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Ecological Studies on the Rain Forest of Southern Nigeria: II. The Atmospheric Environmental Conditions

G. C. Evans
Journal of Ecology
Vol. 27, No. 2 (Aug., 1939), pp. 436-482
DOI: 10.2307/2256374
Stable URL: http://www.jstor.org/stable/2256374
Page Count: 47
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Ecological Studies on the Rain Forest of Southern Nigeria: II. The Atmospheric Environmental Conditions
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Abstract

1. Observations on atmospheric environmental conditions were made as part of a programme of autecological studies of selected species of the undergrowth flora. 2. The methods which have been used for measuring carbon dioxide concentration in tropical forest undergrowth are examined and previous statements of very high concentrations of carbon dioxide there are shown to be based on insufficient evidence. 3. The concentration of carbon dioxide in the undergrowth of the Nigerian rain forest was not observed to exceed 0.06% by volume, and thus is of the same order as that in temperate woods. There is a marked diurnal variation, values much above "normal" being recorded only in the morning, there being a steady fall from sunrise to noon. There are no significant differences between the concentrations observed in primary and in 14-year-old secondary forest. 4. These observations agree in general with those of Stocker at Tjibodas in Java. The differences are discussed; Stocker observed a rise in concentration after sunrise. It is shown that his hypothesis of a nocturnal "binding" of carbon dioxide by alkali on the surface of the foliage will not explain this. 5. Data derived from thermohygrograph records of the conditions of temperature and humidity in the undergrowth and tree tops at the end of the dry and the beginning of the wet seasons are given. 6. From the days covered by these data two are selected for more detailed treatment, and the daily march of temperature, of relative and absolute humidity, and of saturation deficit are considered. Comparisons are made between the tree tops and the undergrowth, and between the dry and wet seasons. 7. A similar analysis is given for days at Buitenzorg and Tjibodas in Java and in the neighborhood of Rio de Janeiro. In general the undergrowth of tropical forest is characterized by a period of saturation during the night, the length of this being very variable, and by a rise of saturation deficit during the morning to a maximum which in no case greatly exceeds 10 mm. Hg and is often much less. There are very small changes in the absolute humidity when the air is not saturated, not only in the undergrowth but also in the open air close to the forest. These small changes, which at Buitenzorg and in the Shasha Forest Reserve are not more than about 2-4 mm. Hg, are in striking contrast to those in the undergrowth of the "Fringing Forest" surrounded by extensive thorn scrub in Northern Nigeria, where at the beginning of the wet season the absolute humidity rises on the average 8 mm. during the day. 8. The difficulties attendant on attempts to survey the conditions of intensity of illumination in forests are discussed. 9. The daily march of intensity of illumination in the undergrowth of primary forest in Nigeria follows a fairly steady course, with few major deviations. Sunflecks are on the whole small both in size and intensity and are confined to a period of about 5 hr. near midday. During this period sunflecks giving a current output from the photocell more than twice that corresponding to the mean shade intensity with the sun unobscured are observed in up to 5% of the total observations, and those giving more than 5 times only in about 2% or less. 10. Conditions of illumination in primary and in 14-year-old secondary forest (which has probably passed the darkest phase of the succession) are similar as regards incidence of sunflecks, absolute intensity as measured by the Weston cell, and spectral composition. The secondary forest appears to be on the whole somewhat darker. 11. When the sun is obscured by cloud the intensity of illumination in 14-year-old secondary forest in Nigeria is roughly 1/2-1% of the intensity in the open near the same forest. The ratio is relatively lower with a blue filter, and higher with a red one. This apparent high intensity of red light is probably due to light of wave-lengths greater than 6800 or 7000 A., in the farther red and near infra-red; presumably in the region of the absorption bands of chlorophyll in the red the intensity is much lower. When the sun is unobscured the spectral distribution is relatively redder in the undergrowth. We have no comparison of total intensities inside and outside the forest under these conditions, but the ratio is probably still about 1/2-1%. 12. These measurements are compared with those of other workers in tropical forests. 13. The possible effects of the environmental conditions observed in Nigeria on the life of the undergrowth plants are discussed.

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