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 CO 2 will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO 2 on production, mortality, and standing crops of fine root length over 2 years in a free-air CO 2 enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient ( $ 370 lmol mol À1 ) and elevated CO 2 ( $ 550 lmol mol À1 ) 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 CO 2 : 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 CO 2 . Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year À1 , and was not significantly different among CO 2 treatments, except for community transect tubes where it was significantly lower for elevated CO 2 . There were no differences in fine root responses to CO 2 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 CO 2 conditions, reduced root systems (compared with ambient CO 2 ) 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 CO 2 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 CO 2 treatment during periods of high water availability.

Effects of Elevated Co2 on Root Function and Soil Respiration in a Mojave Desert Ecosystem

“…Minirhizotron images at the Nevada Desert FACE Facility (NDFF) do not show persistent changes in root length of A. dumosa or L. tridentata with elevated CO 2 treatment (this report, Phillips et al 2006). Because we did not observe a change in specific root length (SRL) of either species (Tables 5 & 6;Figs.…”

“…Effects of elevated CO 2 on AMF of Mojave Desert shrubs -Although shrub aboveground productivity is increased with elevated CO 2 at this site (Smith et al 2000, Housman et al 2006), allocation to roots (Phillips et al 2006) and mycorrhizal biomass (Table 9, Fig. 7) (Clark et al in prep) is not enhanced.…”

“…3 & 4), belowground biomass for these two species is also unaffected by the high CO 2 treatment. Phillips et al (2006) likely have the greatest contributions of roots to soil respiration. Thus, root respiration at the NDFF is unlikely the cause of increased soil or ecosystem respiration observed with elevated CO 2 (this report, Jasoni et al 2005).…”

Root growth and N dynamics in response to multi-year experimental warming, summer drought and elevated CO2 in a mixed heathland-grass ecosystem

“…The direct effects of warming and CO 2 on plant growth in natural ecosystems can be limited if nutrient stocks are gradually depleted (Arnone et al 2000;Gill et al 2006). However, several examples exist that show no evidence of progressive nitrogen limitation even after several years of treatment with elevated CO 2 in forests (Barnard et al 2006;Norby and Iversen 2006;Phillips et al 2006).…”

Effects of elevated CO2 levels on root morphological traits and Cd uptakes of two Lolium species under Cd stress

“…From the viewpoint of water and nutrient uptakes, roots are of particular physiological importance (Bosac et al, 1995;Jia et al, 2008). Fine roots are usually more sensitive to exposure to either excessive metal concentrations (Arduini et al, 1995) or elevated levels of CO 2 (Day et al, 1996;Janssens et al, 1998;Phillips et al, 2006) when compared to coarse roots. Changes in root morphology may therefore serve as an important indicator of environmental changes (Nishizono et al, 1987;Ostonen et al, 2007).…”