2001 Clark Et Al Ecol Apps Measuring Net Primary Production | Forests

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Una buena guía para medir los principales componentes de la productividad primaria neta en bosques tropicales.
  356  Ecological Applications,  11(2), 2001, pp. 356–370   2001 by the Ecological Society of America MEASURING NET PRIMARY PRODUCTION IN FORESTS:CONCEPTS AND FIELD METHODS D EBORAH  A. C LARK , 1 S ANDRA  B ROWN , 2,7 D AVID  W. K ICKLIGHTER , 3 J EFFREY  Q. C HAMBERS , 4 J OHN  R. T HOMLINSON , 5 AND  J IAN  N I 61  Department of Biology, University of Missouri-St. Louis; Mailing address: INTERLINK-341, P.O. Box 02-5635, Miami, Florida 33102 USA 2  Department of Natural Resources and Environmental Sciences, University of Illinois, W503 Turner Hall,Urbana, Illinois 61801 USA 3 The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543 USA 4  National Center for Ecological Analysis and Synthesis, University of California,Santa Barbara, California 93101-3351 USA 5  Institute for Tropical Ecosystem Studies, University of Puerto Rico, P.O. Box 363682,San Juan, Puerto Rico 00936-3682 USA 6  Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Xiangshan Nanxincun 20, 100093 Beijing, P.R. China  Abstract.  There are pressing reasons for developing a better understanding of netprimary production (NPP) in the world’s forests. These ecosystems play a large role in theworld’s carbon budget, and their dynamics, which are likely to be responding to globalchanges in climate and atmospheric composition, have major economic implications andimpacts on global biodiversity. Although there is a long history of forest NPP studies inthe ecological literature, current understanding of ecosystem-level production remains lim-ited. Forest NPP cannot be directly measured; it must be approached by indirect methods.To date, field measurements have been largely restricted to a few aspects of NPP; methodsare still lacking for field assessment of others, and past studies have involved confusionabout the types of measurements needed. As a result, existing field-based estimates of forestNPP are likely to be significant underestimates.In this paper we provide a conceptual framework to guide efforts toward improvedestimates of forest NPP. We define the quantity NPP* as the summed classes of organicmaterial that should be measured or estimated in field studies for an estimate of total NPP.We discuss the above- and belowground components of NPP* and the available methodsfor measuring them in the field. We then assess the implications of the limitations of paststudies for current understanding of NPP in forest ecosystems, discuss how field NPP*measurements can be used to complement tower-based studies of forest carbon flux, andrecommend design criteria for future field studies of forest NPP. Key words: biomass increment; boreal, temperate, and tropical forests; carbon; coarse roots; fine root turnover; forest inventory plots; litterfall; net ecosystem C exchange; net primary production;total belowground carbon allocation. I NTRODUCTION An important current research need is to develop abetter understanding of net primary production (NPP)in the world’s forests, ecosystems that play a majorrole in the global carbon budget (Dixon et al. 1994).While unprecedented atmospheric concentrations(Petitet al. 1999) of the greenhouse gas carbon dioxide (CO 2 )continue to increase due to anthropogenic activities,large uncertainties affect current understanding of theworld’s carbon budget (Melillo et al. 1996). One suchuncertainty is the balance between NPP and heterotro-phic respiration in forests globally. Small shifts be-tween these fluxes can greatly affect atmospheric CO 2 Manuscript received 29 November 1999; revised 16 March2000; accepted 10 April 2000; final version received 10 April2000. 7 Present address: Winrock International, 1611 N. KentStreet, Suite 600, Arlington, Virginia 22209 USA. concentrations. At the same time, changing climate andatmospheric chemistry (e.g., CO 2  levels, nitrogen de-position) are likely to be causing on-going changes inforest NPP. The design and evaluation of global-scalecarbon models require field estimates of forest NPPand how it is responding to these global changes. Inaddition, a better grasp of NPP would help improveassessments of forest-level carbon (C) exchange withthe atmosphere developed from eddy covariance mea-surements (cf. Goulden et al. 1998, Lindroth et al.1998, Running et al. 1999). Such improvements in ourunderstanding of forest carbon dynamics can then beused to develop better policy decisions related to forestproduction or conservation.Progress in understanding NPP and its controls inforest ecosystems is hindered by the limitations of theexisting field data (see recent reviews for boreal andtropical forests, respectively: Clark et al. 2001, Gower  April 2001 357 MEASURING FOREST NPP F IG . 1. The components of (a) forest NPP and (b) NPP*, the sum of all materials that together represent: (1) the amountof new organic matter that is retained by live plants at the end of the interval, and (2) the amount of organic matter that wasboth produced and lost by the plants during the same interval. CHO    carbohydrates. et al. 2001). Although the ecological literature containsa plethora of papers on the topic, reported estimates of forest NPP are based on incomplete, and sometimesinappropriate, field measurements. The substantial ef-forts that are required for NPP field studies, the un-resolved methods challenges, and a frequent lack of conceptual clarity are continuing obstacles to improv-ing our knowledge of NPP.In this paper we examine how forest NPP (above-and belowground) can be estimated based on field mea-surements. We then assess the implications of the lim-itations of past studies for current perceptions of theamount of NPP in forests globally, and we discuss howfield NPP studies can be used to evaluate tower-basedmeasurements of forest carbon flux. We conclude witha set of priorities and design criteria for field studiesaimed at a more complete assessment of forest NPPthan has been achieved to date.E STIMATING  F OREST  NPP  IN THE  F IELD Net primary production is the difference betweentotal photosynthesis (Gross Primary Production, GPP)and total plant respiration in an ecosystem. In the field,however, it is not possible to measure forest NPP interms of this difference (Waring and Schlesinger 1985).GPP cannot be measured directly (Ryan 1991), andestimating total plant respiration at the ecosystem levelremains difficult and involves significant uncertainties(cf. Ryan et al. 1996, Lavigne et al. 1997). Alterna-tively, NPP is defined as the total new organic matterproduced during a specified interval. Although thecomponents of this production are readily conceptu-alized (Fig. 1a), they cannot be directly measured inthe field because of transformations (consumption, de-composition, mortality, export) they undergo during themeasurement interval. Instead, NPP must be estimatedbased on a suite of measurements of various types andnumerous underlying assumptions. To clarify the un-derlying concepts and to provide a complete and in-ternally consistent framework for field studies, we de-fine the quantity NPP*, the field-measurement-based,operational estimate of actual NPP. NPP* (Fig. 1b) isthe sum of all materials that together are equivalent to:(1) the amount of new organic matter that is retainedby live plants at the end of the interval, and (2) the  358  DEBORAH A. CLARK ET AL.  Ecological ApplicationsVol. 11, No. 2 amount of organic matter that was both produced andlost by the plants during the same interval.In practice, few NPP* components are measured infield studies in forest ecosystems. Most frequently,measurements are restricted to fine litterfall and above-ground biomass increment, and their sum is consideredequivalent to aboveground NPP (ANPP). Belowgroundcomponents are often ignored or are estimated as sometheoretical proportion of aboveground values. For ex-ample, among 48 field NPP studies in tropical forests(Clark et al. 2001: Appendix), litterfall and above-ground biomass increment were measured in 94% and60%, respectively, of the studies; no other componentof aboveground NPP* (Fig. 1b) was measured in anystudy, and in only 13% of the studies was any aspectof belowground production assessed. Similarly, Longand Hutchin (1991) reported that in   10% of the In-ternational Biological Programme NPP studies was anybelowground biomass measured. In addition, data re-ported in some past NPP studies are unusable due toinadequate methods and/or inadequate documentationof methods.Reliable assessment of the amount of forest NPP willrequire quantifying all materials that contribute to totalNPP* (Fig. 1b), for at least a benchmark set of sites.When complete accounting is available for represen-tative sites in each major forest type, it will be possibleto identify those materials that can be ignored withoutproducing serious underestimates of site NPP. For allNPP* components in forests, however, there are prac-tical and theoretical challenges for obtaining accurateestimates. Further, the appropriate methods for someof them will differ among forest types. Below we re-view these considerations for all NPP* constituents.  Aboveground increments and losses Aboveground biomass increment  .—In most forestecosystems, aboveground biomass and its incrementare strongly dominated by the overstory trees. For ex-ample, it has been estimated that understory vegetationin mature moist tropical forests generally comprises  3% of the aboveground biomass (Brown 1997); giventhe low light levels close to the ground in these forests,the contribution by this stratum to total abovegroundbiomass increment is negligible. Important exceptionsto this general rule are boreal forests and temperate andtropical woodlands with an open overstory and a denseground cover or shrub layer. In such forests, whereproduction in the understory can be substantial and mayeven exceed that of the trees (cf. Black et al. 1996,Gower et al. 2001), estimating total NPP requires spe-cial techniques for measurement of aboveground pro-duction by nontree vegetation. In most closed forests,however, aboveground biomass production can be re-liably based on the biomass increment by trees abovea carefully chosen minimum size. In large-stature for-est, trees   10 cm in diameter are likely to constitute  90% of vegetation biomass, and would thus sufficefor estimates of aboveground increment. In smallerstature stands such as young second-growth or tropicaldry forest, a lower minimum tree size should be usedand documented (cf. Murphy and Lugo 1986, Brown1997).Aboveground biomass increment is estimated fromtwo successive stand-level biomass estimates. Biomassis estimated by applying harvest-based allometric re-gression equations to measurements of the diametersof all trees in a plot that are above the minimum size.Developing site-specific allometric equations for foresttrees is laborious (harvesting one tropical emergent canrequire   25 person-days). Researchers therefore com-monly use existing allometric equations (cf. Brown1997, Gower et al. 1999). Because of the potential forintersite variation in factors such as tree architectureand wood density, this practice can introduce errors inestimated aboveground increment (see Gower et al.1999). For example, Grier et al. (1984) used both site-specific and generalized regression equations to cal-culate foliage biomass for five  Pseudotsuga menziesii stands, and found the generalized equations producederrors of    24 to   93%. For this reason, when locallyderived equations for the species under study are notavailable, it is important to match the allometric equa-tion as closely as possible to the site under study (i.e.,use an equation based on data from one or more sitesof comparable climatic and edaphic conditions), or,preferably, to test its predictions by first measuring andthen harvesting individual trees or forest plots on site.Two approaches (Fig. 2) can then be used for esti-mating aboveground biomass increment from fieldmeasurements and biomass allometry. Both approachesgive the same estimated aboveground biomass incre-ment, although tree mortality and ingrowth (trees thatgrew past the minimum diameter during the interval)have to be accounted for differently in the two cases.Not understanding the subtleties between these twomethods can lead to erroneous estimates of above-ground biomass increment and thus ANPP or total NPP.Approach 1 is based on tracking individual trees.The increment for each tree is calculated as the dif-ference between its estimated biomass at the beginningand end of the interval. If a tree dies in the measurementinterval and the intercensus interval is short, the treecan be assumed to have no increment and is ignoredin the calculation (but see Approach 2). Thus, for thestand, increments are summed for all trees survivingthe interval. This total is then adjusted for ingrowth;the increment of each new tree is calculated as thedifference between its estimated biomass at the end of the interval and the biomass of a tree of the minimummeasured diameter. The summed increments of the in-growth are then added to the stand increment. A variantof this approach, but with the same calculation methods(Approach 1, Fig. 2), can be used in forests where thetrees make reliable annual rings. All the live trees ina plot are cored, the annual rings they formed over the  April 2001 359 MEASURING FOREST NPP F IG . 2. The two methods for calculating aboveground biomass increment based on measurement of all trees in a plot atthe beginning and end of an interval. Approach 1 is based on tracking individual surviving trees. Approach 2 is based onmeasuring all trees in the stand at each census but also requires measurement of trees (a) that died in the interval and (b)that recruited past the minimum size during the interval. AGB    aboveground biomass of live trees; AGB of a minimum-sized tree is set at 2 units. interval of interest are measured, and these radial in-crements are then converted to biomass increments us-ing the allometric equation.In Approach 2 (Fig. 2), the estimated total above-ground biomass of the stand at the beginning of theinterval is subtracted from the estimated total above-ground biomass of the stand at the end of the interval,with no reference to increments of individual trees.This difference then has to be adjusted for both treemortality and ingrowth. For the mortality correction,the biomass of all trees that died during the interval isestimated from their initial diameters and added to thenet change in stand biomass. If the measurement in-terval is long (  2 yr), attempts should be made toestimate the within-interval growth by all trees thatdied; failure to do this will produce an underestimateof stand increment, especially when large trees diedduring a many-year interval. To correct for ingrowth,the number of new trees is multiplied by the biomassof the minimum-diameter tree being measured (see Ap-proach 1), and this product is subtracted from the mor-tality-adjusted stand increment.Approach 2 is often used to calculate abovegroundbiomass increment from large-scale forest inventorydata (e.g., Schulze et al. 1999) or for stands that areremeasured after a long interval (e.g., 35 yr, Weaverand Murphy 1990). Failing to make the corrections fortree mortality when using Approach 2 will result inunderestimating the aboveground biomass incrementby an amount at least equal to the initial biomass of trees that died in the interval (see above). In tropicalforests, for example, because stand-level mortality of trees  10 cm in diameter is usually 1–3% of stems peryear and is independent of tree size (Swaine et al. 1987,Hartshorn 1990), the underestimate caused by not ac-counting for dead trees will average   1–3% of theinitial aboveground biomass per year of census interval(as found by Carey et al. 1994). Within-interval growthof the trees that died in the interval would augmentthis error. Similarly, in temperate zone plantations orsuccessional forests with high tree mortality due tothinning, either natural or anthropogenic, the ‘‘miss-ing’’ biomass from trees that died in the interval couldbe very large. Thus, failing to correct for tree mortalitywhen using Approach 2 (e.g., Weaver and Murphy1990) can produce a substantial underestimate of NPP.It will also have a high variance, especially in areasaffected by catastrophic disturbance (hurricanes, land-slides), and in small study plots, where not accountingfor the death of one very large tree could result in asevere underestimate of aboveground increment. Formany published estimates of aboveground incrementfrom both boreal and tropical forests, it is not possibleto judge the reliability of the estimates because theauthors failed to state how they dealt with mortality(see Clark et al. 2001, Gower et al. 2001).Even when the biomass accounting is carried outcorrectly, several other sources of error can affect es-timates of aboveground biomass increment. One isover- or underestimating the biomass of very large trees(arbitrarily defined as those of diameters   70 cm). Intropical forests, for example, although densities of suchtrees are low (usually   10% of stems   10 cm in di-ameter), they can comprise 25–50% of the total above-ground biomass (Brown and Lugo 1992, Brown et al.1995, Clark and Clark 1996). If the biomass allometry
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