Coupling Fine Root Dynamics with Ecosystem Carbon Cycling in Black Spruce Forests of Interior Alaska

  • Ruess R
  • Hendrick R
  • Burton A
 et al. 
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Abstract

because mortality tended to peak during fall and be minimal during periods of peak production. Production and mortality were, however, positively correlated across all tubes and time periods. Annual fine root production averaged 2.45 - 0.31, 8.01 ? 1.39, and 2.53 + 0.27 mm.cm-2-yr-1 (means ? 1 SE) among the three sites, when averaged across years. Fine root survival and decomposition were measured by tracking and analyzing the fate of individual fine roots using mark-recapture techniques. Fine root survival was greatest during periods of peak root growth, and least over winter (4time). Roots first appearing in the middle of the growing season had higher survival rates than those first appearing early or late in the growing season, or over winter (cohort), and risk of mortality decreased with root age (kage). Survival estimates -translate to mean life spans of 108 ? 4 d during the growing season. While these values are in striking contrast to needle longevity and rates of aboveground litter decom- position, they are similar to many values found for temperate systems, supporting the notion that there are basic morphological and physiological traits of first-order roots that are common to most woody plant root systems. During the growing season, monthly fine root decomposition rates averaged 0.46 ? 0.01 per month, while decomposition rates over winter averaged 0.73 ? 0.01 per winter. These growing season estimates translate to 49 ? 2 d from the time a root was first observed as dead, to the time it disappeared. For roots that decomposed during the growing season, those with longer life spans decomposed more slowly after death. Comparing these results with other minirhizotron studies suggests that life-history traits of black spruce first-order roots are similar to those from temperate (and perhaps most) forest ecosystems. Annual production of fine roots averaged 228 ? 75 g biomass-m-2.yr-1, constituting --56% of total stand mass-m-2.yr-1, 11%) contributed similarly to total production, while mosses (73 ? 14 g biomass m-2.yr-1, 20%) production. Aboveground production of trees (50 _ 14 g biomass-m-2.yr-1, 13%) and shrubs (40 + 2 g bio- accounted for the largest component of aboveground production. Soil temperature had a strong control over both soil respiration (Q10 = 2.21 ? 0.31) and root respiration (Q10 = 2.30 + 0.37). During the growing season (15 May to 15 September), -56% of soil CO2 efflux (580 ? 40 g C/m2) was derived from fine root respiration (329 ? 54 g C/m2). Although apparent rates of heterotrophic respiration (May through September) and total production did not differ, definitive estimates of net ecosystem production are impossible given the potentially large, unmeasured components of NPP (net primary production), such as root exudation and mycorrhizal pro- duction. Nevertheless, rates of fine root production, mortality, and decomposition indicate that in these black spruce ecosystems, fine roots are much more dynamic than would be predicted from patterns of aboveground processes, and that carbon, and presumably nutrients, are cycling through fine roots at rates several orders of magnitude faster than through aboveground tissues.

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Authors

  • Roger W Ruess

  • Ronald L Hendrick

  • Andrew J Burton

  • Kurt S Pregitzer

  • Bjartmar Sveinbjornssön

  • Michael E Allen

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