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Life Span (Plants) -- Summary
In a study that introduces us to the subject of plant life span, Laurance et al. (2004), as they describe it, "use[d] data from a large-scale demographic study spanning an 18-year period in central Amazonia to infer maximum longevity of 93 tree species, based on measured rates of trunk-growth and tree mortality."  The data were collected on 3,159 individual trees located in 24 1-ha plots of undisturbed forest scattered throughout an approximate 1000-km2 area.  The six scientists determined that about 25% of the species studied attained ages of less than 200 years, nearly 60% had life spans in the range of 200 to 500 years, and the remaining 15% lived from 500 to 1000 years.  Laurance et al. remark, however, that "had we sampled an area 10 times as large, we would have encountered larger individuals of most species, which would have increased their estimated longevities."  Hence, they say their findings "appear to be consistent with the notion that central Amazonia supports ancient (greater than 1000 years old) trees (cf. Chambers et al., 1998, 2001)."

So what, specifically, did the latter investigators do? Working with 20 large trees, selected to represent 13 species harvested in a logging operation near Manaus, Brazil, Chambers et al. (1998) determined their ages by the carbon 14 dating method and related the results to the trunk diameters of the trees.  In doing so, many of the bigger trees were found to be much older than what previously had been believed to be the upper age limit for trees of that region, with some of them having lived for well over a millennium.  And since approximately 50% of the aboveground biomass of tropical rainforests is contained in less than the largest 10% of the trees, very old trees represent the single most important repository of above-ground carbon in these highly productive ecosystems.  This finding, in turn, suggests that atmospheric CO2 removal by large and ancient trees can be an effective means of carbon sequestration for very long time periods, for Chambers et al. point out that some of these trees may continue to grow and sequester significant amounts of carbon for over 1,400 years.  Consequently, since the life span of these massive long-lived rainforest trees is considerably greater than the projected duration of the entire "Age of Fossil Fuels," their cultivation and preservation represents an essentially permanent partial solution to the perceived problem of global warming that many people ascribe to anthropogenic CO2 emissions.

As in the case of very old people and animals, however, it is reasonable to wonder just how metabolically active such ancient trees are, and whether they really add much mass to their trunks, branches and roots each year.

To provide some insight into this question, in what may at first appear to be a rather unrelated commentary, we note that dietary restriction is known to increase lifespan in organisms ranging from yeast to mammals, presumably, in the words of Mair et al. (2003), "by slowing the accumulation of aging-related damage."  But the process need not be a long and drawn out one.  In their studies of Drosophila (the common fruit fly), for example, they found that "dietary restriction extends lifespan entirely by reducing the short-term risk of death."  So powerful is this phenomenon, in fact, they report that only "two days after the application of dietary restriction at any age for the first time, previously fully fed flies are no more likely to die than flies of the same age that have been subjected to long-term dietary restriction."

In an accompanying article entitled "It's Never Too Late," Vaupel et al. (2003) indicate that a similar phenomenon operates in humans.  Following the unification of East and West Germany, for example, they note that "mortality in the East declined toward prevailing levels in the West, especially among the elderly."  After citing a number of other studies that confirm the operation of this phenomenon, they report that long-held "evolutionary theories of aging, which emphasize that senescence is inevitable," are gradually giving way to the realization that "aging is plastic," and that "survival can be substantially extended by various genetic changes and nongenetic interventions," noting that "interventions even late in life [our italics] can switch death rates to a lower, healthier trajectory."

The situation with perennial plants, such as trees, is proving to be very similar, as we note in our Editorial of 9 Jun 2004.  Long-held theory, according to Knohl et al. (2003), maintains that assimilation is "balanced by respiration as a forest stand reaches an 'advanced' stage of development."  Quite to the contrary, however, a number of newer studies are finding this supposition to be as poor a representation of reality as were the early evolutionary theories of aging in animals.

In a recent biomass inventory, for example, Cary et al. (2001) found much larger than expected net primary production in multi-species subalpine forest stands ranging in age from 67 to 458 years, while similar results have been obtained by Hollinger et al. (1994) for a 300-year-old Nothofagus site in New Zealand, by Law et al. (2001) for a 250-year-old ponderosa pine site in the northwestern United States, by Falk et al. (2002) for a 450-year-old Douglas fir/western hemlock site in the same general area, and by Knohl et al. (2003) for a 250-year-old deciduous forest in Germany.

In commenting on their findings, the latter investigators say they found "unexpectedly high carbon uptake rates during 2 years for an unmanaged 'advanced' beech forest, which is in contrast to the widely spread hypothesis that 'advanced' forests are insignificant as carbon sinks."  For the forest they studied, as they describe it, "assimilation is clearly not balanced by respiration, although this site shows typical characteristics of an 'advanced' forest at a comparatively late stage of development."

These observations about trees are remarkably reminiscent of findings of demographers regarding humans: i.e., nongenetic interventions, even late in life, put one on a healthier trajectory that extends productive lifespan.  So what is the global "intervention" that seems to have put the planet's trees on the healthier trajectory of being able to sequester significant amounts of carbon when past theory, which was obviously based on past observations, decreed they should be in a state of no net growth?

The answer, to us, seems rather simple.  For any tree of age 250 years or more, the greater portion of its life (at least two-thirds of it) has been spent in an atmosphere of much-reduced CO2 content.  Up until 1920, for example, the air's CO2 concentration had never been above 300 ppm throughout the entire lives of such trees, whereas it is currently 375 ppm or 25% higher.  And for older trees, even greater portions of their lives have been spent in air of even lower CO2 concentration.  Hence, the "intervention" that has given new life to old trees and allows them to "live long and prosper," in clear contradiction of previous perceived wisdom, would appear to be the flooding of the atmosphere with CO2 that was produced by the Industrial Revolution and is being maintained by its ever-expanding aftermath (Idso, 1995).

Carey, E.V., Sala, A., Keane, R. and Callaway, R.M.  2001.  Are old forests underestimated as global carbon sinks?  Global Change Biology 7: 339-344.

Chambers, J.Q., Higuchi, N. and Schimel, J.P.  1998.  Ancient trees in Amazonia.  Nature 391: 135-136.

Chambers, J.Q., Van Eldik, T., Southon, J., Higuchi, N.  2001.  Tree age structure in tropical forests of central Amazonia.  In: Bierregaard, R.O., Gascon, C., Lovejoy, T., and Mesquita, R. (Eds.).  Lessons from Amazonia: Ecology and Conservation of a Fragmented Forest.  Yale University Press, New Haven, CT, USA, pp. 68-78.

Falk, M., Paw, U.K.T., Schroeder, M.  2002.  Interannual variability of carbon and energy fluxes for an old-growth rainforest.  In: Proceedings of the 25th Conference on Agricultural and Forest Meteorology.  American Meteorological Society, Boston, Massachusetts, USA.

Hollinger, D.Y., Kelliher, F.M., Byers, J.N., Hunt, J.E., McSeveny, T.M. and Weir, P.L.  1994.  Carbon dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere.  Ecology 75: 143-150.

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Department of Soil, Water and Climate, University of Minnesota, St. Paul, Minnesota, USA.

Knohl, A., Schulze, E.-D., Kolle, O. and Buchmann, N.  2003.  Large carbon uptake by an unmanaged 250-year-old deciduous forest in Central Germany.  Agricultural and Forest Meteorology 118: 151-167.

Laurance, W.F., Nascimento, H.E.M., Laurance, S.G., Condit, R., D'Angelo, S. and Andrade, A.  2004.  Inferred longevity of Amazonian rainforest trees based on a long-term demographic study.  Forest Ecology and Management 190: 131-143.

Law, B.E., Goldstein, A.H., Anthoni, P.M., Unsworth, M.H., Panek, J.A., Bauer, M.R., Fracheboud, J.M. and Hultman, N.  2001.  Carbon dioxide and water vapor exchange by young and old ponderosa pine ecosystems during a dry summer.  Tree Physiology 21: 299-308.

Mair, W., Goymer, P., Pletcher, S.D. and Partridge, L.  2003.  Demography of dietary restriction and death in DrosophilaScience 301: 1731-1733.

Vaupel, J.W., Carey, J.R. and Christensen, K.  2003.  It's never too late.  Science 301: 1679-1681.

Last updated 17 August 2005