University Distinguished Professor, Kansas State University

Holistic Consequences of Woody Plant Encroachment in Great Plains Grasslands

Woody plant encroachment is a global phenomenon, and now represents the most widespread threat to grassland conservation after conversion to row-crop agriculture. The consequences of woody plant encroachment on ecosystem structure and function are pervasive. The above ground consequences of encroachment are well studied. For example, woody plant encroachment typically results in reduced plant and animal biodiversity, loss of suitable habitat for obligate small mammals and birds, and increased threat of diseases carried by ticks. Alterations in above ground ecosystem structure also reduces forage availability for range management, resulting in billions of dollars of lost revenue for the cattle industry in the United States alone.

The consequences of encroachment on below ground ecosystem dynamics are less well known. Shifts from grass-dominated to shrub- or tree-dominated at the landscape-scale disrupts C, water, and nutrient cycling throughout grasslands. Ecohydrology studies have shown that woody encroachment alters catchment water budgets and recharge rates, runoff generation mechanisms and amounts, the relative contributions of deeper flow paths to streams, and stream discharge and intermittency. Moreover, the replacement of a grassy ecosystem with a woody ecosystem has uncertain impacts on C sequestration due to alterations in soil carbon dioxide fluxes, increased bedrock weathering rates, and changes in labile soil organic carbon accumulation.

In this presentation, I will use examples from the Konza Prairie over the past decade that illustrate linkages between above- and below-ground ecosystem processes following encroachment. In particular, this research shows the mechanisms by which woody vegetation accelerates water cycling and alters the distribution of C within the soil profile. Coarse woody roots appear to create larger soil macropores that speed up rates of infiltration to the groundwater, a process that could shunt C more deeply into the subsurface. Larger macropores also reduce water residence time in surface soils and along with higher ET result in longer-term drying trends in grassland ecosystems. The results from Konza can be extrapolated to other mesic grasslands of the Great Plains, illustrating the significance of belowground ecosystem change on the functional dynamics of our local grassy systems.