Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment

Abstract

Fine-root production and turnover are important regulators of the biogeochemical cycles of ecosystems and key components of their response to global change. We present a nearly continuous 6-year record of fine-root production and mortality from minirhizotron analysis of a closed-canopy, deciduous sweetgum forest in a free-air CO(2) enrichment experiment. Annual production of fine roots was more than doubled in plots with 550 ppm CO(2) compared with plots in ambient air. This response was the primary component of the sustained 22% increase in net primary productivity. Annual fine-root mortality matched annual production, and the mean residence time of roots was not altered by elevated CO(2), but peak fine-root standing crop in midsummer was significantly higher in CO(2)-enriched plots, especially deeper in the soil profile. The preferential allocation of additional carbon to fine roots, which have a fast turnover rate in this species, rather than to stemwood reduces the possibility of long-term enhancement by elevated CO(2) of carbon sequestration in biomass. However, sequestration of some of the fine-root carbon in soil pools is not precluded, and there may be other benefits to the tree from a seasonally larger and deeper fine-root system. Root-system dynamics can explain differences among ecosystems in their response to elevated atmospheric CO(2); hence, accurate assessments of carbon flux and storage in forests in a globally changing atmosphere must account for this unseen and difficult-to-measure component.

Fig. 1.

Seasonal dynamics of fine roots were determined by analysis of minirhizotron images collected biweekly during the growing seasons from February 1998 to November 2003. (A) Production during an observation interval was calculated as the length of new roots plus increases in length of existing roots. (B) Mortality is the disappearance of roots during an interval, with corrections made for roots that subsequently reappear. (C) Standing crop is the total length of root visible; changes in standing crop generally reflect production minus mortality. Data are expressed as the length of root per square meter of observation window. Pooled SEs for each year are shown.

Fig. 2.

Fine roots comprised an increasing fraction of NPP in CO₂-enriched trees. Annual budgets of production (A), mortality (B), and maximum standing crop (C) were expressed in grams of dry matter per square meter of ground. Increases in root mass between the last measurement in the fall and the first measurement in the spring of the next year were attributed to production in the spring, whereas decreases were attributed to mortality in the fall. Data are the means ± SE of three ambient or two elevated CO₂ plots. Analysis of variance indicated the main effect of CO₂ on production, mortality, and peak standing crop to be significant at P < 0.013, P < 0.068, and P < 0.010, respectively. Probability levels for the effect of year were P < 0.157 (production), P < 0.057 (mortality), and P < 0.028 (standing crop); CO₂–years interaction was never significant (P > 0.20). (D) The contribution of fine-root production to total annual NPP (grams of C per square meter of ground area). NPP was calculated through allometric relationships, leaf litter trap collections, and the data in A, following methods described in ref. . Conversion of dry matter units to C units was based on measured C concentration values of 47.1% in wood (open), 46.3% in leaf litter (vertical hatching), and 39.6% in fine roots (diagonal hatching). The percentage increase in NPP is shown above the bars. The effects of CO₂ on leaf litter C mass and NPP are statistically significant (P < 0.001 and P < 0.002, respectively), but, excluding 1998, there is no statistically significant difference in wood C mass production.

Fig. 3.

Distribution of fine-root length by depth in soil at the time of peak standing-root length in 1998 and 2003. Data are the means ± SE of three ambient and two elevated plots, based on analyses of five minirhizotron tubes per plot and expressed as length per square meter of observation window. CO₂ had a significant effect on root length at 0–15 cm (P < 0.035), 30–45 cm (P < 0.001), and 45–60 cm (P < 0.013) but did not at 15–30 cm. The patterns in intervening years were intermediate to those shown here.

Fig. 4.

Relationship between N uptake and root-length duration. N uptake per plot for each year (1998–2002) was calculated as the N content (dry matter production times N concentration) of woody increment, leaf litter, and annual fine-root production plus net foliar leaching (31). Root-length duration is the area under the plots of fine-root standing crop (Fig. 1C). Regression line: N uptake = 5.74 × root-length duration - 36.3; R² = 0.80.