Effects of elevated CO₂ on fine root dynamics in a Mojave Desert community: a FACE study

Abstract

Fine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO₂ will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO₂ on production, mortality, and standing crops of fine root length over 2 years in a free-air CO₂ enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (∼370 μmol mol⁻¹) and elevated CO₂ (∼550 μmol mol⁻¹) atmospheric concentrations. Twenty-eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought-deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ∼85% of the area). Seasonal dynamics were similar for ambient and elevated CO₂: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO₂. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year⁻¹, and was not significantly different among CO₂ treatments, except for community transect tubes where it was significantly lower for elevated CO₂. There were no differences in fine root responses to CO₂ between evergreen (Larrea) and drought-deciduous (Ambrosia) shrubs. Combined with observations of increased leaf-level water-use efficiency and lack of soil moisture differences, these results suggest that under elevated CO₂ conditions, reduced root systems (compared with ambient CO₂) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO₂ were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO₂ treatment during periods of high water availability.