Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass

Authors: Fay, Philip; ASPINWALL, MICHAEL - Western Sydney University; Collins, Harold; Gibson, Anne; GILL, RICHARD - Brigham Young University; JACKSON, ROBERT - Stanford University; Jin, Virginia; KHASANOVA, ALBINA - University Of Texas; REICHMANN, LARA - University Of Texas; Polley, Herbert

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2017
Publication Date: 3/30/2018
Publication URL: https://handle.nal.usda.gov/10113/6542305
Citation: Fay, P.A., Aspinwall, M.J., Collins, H.P., Gibson, A.E., Gill, R.H., Jackson, R.B., Jin, V.L., Khasanova, A.R., Reichmann, L.G., Polley, H.W. 2018. Flowering in grassland predicted by CO2 and resource effects on species aboveground biomass. Global Change Biology. 24(4):1771-1781. https://doi.org/10.1111/gcb.14032.
DOI: https://doi.org/10.1111/gcb.14032

Interpretive Summary: Ongoing increases in the concentration of carbon dioxide (CO2) in the atmosphere is largely responsible for recent strong increases in global mean temperature, and there is now considerable evidence that species geographic ranges are being affected, moving north with shifts in the climate regime to which they are adapted. CO2 – driven climate changes are also creating novel conditions such as more variable rainfall patterns. For species to evolutionarily adapt to these novel conditions, populations must contain genetic variation among individuals. For plant species, genetic variation arises through flowering and seed production. However, there is considerable concern about whether species can evolve fast enough to successfully adapt to the rapid (in evolutionary terms) changes taking place. We studied rates of flowering in grassland communities grown for ten years under CO2 concentrations ranging from pre-industrial (250 parts per million) to expected mid-21st century (500 parts per million) levels on a range of soil types, to incorporate realistic variation in soils common in grassland ecosystems. Effects of CO2 concentration on flowering in grasslands is poorly understood. We found varying changes in grassland species flowering in response to changing CO2, including increases, decreases, and no response. Flowering was best predicted by species abundance, which in turn varied with light level and soil moisture changes caused by CO2 enrichment, or with soil nutrient content, which was unrelated to CO2 concentration. Therefore, flowering, and the potential to produce the genetically-variable offspring needed for grassland plant populations to adapt to novel conditions, depend largely on current success – as a species, doing well now increases the chance of doing well later.

Technical Abstract: Ongoing enrichment of atmospheric CO2 concentration may increase plant community productivity by changing plant community composition through direct and indirect effects on light, water, or nutrient availability. CO2 enrichment has been predicted to reduce plant reproductive allocation in herbaceous perennials, but field studies incorporating realistic variation in soil properties remain lacking. The allocation of resources to inflorescence production of the five most abundant species, including four C4 grasses and a C3 forb, was assessed in experimental grassland communities established on coarse- to fine-textured soils and exposed to an experimental CO2 concentration gradient spanning pre-industrial to expected mid-21st century levels (250 – 500 uL L-1) for ten growing seasons in an outdoor chamber facility. Inflorescence production, the number of inflorescences per m2, but not the allocation of biomass to inflorescences, was first a function of species abundance, which in turn varied with either indirect CO2 effects mediated by light levels and volumetric soil water content, or with indirect effects of soil nutrients, uncoupled from CO2 enrichment. For species with indirect CO2 effects, total community productivity, a strong predictor of light availability, explained over 80% of the response in species abundance, and included species for which abundance and flowering both increased (Sorghastrum nutans, Solidago canadensis) and decreased (Bouteloua curtipendula) with CO2 enrichment. Variation among soils in soil water content caused negative feedbacks on community productivity, offsetting part of the increases in abundance and inflorescence production in Sorghastrum and Solidago, and variation among soils in soil nitrate availability similarly caused on negative feedbacks on abundance and inflorescence production of Schizachyrium scoparium and Tridens albescens, independent of CO2 concentration. The impacts of CO2 enrichment on flowering must be considered in a context of multiple limiting resources and how they contribute to individual species abundance in multispecies communities across edaphically variable landscapes. Based on inflorescence production, it appears that species favored by higher CO2 gain twice: greater abundance now and greater probability of persistence in the future. Therefore, adapting to climate change means succeeding now.