Janzen, D. 1974.

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Daniel H. Janzen. 1974. Tropical Blackwater Rivers, Animals, and Mast Fruiting by the Dipterocarpaceae. Biotropica, Vol. 6, N° 2 (Jul., 1974), pp. 69-103 doi:10.2307/2989823

Abstract

(tomado del artículo)

It is proposed that tropical nutrient-poor white sand soils produce blackwater rivers, rivers that are rich in humic acids and poor in nutrients, because the vegetation growing on these soils is exceptionally rich in secondary compounds.

The humic acids (= tannins and other phenolics) may even be only the more conspicuous of the secondary compounds that leach out of the living vegetation and the litter. While the water and the soil (including litter) may be expected to have a low productivity and animal biomass solely on the basis of its low nutrient content, it is quite possible that large amounts of secondary compounds are also debilitating to the animal community.

An exceptionally high concentration of secondary compounds is expected in the vegetation growing on white sand soils for two reasons.

  • First, this is an expected outcome in habitats where the loss of a leaf to an herbivore or through deciduous behavior is relatively a much greater loss than on nutrient-rich soils.
  • Second, the plants growing there belong for the most part to families exceptionally rich in secondary compounds, a characteristic which is in turn selected for by the chemical defense requirements of plants growing in low diversity stands.

The small amount of data that is available from Sarawak white sand habitats shows that the carrying capacity for animals is very greatly reduced. The postulated cause is reduced primary productivity and/or much of the productivity being used by the plant for secondary compounds (unharvestable productivity), or stored for seed crops at very long intervals (unavailable productivity).

It is proposed that mast fruiting at the community level, as displayed by trees in the Dipterocarpaceae, is a mechanism of escape from seed predators that is unique to this part of the tropics (S.E. Asia) because this area has reduced animal communities (both on white sand soil sites and in general), and because the climate is sufficiently uniform for such an intra- and inter-population cueing system to evolve.

Without experimentation, it is impossible to know, however, if the animal community is reduced solely due to overall lowered primary and harvestable productivity, or as well to the inevitable reduction in animal numbers when many of the trees in a habitat wait more than a few years for their highly synchronized seed crops.

The occurrence of numerous tropical habitats with a very low diversity of trees inviolates the currently popular dogma that diversity is mandatory for stability in tropical habitats. I propose that the trees in such monotonous habitats are exceptionally well-protected chemically with respect to foliage, and have either very toxic seeds or well-developed mast cycles.

Otros comentarios

En la revisión de literatura para el proyecto Cambio climático y deterioro de hormigón: ¿es predecible y prevenible? se encontraron varias referencias[1] , [2] , [3] a la contribución globalmente significativa de áreas costeras tanto tropicales como extra-tropicales a la producción de CH3Cl y CH3Br, gases responsables parcialmente por la reducción de la capa de O3. Sin embargo, ninguna de las referencias encontradas cita este artículo de Janzen ni hace referencia específica a los manglares ni a los bosques amazónicos desarrollados sobre podzoles whitesand soils de la cuenca del Río Negro. Varias preguntas se hacen necesarias:

  1. ¿la producción de CH3Cl y CH3Br por la vegetación es un mecanismo fisiológico de bajo costo para eliminar sales, particularmente en vegetación de biotopos anfibios: manglares, pantanos, bosques inundables…?
  2. ¿los manglares y biotopos tropicales inundables sobre suelos oligotróficos también contribuyen a la producción de CH3Cl y CH3Br?
  3. ¿se tienen registros o inferencias del tamaño de la capa de O3 en épocas anteriores (e.g., glaciares e interglaciares del pleistoceno, etc.) que puedan asociarse a los cambios en la extensión de los biotopos costeros o continentales sobre suelos oligotróficos? Véase v.gr., Ralph Koppmann, 2007 [4]

Es evidente que estas preguntas pueden ser respondidas o al menos precisadas y formuladas en detalle durante el estudio de Cambio climático y deterioro de hormigón: ¿es predecible y prevenible?

Referencias

  1. ^  Takuya Saito, Yoko Yokouchi, Yoshiko Kosugi, Makoto Tani, Elizabeth Philip, and Toshinori Okuda. 2008. Methyl chloride and isoprene emissions from tropical rain forest in Southeast Asia.
    Atmospheric Composition and Structure: Biosphere/atmosphere interactions; Atmospheric Composition and Structure: Troposphere: composition and chemistry; Biogeosciences: Isotopic composition and chemistry; Biogeosciences: Plant ecology; Atmospheric Composition and Structure: Geochemical cycles
    Emission of CH3Cl is much higher in dipterocarps (66%) than in other taxa (6%). [from other articles] it is also likely to come from coastal salt-marches vegetation and perhaps from mangroves. All in agreemnet with the extremely oligotrophic nature of soils or the low availability of nutrients, given the toxic nature of salt. No mention of D Janzen here.
  2. ^  Y. Yokouchi1, Y. Noijiri1, L. A. Barrie2, D. Toom-Sauntry2, T. Machida1, Y. Inuzuka1, H. Akimoto3, H.-J. Li1,3, Y. Fujinuma1 & S. Aoki4A strong source of methyl chloride to the atmosphere from tropical coastal land. Nature 403, 295-298 (20 January 2000) | doi:10.1038/35002049 https://www.nature.com/nature/journal/v403/n6767/full/403295a0.html
  3. ^  Robert C. Rhew, Benjamin R. Miller & Ray F. Weiss 2008. Naturalmethyl bromide and methyl chloride emissions from coastal saltmarshes NATURE |VOL 403 | 292-295.
    Complementa artículos de Yokouchi 2000 y Takuya Saito 2008 emisiones de Ch3Cl de bosques de diperocarpos y de pantanos salados. No hay referencia a manglares que enfrentan el mismo problema de detoxificación de sal mediante un mecanismo "barato" con CH4? producido en interfase sedimentos-agua en estuarios y quizás en áreas de suelos oligotróficos tales como los podzoles amazónicos. Red. Blackwater rivers, mast fruiting and the diptero carpaceae de D. Janzen
  4. ^  Ralph Koppmann (editor), 2007 Volatile organic compounds in the atmosphere. Blackwell Scientific.