OOS 49-3 - Using root traits to predict the consequences of woody encroachment in grassy ecosystems

Grassy ecosystems are comprised of diverse assemblages of grasses, forbs, and woody species. These species exhibit tremendous diversity in root form and function, with distinct differences in belowground biomass, allocation of roots to varying depths, and differences in water uptake from varying soil layers. Alterations in climate, fire, and grazing have the potential to modify coexistence dynamics and ecosystem processes based on unique belowground traits among diverse species assemblages. In addition, woody encroachment may modify ecohydrological and energy dynamics if root distributions alter infiltration pathways or accelerate substrate weathering. Here, we present data linking the physiological and morphological responses of roots with changes in grassland carbon and water cycling following woody encroachment. These data include long-term observations based on legacies of fire management and grazer abundance, as well as short-term experimental manipulations of drought from the Konza Prairie, KS, USA.

We deployed 1-m deep ingrowth cores in watersheds with varying fire frequencies to measure changes in belowground net primary productivity. Surprisingly, annual production of fine-root biomass was similar among herbaceous and woody plants, even in surface soils. We also excavated entire root systems of clonal shrubs to quantify coarse root biomass. Together, these studies illustrate how the success of encroaching species relies on (1) maintenance of fine root biomass in shallow soils to facilitate surface water uptake when it is available, but (2) production of substantial coarse root biomass to facilitate shifts to deeper water-use when surface conditions are dry. Coarse woody roots also created large soil macropores, increasing water movement to greater depths in the soil (past the grass rooting zone). Resulting changes in infiltration and root respiration deeper in the soil profile may lead to altered ecohydrology, shifts in carbon and nutrient cycling, and enhanced bedrock weathering. In contrast to shrubs, grasses maintained focused water uptake in surface soils (0-30 cm) regardless of burn history, grazing, or drought. Root hydraulic conductivity was also considerably higher in grasses compared to coexisting woody shrubs, reflecting the proportionally larger investment in acquisitive fine roots in grasses. This is also shown by relatively higher specific root length in grass fine roots in surface soils compared to shrubs, despite similarities in overall fine root biomass. Identifying unique belowground root responses of grassy ecosystems to fire, grazing, and climate variability can inform ecosystem management strategies aimed at increasing stress for undesirable encroaching shrubs while promoting diversity within native herbaceous communities.