Ecology, Evolution and Organismal Biology Ecology, Evolution and Organismal Biology Publications 1998 A Global Trend in Belowground Carbon Allocation: Comment Knute J. Nadelhoffer Marine Biological Laboratory James W. Raich Iowa State University, [email protected] J.D. Aber University of New Hampshire Follow this and additional works at: https://lib.dr.iastate.edu/eeob_ag_pubs Part of the Climate Commons, Plant Biology Commons, and the Soil Science Commons The ompc lete bibliographic information for this item can be found at https://lib.dr.iastate.edu/ eeob_ag_pubs/223. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Ecology, Evolution and Organismal Biology at Iowa State University Digital Repository. It has been accepted for inclusion in Ecology, Evolution and Organismal Biology Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. A Global Trend in Belowground Carbon Allocation: Comment Abstract Gower et al. (1996) have questioned the validity of using a global-scale relationship between litterfall and belowground carbon (C) allocation (Raich and Nadelhoffer 1989) at stand and regional scales. We encourage attempts to understand better the controls on C allocation to roots in forests, including efforts to evaluate the potentials and limitations of C budgets for this purpose. However, the tests of our C-balance model that were presented by Gower et al. use inappropriate comparisons and the conclusions they drew are unwar ranted. In addition, they misinterpret and misapply our C-budgeting models and their conceptual bases. Therefore, we clarify our approach to estimating belowground C allocation (Raich and Nadelhoffer 1989, Nadelhoffer and Raich 1992) and highlight problems with the tests of our models as conducted by Gower et al. The issue in question is whether simplified soil C budgets can be used to estimate total root carbon allocation (TRCA, the annual rate at which assimilated C is allocated to producing and maintaining roots and mycorrhizae) in forest ecosystems. The oncc eptual model underlying the statistical model we used to predict TRCA at global scales (Raich and Nadelhoffer1989) is based on the First Law of Thermodynamics (i.e., conservation of mass) and can be expressed as TRCA = soil respiration - litterfall + export + ACroot - ACsoiI (1) where units are grams of C per square meter per year and where soil respiration is C02-C released from the soil surface due to respiration by live roots and het- erotrophs, litterfall is inputs to soil from aboveground production, export is C loss via erosion and leaching, ACroot is the change in root C (fine + oc arse), and AC0oil is the change in soil C (forest floor lusp mineral soil). The ts atistical model (same units) describes a simple linear regression that was derived from a collation of available data in which (export + ACroot - ACSOj) was assumed to be small relative to C fluxes in litterfall and soil respiration. The published statistical model is TRCA = 1.92 X litterfall + 130. (2) This relationship suggests that C allocation to roots (for tissue production plus respiration) in forests increases with litterfall at the global scale. Disciplines Climate | Plant Biology | Soil Science Comments This article is published as Copyright by the Ecological Society of America, Nadelhoffer, Knute J., James W. Raich, and J. D. Aber. "A global trend in belowground carbon allocation: comment." Ecology 79, no. 5 (1998): 1822-1825. 10.1890/0012-9658(1998)079[1822:AGTIBC]2.0.CO;2. Posted with permission. This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/eeob_ag_pubs/223 Wednesday Jun 03 10:00 AM ecol 79 701 Mp 1822 Allen Press x DTPro File # 01sc 1822 COMMENTS Ecology, Vol. 79, No. 5 Ecology, 79(5), 1998, pp. 1822±1825 q 1998 by the Ecological Society of America erotrophs, litterfall is inputs to soil from aboveground production, export is C loss via erosion and leaching, DCroot is the change in root C (®ne 1 coarse), and DCsoil is the change in soil C (forest ¯oor plus mineral soil). A GLOBAL TREND IN The statistical model (same units) describes a simple BELOWGROUND CARBON linear regression that was derived from a collation of available data in which (export 1DCroot 2DCsoil) was ALLOCATION: COMMENT assumed to be small relative to C ¯uxes in litterfall and soil respiration. The published statistical model is Knute J. Nadelhoffer,1 James W. Raich,2 and TRCA 5 1.92 3 litterfall 1 130. (2) J. D. Aber3 This relationship suggests that C allocation to roots Gower et al. (1996) have questioned the validity of (for tissue production plus respiration) in forests in- using a global-scale relationship between litterfall and creases with litterfall at the global scale. belowground carbon (C) allocation (Raich and Nadel- The statistical model (Eq. 2) is useful for estimating hoffer 1989) at stand and regional scales. We encourage C allocation to roots only when certain conditions are attempts to understand better the controls on C allo- met (see Raich and Nadelhoffer 1989). First, reliable cation to roots in forests, including efforts to evaluate measurements of annual litterfall and soil respiration the potentials and limitations of C budgets for this pur- rates are required. These are relatively easily made, but pose. However, the tests of our C-balance model that details of the methods used must be evaluated in order were presented by Gower et al. use inappropriate com- to assess the quality of model inputs. Second, the model parisons and the conclusions they drew are unwar- is applicable only in relatively mature forests with ``. ranted. In addition, they misinterpret and misapply our soils that are near steady state with respect to total C-budgeting models and their conceptual bases. There- organic carbon storage'' (Raich and Nadelhoffer 1989: fore, we clarify our approach to estimating below- 1347). In other words, the terms ``export,'' ``DCroot,'' ground C allocation (Raich and Nadelhoffer 1989, Na- and ``DCsoil'' (Eq. 1) must be small relative to the other delhoffer and Raich 1992) and highlight problems with two terms in the conceptual model. Importantly, the the tests of our models as conducted by Gower et al. model does not apply to young or to experimentally The issue in question is whether simpli®ed soil C manipulated (e.g., fertilized, recently thinned) stands budgets can be used to estimate total root carbon al- where C ¯uxes into and from soils can differ greatly location (TRCA, the annual rate at which assimilated and where changes in soil C pools can be large. We C is allocated to producing and maintaining roots and emphasize that soils should be near, but need not be mycorrhizae) in forest ecosystems. The conceptual at, steady state for application of the model. By stating model underlying the statistical model we used to pre- otherwise, Gower et al. have misrepresented our model. dict TRCA at global scales (Raich and Nadelhoffer Criticism by Gower et al. of the use of C budgets is 1989) is based on the First Law of Thermodynamics directed toward our published statistical relationship (i.e., conservation of mass) and can be expressed as (Eq. 2) and focuses on three points: (1) the lack of a positive correlation between measured and predicted TRCA ø soil respiration 2 litterfall 1 export belowground C allocation; (2) the lack of a positive 1DCroot2DC soil (1) relationship between estimated ®ne-root production and predicted belowground C allocation; and (3) the where units are grams of C per square meter per year potential misuse of our approach even if it is valid. and where soil respiration is CO -C released from the 2 Points (1) and (2) are legitimate attempts to invalidate soil surface due to respiration by live roots and het- the approach. Point (3) is misdirected in that the va- lidity of the approach is questioned based on its po- Manuscript received 9 June 1997; revised 20 December 1997; accepted 5 January 1998. tential for misuse. We respond below to each point. 1 The Ecosystems Center, Marine Biological Laboratory, Point 1.ÐThe comparison between measured and Woods Hole, Massachusetts 02543 USA. predicted TRCA (Gower et al. 1996: Fig. 1) is not an 2 Department of Botany, 353 Bessey Hall, Iowa State Uni- appropriate test of our statistical model (Eq. 2) or of versity, Ames, Iowa, USA. 3 Complex Systems Research Center, University of New the overall C-budgeting approach because it is based Hampshire, Durham, New Hampshire 03824 USA. on inappropriate data. Of the stands used in their com- Wednesday Jun 03 10:00 AM ecol 79 701 Mp 1823 Allen Press x DTPro File # 01sc July 1998 COMMENTS 1823 parison, only 3 of 17 were .30 yr old and were neither marily on uncertainties associated with TRCA esti- fertilized, irrigated, nor thinned (Gower et al. 1996: mates and do not incorporate uncertainties in FRP es- Table 1). Assuming that soil C content is near steady timates into their discussion. Instead, they use the lack state in such stands is not realistic. Furthermore, meth- of a signi®cant correlation as an opportunity to spec- ods used to estimate litterfall and soil respiration in 8 ulate about possible factors that might cause patterns of their 17 stands are unpublished and therefore un- of aboveground and belowground C allocation by trees veri®able. Data from four additional stands were not to vary from stand to stand. The likely possibility that published as of November 1997 and data from two methodological problems might compromise existing others were published in a non-refereed and unavail- FRP estimates was omitted from their abstract in favor able symposium proceedings.