NPP Tropical Forest: Consistent Worldwide Site Estimates, 1967-1999

Data Citation

Cite this data set as follows:

Clark, D. A., S. Brown, D. W. Kicklighter, J. Q. Chambers, J. R. Thomlinson, J. Ni, and E. A. Holland. 2001. NPP Tropical Forest: Consistent Worldwide Site Estimates, 1967-1999. Data set. Available on-line [http://www.daac.ornl.gov] from the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.

Summary

It is clear that the currently available data on net primary production in tropical forests are extremely limited and that even our best estimates for this biome can only be thought of as rough approximations within wide bounds. Nevertheless, this study has provided a basis for evaluating the quality and utility of the data generated by past studies and of the NPP estimates that have been reported in the literature for tropical forests.

Clearly, the most important knowledge gap is with respect to the belowground components of NPP in these ecosystems. In particular, fine root losses are likely to be a substantial proportion of NPP in many tropical forests, and they as yet remain unquantified. It is also clear that there have been many of the previous NPP studies in this biome have involved methodological problems and poor documentation. Henceforward, it is paramount that such studies be carefully designed and reported so as to avoid these pitfalls. A third issue that needs to be addressed is that of sampling bias and the lack of replication in either time or space for NPP studies in these forests. Given the very small and subjectively located plots that are the basis for most of the existing data, it is possible that the existing understanding of NPP processes in tropical forests is highly skewed toward the highest biomass patches (flat sites with large trees, no gaps) and is unrepresentative of the larger forest landscapes.

There is a great need for well-designed field studies of NPP in sites spanning the very broad climatic range that is covered by tropical forests. Of particular importance at this stage will be more studies of old-growth stands of these diverse forest types. While successional and human-impacted tropical forest systems are clearly of great ecological and economic importance, NPP processes in them are highly variable due to successional processes and/or to the varied degrees and qualities of human impacts on them. It thus seems more productive to first concentrate on broadening our base of understanding of the more predictable processes in old-growth stands, which are the basis of many global modeling efforts and which also will be the more interpretable guides to how tropical forest ecosystems are responding to processes of global climatic and atmospheric change. For such studies, we have the following recommendations:

Study plots at a site should be replicated, the number and size of these plots should be determined by pre-sampling with respect to the variance in aboveground biomass, and siting should be based on a random or stratified-random design.
Production should be measured through at least two or three full yearly cycles, to give some bounds on the temporal variance and climatic sensitivity of NPP at each tropical forest site.
For litterfall, large numbers of collectors should be used, the wood component should be restricted to pieces < 1 cm in diameter, data must come from at least a full year cycle and from biweekly collections, components should be quantified and reported separately, and the statistical uncertainty around the litterfall values should be calculated and reported.
For aboveground biomass increment, estimates should be based on re-measurement of at least all woody stems in each plot above a reasonable minimum diameter for that forest type (10 cm in most tropical moist and wet forest, smaller diameters in many tropical dry and montane forests), all boles should be measured above buttresses (and this should be reported), data should be given separately for palms, lianas, and hemiepiphytes, and if possible, appropriate allometries should be used for them, and the details of the biomass accounting should be reported.
A major emphasis should be placed on obtaining reliable estimates for fine root losses, preferably by combining at least two of the best currently available techniques (e.g., minirhizotrons, and sequential coring + decomposition studies), with sampling to at least 1 m depth and with as much replication as logistically possible.
Ancillary studies by other investigators should be sought, to be carried out concurrently at the same site, to obtain at least rough estimates of little-studied components of NPP (coarse root increments, emission of BVOC's, rhizodeposition, losses to consumers).
When publishing values for NPP components, core site descriptions should always be provided, including: mean annual temperature, mean annual precipitation (and preferably the annual temperature and rainfall for the year(s) when production was measured); elevation; soil characteristics; forest age and extent of human impact, and precise site location data (latitude and longitude or more precise GPS data if possible).

References

Clark, D. A., S. Brown, D. W. Kicklighter, J. Q. Chambers, J. R. Thomlinson, J. Ni, and E. A. Holland. 2001. Net primary production in tropical forests: an evaluation and synthesis of existing field data. Ecological Applications 11, 371-384.

Clark, D. A., S. Brown, D. W. Kicklighter, J. Q. Chambers, J. R. Thomlinson, and J. Ni. 2001. Measuring net primary production in forests: concepts and field methods. Ecological Applications 11, 356-370.

Contact Information

Principal Contact: Dr. D. A. Clark
La Selva Biological Station
INTERLINK-341
P.O. Box 02-5635
Miami, FL 33102
U.S.A.
Tel. +506 766-6565 ext. 147
Fax +506 766-6535
Email: daclark@sloth.ots.ac.cr

This work was conducted as part of the Worldwide NPP Working groups supported by the U.S. National Center for Ecological Analysis and Synthesis, a Center funded by NSF (Grant #DEB-94-21535), the University of California at Santa Barbara, and the State of California.