The Substrate Age Gradient of Arizona (SAGA) is a primary succession chronosequence near Flagstaff, Arizona. It is comprised of four semiarid sites that range in age from 1 thousand years (pictured above) to 3 million years.
In the first half of the 20th century, soil scientists and ecologists recognized that soil development occurs due to the interaction of 5 state factors: climate, organisms, relief, parent material, and time. Substrate age gradients seek to hold the first four of these state factors constant, while allowing time to vary. Therefore, substrate age gradients isolate the effect of time on soil formation and associated processes.
The SAGA is a unique study system. It is located in a semiarid ecosystem (~325 mm precipitation y-1), and the microclimate, dominant vegetation, relief, and parent material among sites are all remarkably similar. The sites that comprise the SAGA were formed as part of the larger San Francisco Volcanic Field (SFVF), a 5000 km-2 region in northern Arizona. The SFVF has migrated over a magmatic hotspot in the Earth’s crust at a rate of ~2 cm per year. As a result, the SFVF is home to more than 600 cinder cones volcanoes, four of which are associated with the SAGA.
In the last decade, we have learned a great deal about carbon, nutrient, and water cycles in plants and soils at the SAGA. An ever-growing body of literature has been published regarding the SAGA testing fundamental theories of how primary succession proceeds in arid ecosystems.
Published SAGA literature
Delgado-Baquerizo, M., R.D. Bardgett, P. Vitousek, F.T. Maestre, M.A. Williams, D.J. Eldridge, H. Lambers, S. Newhauser, A. Gallardo, L. García-Velázquez, O, Sala, S. Abades, F.D. Alfaro, A.A. Berhe, M.A. Bowker, C. Currier, N.A. Cutler, S.C. Hart, P.E. Hayes, Z.-Y. Hseu, M. Kirchmair, V.M. Peña-Ramírez, C. A. Pérez, S.C. Reed, F. Santos, C. Siebe, B.W. Sullivan, L. Weber-Grullon, N. Fierer. 2019. Changes in beloground diversity during ecosystem development. PNAS, published ahead of print. https://doi.org/10.1073/pnas.1818400116
Gu, C., S.C. Hart, B.L. Turner, Y. Hu, Y. Meng, M. Zhu. 2019. Aeolian dust deposition and the perturbation of phosphorus transformations during long-term ecosystem development in a cool, semi-arid environment. Geochimica et Cosmochimica Acta 246, 498-514.
Coble, A.A., S.C. Hart. 2016. No evidence of resource limitation to aboveground growth of blue grama (Bouteloua gracilis) on 1 ky-old semi-arid substrate. Biogeochemistry 131(1-2), 243-251.
Kane, J.M., F.L. Dugi, T.E. Kolb. 2015. Establishment and growth of pinon pine regeneration vary by nurse type along a soil substrate age gradient in northern Arizona. Journal of Arid Environments 115:113-119.
Sullivan, B.W., M.K. Nasto, S.C. Hart, B.A. Hungate. 2015. Proximate controls on semiarid soil greenhouse gas fluxes across 3 million years of soil development. Biogeochemistry 125(3): 375-391.
Newman, G.S., S.C. Hart. 2015. Shifting soil resource limitations and ecosystem retrogression across a three million year semi-arid substrate age gradient. Biogeochemistry 124(1):177-186.
Coble, A.A., S.C. Hart, M.E. Ketterer, G.S. Newman, A.L. Kowler. 2015. Strontium source and depth of uptake shifts with substrate age in semiarid ecosystems. Journal of Geophysical Research:Biogeosciences 120(6): 1069-1077.
Coble, A.A., S.C. Hart. 2013. The significance of atmospheric nutrient inputs and canopy interception of precipitation during ecosystem development in piñon-juniper woodlands of the southwestern USA. Journal of Arid Environments 98, 79-87.
Sullivan, B.W., P.C. Selmants, S.C. Hart. 2013. Does dissolved organic carbon regulate biological methane oxidation in semiarid soils? Global Change Biology doi:10.1111/gcb.12201.
Sullivan, B.W., S.C. Hart. 2013. Evaluation of mechanisms controlling the priming of soil carbon along a substrate age gradient. Soil Biology and Biochemistry 58, 293-301.
Sullivan, B.W., P.C. Selmants, S.C. Hart. 2012. High potential nitrification rates occur in soils during dry seasons. Soil Biology and Biochemistry 53, 28-31.
Looney, C.L., B.W. Sullivan, J.M. Kane, T.E. Kolb, S.C. Hart. 2012. Effects of water additions on pinyon pine (Pinus edulis) water relations, growth, and mortality across a three million year old soil age gradient in northern Arizona, USA. Plant and Soil 357, 89-102.
Selmants, P.C., S.C. Hart, 2010. Phosphorus and soil development: Does the Walker and Syers model apply to semiarid ecosystems? Ecology 91, 474-484.
Selmants, P.C., S.C. Hart, 2008. Substrate age and tree islands influence carbon and nitrogen dynamics across a retrogressive semiarid chronosequence. Global Biogeochemical Cycles 22, GB1021.