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Woody Plants Expand Their Ranges, Pumping More Carbon Into the Soils Beneath Them, as the Air's CO2 Content Rises
Woody plant encroachment upon arid and semiarid grasslands and savannas has been an ubiquitous natural phenomenon experienced throughout the entire world over the course of the past century or more (Idso, 1995), driven - at least partially, many believe - by the contemporaneous rise in the air's CO2 concentration (Knapp and Soule, 1998, Soule and Knapp, 1999).  Is it possible this phenomenon may be responsible for sequestering much of the planet's so-called missing carbon, an unidentified but growing repository of organic matter needed to explain the less-than-predicted rate-of-rise of the air's CO2 content that is calculated on the basis of known sources and sinks of this important greenhouse gas?  A recent study sheds new light on this critical subject, suggesting the answer is yes.

Working in the La Copita Research Area southwest of Alice, Texas, Hibbard et al. (2001) analyzed several chemical and physical properties of the top ten centimeters of soils in remnant herbaceous areas and patches of woody vegetation in various stages of invasive development.  Compared to soils beneath herbaceous vegetation, they found that the soils beneath the tree/shrub areas had much greater concentrations of both carbon (C) and nitrogen (N); and a companion study of soil C and N across woody patches ranging in age from 10 to 110 years indicated that these variables had experienced a linear increase through time.

What was the source of these C and N increases?  In a word, roots.  The authors write they "were surprised by the magnitude of root biomass in surficial soils of woody patches, which greatly exceeded that of herbaceous patches and which greatly exceeded that of foliar litter inputs."  Citing a number of studies of rates of root turnover in herbaceous and woody-plant ecosystems, they concluded that "the role of belowground inputs in fueling changes in surficial soil C and N stocks ... accompanying shifts from grass to woody plant domination may therefore be more substantial than previously appreciated."

How much more substantial?  In broaching this question, the authors began by noting that "the contrasts between woody and herbaceous patches reported here are conservative in that they do not include an assessment of whole plant C and N stocks," i.e., root biomass below ten centimeters depth and woody biomass aboveground.  With respect to the first of these factors, they cite several studies that have detected greater soil C concentrations beneath woody vs. herbaceous vegetation to depths of 100 to 400 centimeters.  With respect to the second factor, they likewise cite evidence suggesting that "plant C mass has increased tenfold with the conversion of grassland to savanna woodland over the past 100 years."

So what do these findings imply about the world as a whole?  The authors note that since "woody plant expansion into drylands has been geographically widespread over the past century," and since "40% of the terrestrial biosphere consists of arid and semiarid savanna, shrubland, and grassland ecosystems, this type of vegetation change may be of significance to the global C and N cycle."  To fully quantify the significance of this phenomenon, however, they say we must obtain better information on "the historic or modern rate, areal extent, and pattern of woody plant expansion in the world's drylands."

Vigorous pursuit of this information via remote sensing techniques that show promise of quantifying grass vs. woody plant biomass in grasslands and savannas, coupled with ever-evolving ecosystem modeling techniques, may soon provide the answers we seek.  From what we already know, however, it's a good bet that Hibbard, Archer, Schimel and Valentine have laid the necessary groundwork for resolving the dilemma of the world's missing carbon.  It's likely to be found in the soils and standing biomass of woody plants that have invaded earth's grasslands and savannas over the period of rising atmospheric CO2 concentration that has accompanied the progression of civilization since the dawn of the Industrial Revolution.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Hibbard, K.A., Archer, S., Schimel, D.S. and Valentine, D.W.  2001.  Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna.  Ecology 82: 1999-2011.

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Special Publication, Kuehnast Lecture Series. Department of Soil, Water & Climate, University of Minnesota, St. Paul, MN.

Knapp, P.A. and Soule, P.T.  1998.  Recent Juniperus occidentalis (Western Juniper) expansion on a protected site in central Oregon.  Global Change Biology 4: 347-357.

Soule, P.T. and Knapp, P.A.  1999.  Wester juniper expansion on adjacent disturbed and near-relict sites.  Journal of Range Management 52: 525-533.