Background
The authors report that "colonization of grasslands or savannas by trees over the last 50-100 years, often described as 'thickening', has received attention due to the large potential for carbon sequestration in woody biomass."  It is also a hot topic because many studies, in their words, "attribute thickening to the increase in atmospheric CO2, causing CO2 fertilization and resulting in increased water-use efficiency in C3 plants (Berry and Roderick, 2002; Grunzweig et al., 2003)."  In addition, they say that "much of the change in atmospheric CO2 occurred over the last 50 years [1953-2003 = 64 ppm] with the most significant changes being in the last 20 years [1983-2003 = 33 ppm]."  For more on this phenomenon, see Trees (Range Expansions) in our Subject Index.

What was done
Working at a site some 40 km northwest of Longreach, Queensland, Australia, Krull et al. measured vertical profiles of δ¹³C and ¹⁴C of bulk and size-separated soil organic matter to infer the time course of changes in these parameters along a transect spanning the dynamic transition zone between C4-dominated grassland and C3-dominated woodland, which ecotone is comprised of different-age specimens of leguminous gidyea trees (Acacia cambagei) interspersed with occasional whitewood trees (Atalaya hemiglauca).  Then, since the long-time landholder indicated that thickening by the Acacia trees had occurred "at least since the 1950s," they tested whether the observed changes in soil carbon stocks could be reproduced by the Roth-Carbon turnover model over a 50-year time period.

What was learned
It was determined, in the words of the authors, that "much of the vegetation change at this site occurred over the last 50 years."  In addition, they measured approximately twice as much total organic carbon in the soil beneath the fully established woodland as what they measured in the soil beneath the pristine grassland.

What it means
Krull et al. conclude their paper by saying their findings "stress the importance of viewing soils as dynamic systems and indicating the potential for soil organic carbon sequestration in grazed semi-arid woodlands," which land use represents a form of agroforestry whose virtues have recently been touted by Mutuo et al. (2005).  Also, their findings suggest the operation of an important negative feedback phenomenon that has the potential to slow the rate-of-rise of the air's CO2 content, wherein the ongoing enrichment of the air with CO2 from the burning of fossil fuels enables woody species to more readily colonize less productive grasslands and thereby extract greater amounts of CO2 from the atmosphere, which tends to retard atmospheric CO2's upward concentration trend while simultaneously providing many benefits to the soil and the plants that grow upon it.

References
Berry, S.L. and Roderick, M.L.  2002.  CO2 and land-use effects on Australian vegetation over the last two centuries.  Australian Journal of Botany 50: 511-531.

Grunzweig, J.M., Lin, T., Rotenberg, E., Schwartz, A. and Yakir, D.  2003.  Carbon sequestration in arid-land forest.  Global Change Biology 9: 791-799.

Mutuo, P.K., Cadisch, G., Albrecht, A., Palm, C.A. and Verchot, L.  2005.  Potential of agroforestry for carbon sequestration and mitigation of greenhouse gas emissions from soils in the tropics.  Nutrient Cycling in Agroecosystems 71: 45-54.