Forest Expansion and Soil Carbon Changes in the Loess Hills of Eastern South Dakota
Spencer, Craig N., Matzner, Steven L., Smalley, Jason, Bukrey, Matthew, Onberg, Jennifer, Chapman, Michael, The American Midland Naturalist
ABSTRACT.-We evaluated the status of remnant prairie patches in the Loess Hills of southeastern South Dakota using three parallel approaches. Aerial photograph analysis, vegetation surveys and stable carbon isotope analysis of soil organic matter all yielded evidence of woody plant encroachment. Time series analysis of aerial photos indicated that forest cover expanded by 37.5% between 1941 and 2000. Vegetation surveys revealed several distinct community types ranging from forested ravines supporting basswood, American elm and black walnut trees to upland prairie remnants and oak savannas that now include encroaching eastern red cedar trees, and/or a dense understory of prickly ash, ironwood and other woodland species moving up from the ravines. Finally, carbon isotope values (δ^sup 13^C) in soil cores decreased as much as 5-7[per thousand] towards the soil surface consistent with increased prevalence of C^sub 3^ forest species over C^sub 4^ prairie species in recent years. One consequence of forest encroachment appears to be an increase of 35% in soil organic carbon (SOC) content in surface soils. At the current rate of encroachment and unless management techniques are employed to keep the forests in check as fires once did, we expect the existing prairie remnants to be completely replaced by forest within the next several decades.
At one time, the Great Plains of North America were characterized by native prairies extending from north-cenual Mexico to southern Canada and from the foothills of the Rocky Mountains to western Indiana and Wisconsin (Risser et al, 1981; Samson and Knopf, 1994). Following European settlement, the mixed grass prairies of North Dakota and Nebraska have declined by 71.9 and 77.1% respectively (Sampson & Knopf, 1994). Similar declines have occurred in the Loess Hills of Iowa and southeastern South Dakota (Mutel, 1989). While much of the prairie loss has resulted from agricultural development, remaining prairie areas face an additional threat. Prior to European settlement, fires were common on the grasslands and frequent fires kept forests from encroaching onto the prairies as first noted by Lewis and Clark in 1805. "This country on both sides of the river, except some of its bottom lands, ... is one continued open plain, in which no timber is to be seen except a few ... clumps of trees, which from their moist situation, or the steep declivities of the hills, are sheltered from the effects of fire" (Thwaites, 1905). With the control of prairie fires, the prairie sites that remain today are increasingly threatened by forest encroachment (see reviews by Collins and Wallace, 1990; Sampson & Knopf, 1994).
The Loess Hills of Newton Hills State Park in eastern South Dakota contains prairie sites threatened by forest encroachment. In 1863, land surveyors noted crossing grassy prairies on the ridge tops of the hills with dense to scattered timber stands in the ravines (Rogers, 1972). The dissected topography of Newton Hills precluded these ridgetop prairies from being converted to row-crop agriculture that dominates most of eastern South Dakota. However, these ridgetop prairies are currently threatened by woody plant invasion. Wildfires have not been recorded in the Newton Hills area for at least 150 y, and one consequence appears to be forest expansion.
Various techniques for documenting woodland expansion and other plant community changes have been developed over the years, with three methodologies (vegetation surveys, aerial photography and carbon isotope ratios) receiving the most widespread use.
Aerial photography. - Time series analysis of aerial photographs has been widely used to quantify plant community change, including woody plant invasion of prairies. For example, Briggs et al., (2002) studied aerial photos of the Flint Hills of Kansas between 1958 and 1996 and reported an average increase in forest cover of 2.3% per year. …