Southern oak savanna may be especially responsive to rainfall redistribution and climate warming during tree establishment as tree and grass species compete for water and other resources. Quercus stellata (post oak) and Juniperus virginiana (juniper) are the dominant C₃ trees and Schizachyrium scoparium (little bluestem) the dominant C₄ grass. We warmed monoculture and tree-grass plots and manipulated rainfall events to intensify summer drought and augment cool season rainfall compared to the long-term mean and measured growth and water potential for four years. Initially, the presence of grass increased water stress and reduced aboveground growth in seedlings of both tree species, however, the presence of juniper reduced water stress in grass plants compared to grass monocultures. As the trees grew in size, this pattern changed and trees performed better in mixtures with grass than in monocultures. To assess the role of belowground interactions we obtained root morphological data from in-growth cores collected three times per year over a four-year period.

Results/Conclusions

Initially, root length density (RLD, km_root m^-3_soil) of the grass tripled when grown with juniper compared to grass monocultures. This was partially due to an increase in specific root length (SRL, m_root g^-1_root) from 128 m g^-1 in monoculture to 200 m g^-1 in the plots with juniper, as well as an increase in root dry mass production. Juniper root characteristics were unaffected by the presence of the grass and juniper RLD was generally higher in the monocultures. Grass root characteristics were generally not affected by the presence of oak. As the juniper trees increased in size, grass RLD in mixtures with junipers decreased from a high of 15 km m^-3 in March 2005 to 2.6 km m^-3 in February 2008, likely as a result of a decreased number of grass tillers and a corresponding reduction in root length production in the plots with a grass-juniper mixture. In February 2008, RLD of the grass in plots with juniper was one third of the RLD of grass in monoculture plots, while SRL of grass roots in plots with juniper was still higher than that of roots of grass grown in monocultures, 205 m g^-1 versus 155 m g^-1. Occasional effects of either warming or precipitation distribution were found, but patterns were not consistent and varied seasonally and by species. These results indicate that there are strong effects of interspecies competition on root mass and length production and root characteristics of dominant oak-savanna species.