Three-Dimensional Spatial Patterns of Fine Roots at the Landscape-Scale in a Subtropical Savanna.

Grasslands and savannas in the arid and semi-arid regions of the world have experienced proliferation of woody plants during the past century. In these resource-limited areas, the distribution and abundance of fine roots play an important role in acquiring essential resources and structuring communities. However, little is known regarding the depth distribution of fine root biomass at the landscape scale. We quantified spatial patterns of fine root biomass across soil depths in a subtropical savanna landscape in southern Texas by intensively sampling to a depth of 1.2 m in a 160 m × 100 m georeferenced plot subdivided into 10 m × 10 m grids.      Fine root biomass decreased with soil depth and differed significantly among plant communities. Kriged maps of fine root biomass showed fine root biomass is highest at the center of woody patches, decreasing towards the canopy edges of woody patches, and reaching lowest values within the grassland matrix across all soil depths. These spatial patterns were further supported by strong positive correlations between fine root biomass and NDVI as an index of aboveground biomass.  Spatial heterogeneity of fine root biomass across this landscape, quantified by lacunarity analysis, decreased continuously to a depth of 50 cm and then increased. This inconsistency is related to the presence/absence of an argillic horizon across this landscape. Anisotropic autocorrelation analyses showed fine root biomass distribution was strongly directional, with highest significant r values at the direction of NW-SE, perpendicular to the slope gradient of this landscape. The greater abundance of fine root biomass beneath woody plants in both upper and lower soil layers suggests an ability to acquire disproportionately more resources than herbaceous species, which may facilitate the encroachment of woody plants   across this landscape.