Root dynamics in response to elevated temperatures and altered rainfall regimes in oak savanna: A global change experiment

Background/Question/Methods

Increasing concentrations of greenhouse gases are projected to elevate global surface air temperatures by 1.1 to 6.4°C by the end of the century, and potentially magnify the intensity and variability of seasonal precipitation distribution. The mid-latitude grasslands and savannas of North America are predicted to experience a 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 (WaRM) Site in College Station 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 (long term mean vs. 40% of summer redistributed to fall and spring with same annual total). 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 within each shelter in native soil.  Fine (<2 mm), coarse (>2 mm), and total root biomass were quantified seasonally for 2 yrs on soil cores taken to 10 cm depth. 

Results/Conclusions

Total root biomass ranged from approximately 500-1500 g m^-2 across all treatment combinations for the duration of the study.  Root biomass varied seasonally, with peak values occuring in late winter/early spring and again in late summer/fall.  Root biomass was dominated by fine roots, which generally accounted for about 60-70% of total root mass.  Total root biomass was lower in redistributed rainfall treatments that resulted in drier summer and wetter spring and fall periods, and this difference was driven primarily by the fine root response.  Elevated temperatures reduced total, fine, and coarse root biomass in the juniper and juniper/grass plots, but not in the other vegetation types.  Total root biomass was generally 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 drivers may have important implications for net primary production, soil fertility, carbon storage, interspecific interactions, and vegetation dynamics in the oak savanna region of North America.