Global temperatures are predicted to increase 1.1 to 6.4°C by the end of the century, and will potentially magnify the intensity and variability of seasonal precipitation distribution. Climate regimes within the mid-latitude grasslands and savannas of North America are projected to shift towards drier summers and wetter spring and fall seasons. The purpose of this study was to quantify seasonal variation in root biomass under experimental warming and rainfall manipulation. 

Research was conducted at the Texas A&M Warming and Rainfall Manipulation Site where eight 9x18m rainout shelters and two unsheltered controls were established in post oak savanna. Replicate (n = 4) annual rainfall redistribution treatments are applied at the shelter level (50-yr mean vs. 40% of summer redistributed to fall and spring). Warming treatments (ambient vs. 24-hr IR canopy warming of 1-3°C) are applied to planted monocultures of the dominant tree (post oak and juniper) and grass (little bluestem) species and two tree-grass combinations in native soil.  Fine (<2 mm), coarse (>2 mm), and total root biomass (0-10 cm) were quantified seasonally for 2 years. 

Total root biomass ranged from approximately 500-1500 g m^-2, with fine roots comprising 60-70% of the total.  Root biomass varied seasonally, with peak values occuring in late winter/early spring and late summer/fall.  Total root biomass was lower in redistributed rainfall treatments that resulted in drier summer and wetter spring and fall periods, primarily driven by decreases in fine roots.  Elevated temperatures reduced root biomass in the juniper and juniper/grass plots.  Total root biomass was lowest in the little bluestem monocultures, and highest in the juniper and juniper/little bluestem plots.  Changes in root biomass in response to global change forcing factors may influence net primary production, soil fertility, carbon storage, and vegetation dynamics in the savanna region of North America.