Steeper declines in forest photosynthesis than respiration explain age-driven decreases in forest growth

Abstract

The traditional view of forest dynamics originated by Kira and Shidei [Kira T, Shidei T (1967) Jap J Ecol 17:70-87] and Odum [Odum EP (1969) Science 164(3877):262-270] suggests a decline in net primary productivity (NPP) in aging forests due to stabilized gross primary productivity (GPP) and continuously increased autotrophic respiration (R a ). The validity of these trends in GPP and R a is, however, very difficult to test because of the lack of long-term ecosystem-scale field observations of both GPP and R a . Ryan and colleagues [Ryan MG, Binkley D, Fownes JH (1997) Ad Ecol Res 27:213-262] have proposed an alternative hypothesis drawn from site-specific results that aboveground respiration and belowground allocation decreased in aging forests. Here, we analyzed data from a recently assembled global database of carbon fluxes and show that the classical view of the mechanisms underlying the age-driven decline in forest NPP is incorrect and thus support Ryan's alternative hypothesis. Our results substantiate the agedriven decline in NPP, but in contrast to the traditional view, both GPP and R a decline in aging boreal and temperate forests. We find that the decline in NPP in aging forests is primarily driven by GPP, which decreases more rapidly with increasing age than R a does, but the ratio of NPP/GPP remains approximately constant within a biome. Our analytical models describing forest succession suggest that dynamic forest ecosystem models that follow the traditional paradigm need to be revisited.chronosequence | plant respiration | carbon use efficiency | ecosystem development | eddy covariance I t has been long observed and well established that forest net primary production (NPP), particularly aboveground NPP, increases during initial stand development, peaks at maturity, and then gradually declines as forests age (1-8). Kira and Shidei (9) were the first to analyze 10 y of empirical data and developed the long-accepted theory that forest NPP declines with age because wood respiration increases in response to the accumulating wood biomass, whereas gross primary production (GPP) remains quasiconstant (Fig. 1). Similarly, in his theory of ecosystem succession, Odum (10) postulated that ecosystem respiration (i.e., the sum of autotrophic and heterotrophic respiration) increases with age and eventually balances GPP such that the net ecosystem carbon balance approaches zero at a dynamic steady state. Odum did not specify the successional pattern of autotrophic respiration (R a ) and NPP, but the underlying assumption is similar to that of Kira and Shidei, i.e., the difference between carbon uptake and release declines with age primarily because respiratory losses increase.Ryan et al. have disproved these earlier hypothesized patterns and contended that total stem respiration, including growth and maintenance respiration, in a chronosequence of subalpine lodgepole pine (Pinus contorta ssp. latifolia) stands in Colorado did not increase with forest age (11), and that the observed decrease in aboveground NPP wi...

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