trees to capture carbon, but only on specific sites. For a net increase of carbon fixation this study indicates tree plantations on sites with less than 200 mm annual precipitation (major carbon fixation occurring as woody biomass), and managed pastures on sites with more than 200 mm annual precipitation (major carbon fixation occurring as soil organic carbon) (Jackson et al. 2002).

This study was done in the south-eastern United States with species native to that area, so the information gained in this study may not apply to areas with significantly different species and climatic conditions. It would be interesting to do a similar study in the Kamloops area on a grassland site that is directly adjacent to a ponderosa pine stand.

Qualifications of the lead author

Robert B. Jackson has a B.S. degree in chemical engineering from Rice University, M.S. degrees in both Ecology and Statistics, and a Ph.D. in Ecology from Utah State University. He has taught at the University of Texas, and is currently oversees the Ecology program at Duke University as well as being the director of Duke’s Center on Global Change. Dr. Jackson is also the director of the National Institute for Climate Change Research for the Southeastern US and co-directs the Climate Change Policy Partnership. He is among the top 0.5% of the most cited scientific researchers, has more than 100 peer-reviewed scientific publications, and was the recipient of the Murray F. Buell Award from the Ecological Society of America. (Jackson 2007)

References

Jackson, R.B. 2007. Robert B. Jackson Departmental Biography. 2pp. [online]. Available: http://fds.duke.edu/db/aas/Biology/faculty/jackson [Feb.23, 2007].

Jackson, R.B., J.L. Banner, E.G. Jobbagy, W.T. Pockman, and D.H. Wall. 2002. Ecosystem carbon loss with woody plant invasion of grasslands, Nature. 418: 623-626.