Shrub Expansion Along a Coastal Soil Chronosequence

Authors Brantly and Young (2010) write that woody plant encroachment in herbaceous ecosystems "represents a key shift in community structure that has [the] potential to alter regional and global C [carbon] and N [nitrogen] cycling." However, they say "there is considerable uncertainty regarding the effects of woody [plant] encroachment on global terrestrial C storage," due to the possibility that "increases in C sequestration in woody biomass may be offset by associated increases in soil CO₂ efflux (i.e., combined heterotrophic respiration and root respiration) resulting from increased litterfall, increased soil moisture, and associated increases in microbial activity that often accompany woody [plant] encroachment."

To further investigate this situation by determining "if young, sandy soils on a barrier island became a sink for C after encroachment of the nitrogen-fixing shrub Morella cerifera, or if associated stimulation of soil CO₂ efflux mitigated increased litterfall," Brantley and Young "monitored variations in litterfall in shrub thickets across a chronosequence of shrub expansion and compared those data to previous measurements of ANPP [annual net primary production] in adjacent grasslands," after which they "quantified standing litter C and N pools in shrub thickets and soil organic matter (SOM) , soil organic carbon (SOC), soil total nitrogen (TN) and soil CO₂ efflux in shrub thickets and adjacent grasslands," which field work was conducted on the north end of Hog Island (37°27'N, 75°40'W), a barrier island just east of the Virginia portion of the DelMarVa peninsula, USA.

The two researchers report that although soil CO₂ efflux was indeed stimulated by shrub encroachment in the younger soils, "soil CO₂ efflux did not vary between shrub thickets and grasslands in the oldest soils, and increases in CO₂ efflux in shrub thickets did not offset contributions of increased litterfall to SOC." In fact, they found that "SOC was 3.6-9.8 times higher beneath shrub thickets than in grassland soils, and soil TN was 2.5-7.7 times higher under shrub thickets."

According to the scientists who performed the work, the expansion of shrubs on barrier islands -- which often have low levels of soil carbon but a high potential for ANPP -- has the ability "to significantly increase ecosystem C sequestration." What is more, they say that "stimulation of N storage beneath shrub thickets will also favor future growth of species with lower nutrient use efficiencies than native grasses, including climax maritime forest species that could sequester additional C in biomass," citing the work of Ehrenfeld (1990) and Vitousek et al. (2002), all of which phenomena presaged by their work bode well for earth's barrier islands, and for the planet's less productive grasslands as well.

Additional References
Ehrenfeld, J.G. 1990. Dynamics and processes of barrier island vegetation. Reviews in Aquatic Sciences 2: 437-480.

Vitousek, P.M., Cassman, K., Cleveland, C., Crews, T., Field, C.B., Grimm, N.B., Howarth, R.W., Marino R., Martinelli, L., Rastetter, E.B. and Sprent, J.I. 2002. Towards an ecological understanding of biological N fixation. Biogeochemistry 57: 1-45.