Introduction

The Oak Ridge DAAC Net Primary Production (NPP) Database includes field measurements from grassland study sites worldwide. The following brief review and discussion is intended to explain the complexity of NPP estimates derived from grassland measurements. There is no single answer to the question, "What is the productivity of the ecosystem at study site A?"; rather there may be range of estimates of NPP, depending upon what data were actually collected and how these data are processed. Although some of these methods for determining NPP for grasslands may be applicable to other vegetation types (e.g., semi-deserts, tundra, or some crops), methods for forests, in particular, are significantly different. Nevertheless, it should be possible to answer the question, "Is this modelled value of NPP reasonable for this ecosystem type at this location?"

Since the field measurements were collected by research workers using a variety of methods, secondary users of these data such as the modelling community are encouraged to consult the general scientific literature as well as the published study site descriptions (see References) in order to more fully understand the data from each site.

Brief Literature Review

Net primary production, sensu stricto, is the total photosynthetic gain (less respiratory losses) of vegetation per unit ground area. For a given period, this is equal to the change in plant mass plus any losses due to death and decomposition, measured for both above ground and below ground plant parts. Earlier estimates of grassland NPP were based on peak standing dry matter only, and the studies of the International Biological Programme (IBP) in the late 1960s and early 1970s were based mainly on above-ground biomass changes, with few estimates of below-ground production.

Peak above-ground live biomass (or in some cases, the difference between maximum and minimum biomass) has been used as an estimate of net primary production - usually where only one or two measurements per year are available. Sometimes a conversion factor has been applied to take into account the estimated turnover and the estimated ratio of above-ground to below-ground dry matter.

The "IBP Standard Method" of Milner and Hughes (1968) assumes that where live biomass increases between successive samples, production equals this increase; where biomass decreases or remains the same, production is assumed to be zero. Annual production is then obtained by summing the estimates for each sample interval.

Essentially, this method was used for the IBP synthesis by Singh and Joshi (1979), in particular for their estimates of below-ground production. A modified method was used for above-ground production, determined by a decision matrix (Singh et al., 1975). In cases where increments in live biomass coincided with increases in standing dead matter, the latter were added to the monthly production.

The limitations of the above methods are discussed in detail by Long et al. (1989) and Long et al. (1992). In particular, the peak biomass method and variations on the IBP method underestimate production by not accounting for simultaneous growth and death. This may be significant in temperate grasslands with a long growing season, and is particularly a problem in tropical grasslands where the growing season may extend over much of the year. Some limited overestimation may occur by not accounting for periods of negative NPP (due to stress, or translocation between above and below ground plant parts) but underestimation of root turnover is probably the largest source of error. Long et al. (1989) estimated NPP for three terrestrial tropical grassland sites by summing monthly changes in live biomass plus losses due to death and decomposition for above and below ground vegetation. Monthly losses were determined as the change in dead matter plus the estimated disappearance of dead matter through decomposition. Dead matter disapperance was calculated each month as the product of relative decomposition rate and mean amount of dead matter.

Although some correlation between estimates obtained using different methods has been reported (Singh et al., 1975), the degree of underestimation may be strongly site-specific (Linthurst and Reimold, 1978; Long and Mason, 1983). Where sufficient data are available for a given grassland site, it may be possible to estimate NPP according to the different methods for the purposes of comparison. This may involve entry of data into algorithms or a spreadsheet containing these algorithms.

Methods and Algorithms for Estimating NPP

Annual NPP may be calculated for a given year (Jan-Dec) or for any appropriate 12-month growing cycle, depending on latitude (Northern or Southern hemisphere growing season) and environmental or management factors which determine this cycle. For example, annual burning during a short dry season is an important "scheduling" factor in many humid savannas. Note that the length of the growing season varies widely, from as little as 3 months in extreme continental or semi-arid conditions to as much as 12 months in some humid tropical regions. In some cases (e.g. East Africa) precipitation may be bi-modal, leading to two growing seasons per year.

Changes in live biomass and dead matter, above and below ground, are measured according to the general methodology described in the paragraphs that follow:

Dry weight of each above-ground category is determined at intervals of one month or less (preferably) within a specified number of randomly located quadrats or in a randomized block design of quadrats. Dry weight is determined by clipping standing biomass to ground level and collecting the litter (fallen dead matter) from the area of each quadrat. Clipped material is sorted into live leaves and standing dead matter, and stems are sorted likewise, paying attention to removal of dead sheaths from live stems.

Below-ground plant matter is sampled by removing soil cores from the center of each quadrat, to a depth determined by trial sampling to retrieve at least 80% of below-ground matter (usually 15-30 cm). Where the soil structure or hardness does not permit the use of corers (gouges) or augers, soil pits may be dug to specified dimensions. Soil samples are washed over a 2 mm sieve, because the ability to pass through a 2 mm mesh is generally taken as the arbitrary division between recognisable dead matter and particulate organic matter. Larger roots may be removed and weighed separately from fine roots (less than about 1 mm diameter). Fine roots may be sub-sampled before separation into live and dead matter on the basis of tissue necrosis, using vital staining such as tetrazolium salts where visual discrimination is difficult. All sorted plant matter is washed and dried to constant weight at 90-95 degrees Celsius. Below-ground sampling is time-consuming and can be expensive, so data is often of poor quality or absent altogether.

Decomposition or disappearance of dead matter may be determined using "paired plots" or litter bags (Weigert and Evans, 1964; Long et al., 1989). The paired plots technique involves measuring dead matter present at the start and end of a sampling interval, whereas the latter comprises the use of (typically) 2 mm nylon mesh bags. Bags of dead above-ground matter are placed at the ground surface, and dead below-ground matter is buried in the soil. Bags are recovered at the end of the sample interval, and the loss of material determined and expressed as a relative decomposition rate.

Statistical procedures exist for determining the optimum number and size of quadrats for sampling within desired tolerances. However, it is generally not practicable to maximise the number of quadrats so as to obtain a statistically significant difference in biomass between consecutive samples (Long et al., 1992).

Methods for calculating NPP are summarised below (refer to the Directory of Terms and Definitions for further details):


1a. Peak Biomass Method A

ANPP = max {AGbiomass}

Assumptions

  • any standing dead matter or litter was carried over from previous year, and death in current year is negligible
  • live biomass was not carried over from previous year
  • below-ground production is ignored, or estimated only as a fraction of above-ground production using a crude root/shoot ratio

Conclusion

  • may be applicable to annual arable crops, but clearly a poor estimate of production for perennial vegetation (i.e. most natural plant communities), especially where below-ground turnover may be significant. May be useful for crude comparisons between seasonal temperate grasslands, but has little meaning for tropical grasslands, and should definitely not be used to compare temperate and tropical grasslands.


1b. Peak biomass method B

ANPP = max {AGTotclip}

Assumptions

  • any standing dead matter was formed by death in current year, hence counts as part of this year's plant production
  • no standing dead matter has yet fallen as litter or decomposed
  • neither live biomass nor standing dead matter were carried over from previous year
  • below-ground production is ignored, or estimated only as a fraction of above-ground production using a crude root/shoot ratio

Conclusion

  • as for Method (1a) above; may be a slightly better estimate of NPP where significant death occurs during the growing season.


1c. Max-Min Method

NPP = max {AGbiomass} - min {AGbiomass}

Assumptions

  • as for Method (1a), but any live biomass carried over from the previous year is excluded

Conclusion

  • as for Method (1a) above; subtraction of minimum biomass is likely to be a useful correction only under limited conditions.


2a. IBP Standard Method (Milner and Hughes, 1968)

NPP = sum {positive increments in AGbiomass}

Assumptions

  • most growth occurs between successive sample intervals, i.e. simultaneous growth and death do not occur
  • NPP is never negative during a sample interval
  • below-ground production may be similarly measured, ignored altogether, or estimated only as a fraction of above-ground production using a crude root/shoot ratio

Conclusion

  • this allows for several distinct phases of growth within a year, but still fails to account for new shoot growth during periods of high mortality, and vice versa. However, for sites where data on biomass dynamics are available (preferably both above and below ground) a more dynamic comparison of net primary production may be possible. Nevertheless, comparisons between temperate grasslands displaying marked seasonal changes in biomass and tropical grasslands (where biomass may not change much despite high turnover) should be avoided.


2b. Modified IBP Standard Method ("Smalley's method" Singh et al., 1975)

NPP = sum {growth increment}

where "growth increment" = positive increment in AGbiomass

UNLESS {AGTotdead} increases for that sample interval, IN WHICH CASE:
"growth increment" = positive increment in AGbiomass + positive increment in AGTotdead

Assumptions

  • simultaneous growth, death and decomposition (i.e. continuous turnover) does not occur
  • NPP is never negative during a sample interval
  • below-ground production may be similarly measured, ignored altogether, or estimated only as a fraction of above-ground production using a crude root/shoot ratio

Conclusion

  • as for Method (2a), although the correction for material lost by death during periods of biomass increase will reduce the degree of underestimation of NPP

Note: AGTotdead = (Stdead + litter)


3. "UNEP Project" Method (after Weigert and Evans, 1964)

NPP = sum {change in AGbiomass + change in AGTotdead
+ (AGr x AGTotdead)}

Note: AGr = above-ground relative rate of decomposition

Assumptions

  • measured changes in parameters are statistically significant over each sample interval (in practice, this may be very hard to achieve, since an impractically large number of samples would be required to detect real but modest changes over each sampling interval)
  • decomposition rate is independent of the composition of dead matter (in fact, it will decline exponentially as a function of lignin:N ratio)
  • losses of AGbiomass and AGTotdead by grazing, root exudation, etc. are negligible
  • below-ground production may be similarly measured, or estimated only as a fraction of above-ground production using a crude root/shoot ratio

Conclusion

  • this is the only method which incorporates all components required for an accurate estimate of NPP (and then only if both above and below-ground production are measured). Although such detailed data are not available for all study sites, it provides a useful benchmark against which to check the possible degree of underestimation using other methods. Where detailed biomass dynamics are available but no data exists on decomposition or disapperance of dead matter, it may be possible to improve on estimates by modelling decomposition using data from other similar sites. However, such applications are outside the scope of this summary.

References

Linthurst, R. and R.J. Reimold (1978) An evaluation of methods for estimating the net primary production of estuarine angiosperms. J. Applied Ecology 15, 919-932.

Long, S.P. and Mason, C.F. (1983) Saltmarsh Ecology. Blackie, Glasgow.

Long, S.P., E. Garcia Moya, S.K. Imbamba, A. Kamnalrut, M.T.F. Piedade, J.M.O. Scurlock, Y.K. Shen and D.O. Hall (1989) Primary productivity of natural grass ecosystems of the tropics: a reappraisal. Plant and Soil 115, 155-166.

Long, S.P., M.B. Jones and M.J. Roberts, eds. (1992) Primary Productivity of Grass Ecosystems of the Tropics and Sub-tropics. Chapman and Hall, London. 267 pp.

Milner, C. and R.E. Hughes (1968) Methods for the Measurement of the Primary Production of Grassland. IBP Handbook No.6. Blackwell, Oxford.

Singh, J.S. and M.C. Joshi (1979) Tropical grasslands primary production. IN: Grassland Ecosystems of the World (R.T. Coupland, ed.) Cambridge University Press. pp. 197-218.

Singh, J.S., W.K. Lauenroth and R.K. Sernhorst (1975) Review and assessment of various techniques for estimating net aerial primary production in grasslands from harvest data. Botanical Review 41, 181-232.

Weigert, R.G. and F.C. Evans (1964) Primary production and the disappearance of dead vegetation on an old field in south-eastern Michigan. Ecology 45, 49-63.