Skip to main content
  • Submit
  • About
    • Editorial Board
    • PNAS Staff
    • FAQ
    • Rights and Permissions
    • Site Map
  • Contact
  • Journal Club
  • Subscribe
    • Subscription Rates
    • Subscriptions FAQ
    • Open Access
    • Recommend PNAS to Your Librarian
  • Log in
  • My Cart

Main menu

  • Home
  • Articles
    • Current
    • Latest Articles
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • Archive
  • Front Matter
  • News
    • For the Press
    • Highlights from Latest Articles
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Purpose and Scope
    • Editorial and Journal Policies
    • Submission Procedures
    • For Reviewers
    • Author FAQ
  • Submit
  • About
    • Editorial Board
    • PNAS Staff
    • FAQ
    • Rights and Permissions
    • Site Map
  • Contact
  • Journal Club
  • Subscribe
    • Subscription Rates
    • Subscriptions FAQ
    • Open Access
    • Recommend PNAS to Your Librarian

User menu

  • Log in
  • My Cart

Search

  • Advanced search
Home
Home

Advanced Search

  • Home
  • Articles
    • Current
    • Latest Articles
    • Special Features
    • Colloquia
    • Collected Articles
    • PNAS Classics
    • Archive
  • Front Matter
  • News
    • For the Press
    • Highlights from Latest Articles
    • PNAS in the News
  • Podcasts
  • Authors
    • Information for Authors
    • Purpose and Scope
    • Editorial and Journal Policies
    • Submission Procedures
    • For Reviewers
    • Author FAQ

New Research In

Physical Sciences

Featured Portals

  • Physics
  • Chemistry
  • Sustainability Science

Articles by Topic

  • Applied Mathematics
  • Applied Physical Sciences
  • Astronomy
  • Computer Sciences
  • Earth, Atmospheric, and Planetary Sciences
  • Engineering
  • Environmental Sciences
  • Mathematics
  • Statistics

Social Sciences

Featured Portals

  • Anthropology
  • Sustainability Science

Articles by Topic

  • Economic Sciences
  • Environmental Sciences
  • Political Sciences
  • Psychological and Cognitive Sciences
  • Social Sciences

Biological Sciences

Featured Portals

  • Sustainability Science

Articles by Topic

  • Agricultural Sciences
  • Anthropology
  • Applied Biological Sciences
  • Biochemistry
  • Biophysics and Computational Biology
  • Cell Biology
  • Developmental Biology
  • Ecology
  • Environmental Sciences
  • Evolution
  • Genetics
  • Immunology and Inflammation
  • Medical Sciences
  • Microbiology
  • Neuroscience
  • Pharmacology
  • Physiology
  • Plant Biology
  • Population Biology
  • Psychological and Cognitive Sciences
  • Sustainability Science
  • Systems Biology

Evidence for a recent increase in forest growth is questionable

Jane R. Foster, Julia I. Burton, Jodi A. Forrester, Feng Liu, Jordan D. Muss, Francesco M. Sabatini, Robert M. Scheller, and David J. Mladenoff
PNAS May 25, 2010 107 (21) E86-E87; https://doi.org/10.1073/pnas.1002725107
Jane R. Foster
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Julia I. Burton
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Jodi A. Forrester
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Feng Liu
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Jordan D. Muss
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Francesco M. Sabatini
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
Robert M. Scheller
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
David J. Mladenoff
  • Find this author on Google Scholar
  • Find this author on PubMed
  • Search for this author on this site
  • Article
  • Figures & SI
  • Info & Metrics
  • PDF
Loading

In a recent article, McMahon et al. (1) examined forest-plot biomass accumulation across a range of stands in the mid-Atlantic United States and suggest that climate change and trends in atmospheric CO2 explain an increase in forest growth. To show this increase, they fit a simple model to live above-ground forest biomass (AGB) as a function of stand age, and then propose that the derivative of this model is the expected rate of ensemble biomass change (Embedded Image). They conclude that biomass changes within census plots that exceed the ensemble expectation constitute recent increases in growth rates.

We disagree with this conclusion, and instead, we suggest that (i) Embedded Image is incorrectly equated with forest growth, because it ignores past mortality that could explain the difference in rates, (ii) stated trends in plot-species composition could account for plot AGB trajectories that differ from ensemble-model expectations, and (iii) the authors’ model and confidence bounds (CIs) are overly conservative (2), making overlap of rates unlikely.

McMahon et al. (1) equate the derivative of their model with annual growth, but a more complete definition of Embedded Image is described by Caspersen et al. (3) as

Embedded Image

Biomass change within plots may deviate from the median expectation because of differences in growth or mortality rates (Fig. 1). Either rate can vary among plots and through time because of species composition, age structure, herbivory, defoliation, competition, developmental stage, disturbances, etc. Based on Embedded Image data alone, there is no way to determine if growth rates were higher, as the authors assert, or if mortality rates were lower than expected over the last 22 years (Fig. 1). Excluding plots with significant mortality (−Embedded Image) (1) only exacerbates this problem.

Fig. 1.
  • Download figure
  • Open in new tab
  • Download powerpoint
Fig. 1.

(A) Derivative of McMahon et al.’s (1) AGB model (black) with 95% CIs (black dashes) (table 2 in ref. 1). We calculated above-ground net primary productivity (growth; blue) as a function of AGB using equations derived from Forest Inventory and Analysis plot data for the Mid-Atlantic states region (figure 9 in ref. 4). We estimated biomass lost to mortality assuming a constant annual mortality rate of 2.1% (3) of ensemble AGB (red dotted line). The sum of growth and mortality gives us an estimate of biomass change (green dashes), which corresponds with the authors’ model and confirms the definition of Embedded Image (1). We note that temporal variability in either mortality or growth rates could cause point estimates of Embedded Image from census plots to deviate from the model CIs. (B) To illustrate this, we allow the mortality rate to vary randomly over time, assuming that it is ~(0.021,0.005) We find that varying mortality rates (red) cause the observed Embedded Image (green) to frequently fall outside of the model expectations (black lines). If mortality rates decline to 1% for remeasurement intervals similar to those cited by McMahon et al. (1), the resulting Embedded Images exceed the upper confidence bound by approximately the amount observed by the authors (3–4 Mg·ha−1·yr−1).

Mortality also factors into the authors’ assumed succession sequence from young stands dominated by tulip poplar to older oak–hickory stands (1). Because tulip poplar grows very fast, gets very large, and is long-lived (~250 years), young tulip poplar stands cannot transition to oaks over the time frame discussed without experiencing significant biomass loss. In the classic ecosystem-development context (5), aggrading forest stands do accumulate biomass faster than an asymptotic model would suggest, precisely because old stands in the model experienced substantial past mortality before measurement.

Thus, although simple models such as the Monod function (1) can approximate the median relationship in AGB data, they are considered inadequate to fully capture the dynamics of forest-biomass accumulation (5). Furthermore, Clark et al. (2) recommend wider Bayesian CIs that would encompass most of the plot-level variability in AGB and Embedded Image observed in ref. 1. Instead, the authors described applying CIs from parameter estimation to plot-level Embedded Image (figure 2b in ref. 1) when much wider prediction intervals were necessary. We suspect that more appropriate CIs and a complete accounting of uncertainty in plot AGB and stand-age estimation may eliminate the difference between observed and expected Embedded Image that is attributed to recent increases in growth.

The evidence cited by McMahon et al. (1) is unconvincing and certainly does not prove an actual change in recent forest-growth rates; thus, their subsequent speculation on causes including possible CO2 fertilization, although interesting, is premature.

Footnotes

  • 1To whom correspondence should be addressed. E-mail: jrfoster{at}wisc.edu.
  • Author contributions: J.R.F. performed research; J.R.F. analyzed data; and J.R.F., J.I.B., J.A.F., F.L., J.D.M., F.M.S., R.M.S., and D.J.M. wrote the paper.

  • The authors declare no conflict of interest.

    References

    1. ↵
      1. McMahon SM,
      2. Parker GG,
      3. Miller DR
      (2010) Evidence for a recent increase in forest growth. Proc Natl Acad Sci USA 107:3611–3615.
      OpenUrlAbstract/FREE Full Text
    2. ↵
      1. Clark JS,
      2. Mohan J,
      3. Dietze M,
      4. Ibanez I
      (2003) Coexistence: How to identify trophic trade-offs. Ecology 84:17–31.
      OpenUrlCrossRef
    3. ↵
      1. Caspersen JP,
      2. et al.
      (2000) Contributions of land-use history to carbon accumulation in U.S. forests. Science 290:1148–1151.
      OpenUrlAbstract/FREE Full Text
    4. ↵
      1. Jenkins JC,
      2. Birdsey RA,
      3. Pan Y
      (2001) Biomass and NPP estimation for the Mid-Atlantic region (USA) using plot-level forest inventory data. Ecol Appl 11:1174–1193.
      OpenUrlCrossRef
    5. ↵
      1. Bormann FH,
      2. Likens GE
      (1979) Pattern and Process in a Forested Ecosystem (Springer, New York), pp 164–191.
    View Abstract
    PreviousNext
    Back to top
    Article Alerts
    Email Article

    Thank you for your interest in spreading the word on PNAS.

    NOTE: We only request your email address so that the person you are recommending the page to knows that you wanted them to see it, and that it is not junk mail. We do not capture any email address.

    Enter multiple addresses on separate lines or separate them with commas.
    Evidence for a recent increase in forest growth is questionable
    (Your Name) has sent you a message from PNAS
    (Your Name) thought you would like to see the PNAS web site.
    Citation Tools
    Evidence for a recent increase in forest growth is questionable
    Jane R. Foster, Julia I. Burton, Jodi A. Forrester, Feng Liu, Jordan D. Muss, Francesco M. Sabatini, Robert M. Scheller, David J. Mladenoff
    Proceedings of the National Academy of Sciences May 2010, 107 (21) E86-E87; DOI: 10.1073/pnas.1002725107

    Citation Manager Formats

    • BibTeX
    • Bookends
    • EasyBib
    • EndNote (tagged)
    • EndNote 8 (xml)
    • Medlars
    • Mendeley
    • Papers
    • RefWorks Tagged
    • Ref Manager
    • RIS
    • Zotero
    Request Permissions
    Share
    Evidence for a recent increase in forest growth is questionable
    Jane R. Foster, Julia I. Burton, Jodi A. Forrester, Feng Liu, Jordan D. Muss, Francesco M. Sabatini, Robert M. Scheller, David J. Mladenoff
    Proceedings of the National Academy of Sciences May 2010, 107 (21) E86-E87; DOI: 10.1073/pnas.1002725107
    del.icio.us logo Digg logo Reddit logo Twitter logo CiteULike logo Facebook logo Google logo Mendeley logo
    • Tweet Widget
    • Facebook Like
    • Mendeley logo Mendeley
    Proceedings of the National Academy of Sciences: 116 (8)
    Current Issue

    Submit

    Sign up for Article Alerts

    Jump to section

    • Article
      • Footnotes
      • References
    • Figures & SI
    • Info & Metrics
    • PDF

    You May Also be Interested in

    News Feature: Cities serve as testbeds for evolutionary change
    Urban living can pressure flora and fauna to adapt in intriguing ways. Biologists are starting to take advantage of this convenient laboratory of evolution.
    Image credit: Kristin Winchell (Washington University in St. Louis, St. Louis).
    Several aspects of the proposal, which aims to expand open access, require serious discussion and, in some cases, a rethink.
    Opinion: “Plan S” falls short for society publishers—and for the researchers they serve
    Several aspects of the proposal, which aims to expand open access, require serious discussion and, in some cases, a rethink.
    Image credit: Dave Cutler (artist).
    Featured Profile
    PNAS Profile of NAS member and biochemist Hao Wu
     Nonmonogamous strawberry poison frog (Oophaga pumilio).  Image courtesy of Yusan Yang (University of Pittsburgh, Pittsburgh).
    Putative signature of monogamy
    A study suggests a putative gene-expression hallmark common to monogamous male vertebrates of some species, namely cichlid fishes, dendrobatid frogs, passeroid songbirds, common voles, and deer mice, and identifies 24 candidate genes potentially associated with monogamy.
    Image courtesy of Yusan Yang (University of Pittsburgh, Pittsburgh).
    Active lifestyles. Image courtesy of Pixabay/MabelAmber.
    Meaningful life tied to healthy aging
    Physical and social well-being in old age are linked to self-assessments of life worth, and a spectrum of behavioral, economic, health, and social variables may influence whether aging individuals believe they are leading meaningful lives.
    Image courtesy of Pixabay/MabelAmber.

    More Articles of This Classification

    • Need for early, minimally invasive cancer diagnosis
    • Reply to Schellenberg: Is there more to auditory plasticity than meets the ear?
    • Reply to Rajendran and Schnupp: Frequency tagging is sensitive to the temporal structure of signals
    Show more

    Related Content

    • No related articles found.
    • Scopus
    • PubMed
    • Google Scholar

    Cited by...

    • Reply to Foster et al.: Using a forest to measure trees: Determining which vital rates are responding to climate change
    • Scopus (13)
    • Google Scholar

    Similar Articles

    Site Logo
    Powered by HighWire
    • Submit Manuscript
    • Twitter
    • Facebook
    • RSS Feeds
    • Email Alerts

    Articles

    • Current Issue
    • Latest Articles
    • Archive

    PNAS Portals

    • Classics
    • Front Matter
    • Teaching Resources
    • Anthropology
    • Chemistry
    • Physics
    • Sustainability Science

    Information

    • Authors
    • Editorial Board
    • Reviewers
    • Press
    • Site Map

    Feedback    Privacy/Legal

    Copyright © 2019 National Academy of Sciences. Online ISSN 1091-6490