IAWA Journal, Val. 20 (3), 1999: 255-260

HOW OLD ARE TROPICAL TREES? THE PERSISTENCE OF A MYTH by

Martin Worbes 1 & Wolfgang Johannes Junk2

SUMMARY

The recent report of ancient trees in the Amazon region (Chambers et al. 1998) with a maximum radiocarbon dated age of about 1400 years for the long-living pioneer species Cariniana micrantha is dis­ cussed in the light of dendrochronological age determinations from Africa and South America together with the results of indirect age estimations from other sources. There is a tendency in the literature to considerably overestimate the maximum ages of tropical trees. Age determination by the direct counting of annual rings and mak­ ing estimations for hollow trees by measuring growth rates and diam­ eters result in ages between 400 and 500 years for the largest trunk dimensions, e.g. in Cariniana legalis. Key words: Age determination, dendrochronology, radiocarbon dat­ ing, tropical trees.

INTRODUCTION

Early scientists, such as Alexander von Humboldt (1815-1832) who visited tropical forests, brought controversial impressions horne to Europe. On the one hand they reported the inexhaustible richness of plants and animals and the rapid growth of vegetation. On the other hand they found impressive tree giants which seemed to be thousands of years old. In the 1960s and early 70s the findings of the large productivity of some tropical herbaceous plant communities were supported by reports of high net primary produc­ tion (NPP). Measurements of photosynthethic rates and climate modeling were ex­ trapolated to tropical forests (Lieth 1975) and strengthened the myth of highly pro­ ductive tropical forests. This raised the hopes of foresters, the logging industry, and politieians that wood production eould be used as a persistent, quiekly renewable resouree in tropical eountries. Deductions from NPP to wood production resuIted in values of up to 18 t ha y-I (Bruenig 1996) whieh are three times higher than the pro­ duetion in temperate zones (Ellenberg et al. 1986). Only a few ealculations from repeated diameter measurements (Jordan 1983) and from tree-ring analyses (Worbes 1997) indicated that the total wood produetion at only the most produetive sites in the natural tropical forests equals the produetion of 6-7 t ha y -I in the temperate zones.

1) Forstbotanisches Institut, Büsgenweg 2, D-37077 Göttingen, Germany. 2) AG Tropenökologie, Max-Planck-Institut für Limnologie, D-24302 Plön, Germany.

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More recent research has focussed on the search for the oldest tropical forest tree, stimulated by discussions on sustainable forest management, palaeoclimate, and car­ bon fluxes. The recent tendency is that the maximum age of tropical trees increases rapidly from report to report, while the assumed growth rates decrease in the same order (Clark & Clark 1992; Camargo et al. 1994; Koming & Balslev 1994; Condit et al. 1995). The youngest report of a 1400-year-old Amazonian Cariniana micrantha 'tree in a terra firme forest (Chambers et al. 1998) follows this tendency. We feel that the whole issue requires a critical discussion of different methods used and of the results obtained in age determination of tropical trees.

METHODS TO ESTIMATE THE AGE OF TROPICAL TREES

There are four methods of age determination of living trees, one direct and three indirect ones: age estimation by repeated diameter measurements (Lieberman et al. 1985), radiocarbon dating (e. g. Camargo et al. 1994), a mathematical approach based on the estimation of mortality rates (e. g. Condit et al. 1995), and direct annual tree­ ring counting (e.g. Mariaux 1969).

• Repeated diameter measurements give information about the growth during the measured period. Upscaling the data to the whole life span of the tree allows only estimations because tree growth changes considerably depending on age and envi­ ronmental conditions (Worbes 1989). • The interpretation of radiocarbon datings can be problematic. Ages of wood sam­ pIes oider than 50 years and younger than 350 years cannot be dated. The high variation of atmospheric radiocarbon due to the Suess Effect results in up to five possible ages for one radiocarbon age (Stuiver & Becker 1986). Five trees from theAmazonian terra firme dated with the 14C method by Chambers et al. (1998) to be between 200 and 300 years old could be much younger. Moreover, the variation of the radiocarbon age cannot be added to the mean as it was done for the giant Bertholletia excelsa tree from Para. This tree was originally reported by Camargo et al. (1994) to be 440 ± 60 years old and not 500 years as was reported by Cham­ bers et al. (1998). Moreover, exceptional findings like the age of 1400 years for the broadleaf Cariniana micrantha needs a confirmation of at least a repeated radio­ carbon measurement from the same tree sampie which apparently was not done. • The estimation of a hypothetical age of 2000 years on the basis of mortality rates in a tropical tree population (Swartzia simplex in Panama) is a mathematical exer­ cise which has never been confirmed by direct measurements (Condit et al. 1995). • Growth rates and ages of tropical trees often give occasion to controversial dis­ cussions due to the assumed absence of annual tree rings. This is another of the many myths about tropical forests, because in fact the existence of annual rings in tropical trees under seasonal precipitation conditions has been proven since the beginning of this century (Geiger 1915) and has meanwhile been confirmed in numerous publications (Worbes & Junk 1989; overviews by Worbes 1992, 1995).

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A seasonal climate with one distinct dry season is widely distributed in the humid tropics (Worbes 1995), e.g. in Central Amazonia. Tree-ring analyses give informa­ tion on individual growth rates and growth conditions during the whole life period of the individiual tree. Therefore we are in a position to compare results from direct tree-ring analyses with those from indirect methods.

RESULTS AND DISCUSSION

Estimations in Costa Rican (Lieberman et al. 1985) and Ecuadorian rain forests (Kor­ ning & Balslev 1994) derived from repeated diameter measurements used the slowest observed growth trajectory to calculate a 'simulated life span' from dbh = 10 cm to the largest observed diameter within one species. The hypothetical maximum age for a small-stemmed mid-canopy species in Ecuador was 529 years (Koming & Balslev 1994). In reality, however, the cited investigations and our own findings (Klinge et al. 1996) point out that the largest diameters and highest growth rates were observed in individuals of the upper storey, and that these trees must therefore be hundreds of years younger than the artificiallifetime calculation suggests.

We directly counted on intact stern discs a maximum of 260 years for Guibourtia tessmanii in Gabon (dbh = 84 cm) and 204 years for a Piranhea trifoliata (dbh = 60 cm) in the Central Amazonian floodplains. The trunks of the thickest (dbh > 1.2 m) and probably oldest trees were generally hollow, even in trees with high wood density and a high amount of heartwood substances (e.g., Piranhea trifoliata in Amazonian floodplain forest, Manilkara sp. in the Venezuelan Gran Sabana). From some twenty thousand cored trees in different forests between 34 and 62% showed damage in the heartwood or were hollow due to attacks by aggressive fungi and high decomposition rates in tropical environments (Lamprecht 1986). Therefore it is necessary to use mean growth rates for a rough age estimation of hollow trees. Also the fact that the trees investigated by us were comparably 'thin' needs an extrapolation to the observ­ ed diameters in the Amazonian terra firme.

On extremely nutrient-poor sites in the Rio Negro floodplains and in Venezuelan highland forests we found a mean diameter increment of about 0.4 cm y-l in trees of the upper crown layers. The growth rates of Cariniana micrantha, Dipteryx odorata and Lecythis poiteaui (Chambers et al. 1998) in the Amazonian terra firme are in the range (0.1 cm y-l ) ofthe slowest growing understorey shrubs (Psidium acutangulum) in the Amazon floodplains which are exposed to a mean flood period of eight months per year without any wood formation. Cariniana micrantha in the terra firme how­ ever is an emergent species and probably belongs to the group of long-living pio­ neers, such as Bombacopsis quinata, Swietenia macrophylla, Terminalia amazonica in the Neotropics, Triplochiton scleroxylon in Africa or its South Brazilian counter­ part Cariniana legalis. One characteristic of this group is the moderate wood density of up to 0.7 g cm-3 as in C. micrantha (Loureiro & Silva 1968) which is usually as­ sociated with relatively high growth rates (Worbes 1989). For C. legalis we measured 0.5 cm diameter growth per year in a natural stand in the state of Säo Paulo, much

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higher than the C micrantha tree in the Amazonian terra firme. The oldest tree we found was 2.75 m in diameter and hollow. We calculated from the growth rates of the surrounding trees of this species a maximum age of between 400 and 500 years. It seems to be impossible that an emergent long-living pioneer as Cariniana micrantha grows much slower than all other species of this group and persists 2-3 times longer than any other broadleaf tree species in the world.

These species never regenerate in the shade. From the time this Cariniana micrantha was a seedling until an extremely old age, the tree must have been exposed to full sunlight. Extremely low growth rates as estimated for this tree (Chambers et al. 1998) are bound to a high mortality (Swaine et al. 1987) especially of light demanding species and therefore do not lead to exceptional old ages. Considering the community development it is evident that old individuals of large, long-living pioneer species increase the survival chance of the population. Seedling establishment depends on events of large-scale forest disturbance which occur at intervals of hundreds of years.

250~-----,------r-----~-----r-----,------~----~~ •BE I I I I I I 200 --- -1- --- -1- --- -1- --- -1- --- -1- --- -1- --- -I-- I I I I I I I I I I I I I I I I eC,"", E'150 1------I I I I .:=.. I I I I I Oi I aJ E I I I I ~ 100 -- -1- --- _I _____I _____ 1 _____ 1_____ 1__ TSI ...... GT I I I I I I I • CZI I y=0.481<-30.18 I I ~ I R2-1073 I TS. I.. I -I . I I I -EC I 50 --- 11; --- -1- --- _1- --- _I ---- _1- ____ 1_____ 1__ .1I PT I I I I I I MAI

04-----~----_+----~------r_----+_----~----~~ o 200 400 600 800 1000 1200 1400 age (years) Fig. 1. Age-diameter relation in emergent tropical trees from different environments. Age determination by radiocarbon analysis: CM, Cariniana micrantha, Amazonian terra firme (Chambers et al. 1998); BE, Bertholletia excelsa from Para (Camargoet al. 1994). Tree-ring analysis: CH, Calophyllum brasiliense; MA, Myrciaria amazonica; PT, Piranhea trifoliata from Amazonian floodplains; CZ, Celtis zenkeri; EC, Entandophragma cylindricum; TS, Tri­ plochiton scleroxylon (Stasehel et al. 1996) from Cameroon; GT, Guibourtia tessmanii from Gabon; CL, Cariniana legalis from Säo Paulo state, Brazil. Trees of which the age was ca1cu­ lated by mean specific growth rates and diameter are underlined, for Piranhea trifoliata the standard deviation is marked by an arrow. Other ages by direct ring counting.

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This strategy requires fast growth in the youth in order to outcompete other pioneer species until becoming established in the canopy. Many adult trees must survive for a long time to produce a huge number of seeds for the reproduction in unpredictably formed gaps in time. The combination of fast growth and longevity leads to enormous trunk diameters of almost 2 m, as shown for the examples above, but not to excep­ tionally high ages. This is shown in Figure 1, where we plotted the diameter against the age ofbig trees from our estimations, from Camargo et al. (1994) and from Cham­ bers et al. (1998). Our trees together with Bertholletia excelsa (Camargo et al. 1994) form a relative small cluster with a regression coefficient of 0.73. The Carinana micrantha tree however lies far outside of this regression due to its calculated excep­ tionallow growth rate. Assuming a realistic growth rate for the given species the tree would not be older than 400-600 years. We cannot explain the difference between our findings and the findings of Chambers et al. (1998) but propose arepetition of the analysis or a dendrochronological confirmation. In general the ages of individual Methuselahs provide little evidence for the inter­ pretation of the dynamics of any population. In the oldest growing broadleaf tree species in North America (Quercus alba) and Central Europe (Quercus robur) trees with an age of 600 years were found only exceptionally. The typical age in both cases is only 300 years (Loehle 1988; Becker 1983). The rotation period of more or less natural stands of Fagus sylvatica is about 200 years whereas the oldest individuals have ages of about 400 years (Koop 1989).

The examples show that investigations on the growth of individual trees in the tropics require direct age determinations by tree-ring analysis to confirm age estimations achieved by indirect methods, and that a discussion of forest dynamics requires a large number of growth data about the whole population.

REFERENCES

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