Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ 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.R esearchers who attempt to quantify C pools and fluxes in terrestrial ecosystems often find it expedient to measure only the aboveground components despite the wide recognition that a substantial fraction of net primary production occurs belowground (1). Fine roots (ephemeral, nonwoody roots generally of diameters Ͻ1 mm) are important regulators of biogeochemical cycling (2), and their turnover is a key component of C sequestration in soil (3). Root production often is enhanced when the plants are grown in a CO 2 -enriched atmosphere (4, 5). However, the difficulty in measuring these roots, which cannot generally be seen, are not easily sampled, and can grow and die rapidly and unpredictably, means that they are left out of both retrospective (6) and prospective (7) studies of the potential for terrestrial ecosystems to sequester additional C in response to a changing atmosphere and climate. Fine-root production and distribution in forests exhibit high spatial and temporal variation, making their characterization especially difficult, and most of the experimental evidence on tree root responses to elevated CO 2 comes from static or short-term observations that are strongly confounded with plant developmental trajectories (5). Free-air CO 2 enrichment (FACE) experiments (8) provide an opportunity for longer-term observations under realistic conditions in fully developed forests stands.We used minirhizotron observation tubes and video imaging (9, 10) to quantify fine-root production and mortality in a FACE experim...

Forest response to elevated CO2is conserved across a broad range of productivity

“…Experimental results demonstrate that the partitioning of C between plant organs with different turnover rates determines the potential of an ecosystem to store additional C and whether the storage occurs in plant biomass or in soil. At Duke-FACE, increased NPP was associated with C sequestration in stem wood (18,42), whereas at ORNL-FACE increased NPP was partitioned to production of fine roots (28). Fine roots decompose rapidly and add C to soil, where most is respired by microbes, but a fraction may be sequestered into soil organic matter pools (28).…”

“…At ORNL-FACE (28) and PopFACE (29), fine-root production was measured directly by using minirhizotrons and in-growth cores. The DukeFACE experiment used a compartment flow model for estimating fine-root production (30); AspenFACE calculated fine-root turnover from published rates of aspen fine-root production and mortality (31) that were then applied to allometrically determined peak standing fine-root biomass (20).…”

Climate and species affect fine root production with long-term fertilization in acidic tussock tundra near Toolik Lake, Alaska

“…This may be particularly advantageous when the vegetation is dominated by plant species with roots that grow predominantly downward, such as Eriophorum vaginatum in tussock tundra. Second, angular cores and volumetric scaling facilitate comparison with angular minirhizotrons, which can be scaled using the same volumetric approach (e.g., Norby et al 2004). …”

“…The mid-summer sampling date reduced the likelihood of fine root production and death between sampling dates. Root dimensions measured with minirhizotrons are often scaled to the square meter by assuming an image depth of field and scaling the data volumetrically (e.g., Norby et al, 2004).…”

Effects of salt stress on eco-physiological characteristics in Robinia pseudoacacia based on salt-soil rhizosphere

“…For an individual tree, root biomass is roughly 18-45% of the total biomass (Fogel, 1983). Roots are predominantly responsible for the uptake of water and mineral nutrients from the soil (Norby et al, 2004), nutrient storage and transport (Ouimet et al, 2008), etc. Through the same scion cultivar of Prunus persica growing on the rootstocks with differing sizecontrolling potentials, Solari et al (2006) found that rootstocks had a pronounced effect on shoot growth and photosynthetic capacity.…”