Elevated CO₂ and conifer roots: effects on growth, life span and turnover

Abstract

 Elevated CO# increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO # changes the proportion of C allocated to root biomass, measured as either the root : shoot ratio or the fine root : needle ratio. Elevated CO # tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO # on conifer fine-root life span, and there is no clear trend. Elevated CO # increases the absolute fine-root turnover rates ; however, the standing crop root biomass is also greater, and the effect of elevated CO # on relative turnover rates (turnover : biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO # can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO # concentration initially increases biomass, but this stimulation declines with the response to elevated CO # because increases in assimilation are not matched by increases in nutrient supply.

Growth, physiological and biochemical responses of Meliaceae species - Azadirachta indica and Melia dubia to elevated CO2 concentrations

“…The increase in the percent of the total biomass in these two plants grown in the elevated CO 2 treatment (Fig 2) was consistent with a previous report of Quercus cerrioides and Q. ilex (27). It is postulated that elevated CO 2 atmosphere might show a preferential allocation to roots, thus increasing their root/shoot ratio [28]. However, in the present study, both species showed lower ratios than the ambient indicating a relatively higher allocation of biomass to the shoots than the roots.…”

Root dynamics and global change: seeking an ecosystem perspective

“…In six studies, fine-root density (the mass of roots per unit ground area) increased from 60 to 140% in elevated CO # (Norby et al, 1999). Contrasting with the conclusion of Tingey et al (2000), fine-root density increased more than leaf area in every case, suggesting that the stimulation of fineroot production was a specific response to elevated CO # . In addition to these static measures of root response, fine-root production and mortality have increased in CO # -enriched deciduous trees in the few studies that used minirhizotrons to measure root dynamics.…”

“…They are not without problems, however. For an ecosystem C budget it is necessary to convert data on root length production per minirhizotron to grams of C per square meter ; a method for doing this is described by Tingey et al (2000). Another problem can be the definition of root death, especially in minirhizotron images where tactile cues and staining techniques are not possible.…”

“…It then becomes especially important to focus on the dynamics of the root system -the longevity of roots and the turnover of root systems. The few observations of CO # effects on root turnover in field experiments with conifers have shown an increased rate of root loss, but the response of relative turnover has been inconsistent (Tingey et al, 2000). In the free-air CO # enrichment (FACE) experiment in a 15-yr-old Pinus taeda stand in North Carolina, USA, Allen et al (2000) reported that fineroot production increased 37% in elevated CO # , and mortality plus decomposition (or absolute turnover) increased 26% (not statistically significant), but in this system turnover rate relative to production did not change.…”

Considerations in the study of tree fine-root turnover with minirhizotrons

“…In the minirhizotron methods, the term 'root turnover' has various definitions associated with the dynamic processes of root production, death, and decomposition (Tingey et al, 2000). There are two dry-matter transfer compartment models, each depending on the combinations of dry matter compartments and dry matter transfer processes: the live fine-root abundance-based model (Fig.…”