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A Comparison of Montane and Lowland Forest in Ecuador: III. The Light Reaching the Ground Vegetation

P. J. Grubb and T. C. Whitmore
Journal of Ecology
Vol. 55, No. 1 (Mar., 1967), pp. 33-57
DOI: 10.2307/2257715
Stable URL: http://www.jstor.org/stable/2257715
Page Count: 26
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A Comparison of Montane and Lowland Forest in Ecuador: III. The Light Reaching the Ground Vegetation
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Abstract

A comparison of the light reaching the forest ground vegetation in two rain forests, one montane and one lowland, has been made using four sets of data from each site: (i) an area survey of the type described by Evans (1956) at 28 in. (70 cm) above the ground; (ii) a daily march of the light intensity in one position in the forest on two days of contrasting weather conditions, and a similar daily march in the large clearing nearby on two contrasting days, all using the Evans apparatus; (iii) hemisphere photographs of the canopy over the area survey plots taken with a Hill camera (Hill 1924); and (iv) a detailed record of the weather conditions during 22 days, including a record of bright sunshine made with a Campbell-Stokes recorder. The results of the area survey have suggested that in equivalent weather conditions at the two sites more light energy would reach the ground vegetation of the montane forest than that of the lowland forest as diffuse light in cloudy and sunny conditions but, curiously, less as sunflecks (Table 2). Further, the montane canopy transmits relatively much more light in the 4 h around noon; and this is correlated with the finding from the hemisphere photographs that the montane canopy has more holes around the zenith than the lowland. Close examination of the distribution on the photographs of canopy holes along the sun's track on the days of the survey and of the distribution of observations through the day suggest that insufficient sunfleck readings were made in the montane forest to be representative. It is not possible therefore to extrapolate to a daily total for sunfleck light. The curious difference between the forests is an artefact. The diffuse light totals are extrapolated to 22 days of weather observations kept at each site; hemisphere photographs are used to check the validity of the extrapolation. Over this period the montane forest ground vegetation received during an 8-h day 40% more diffuse light in sunny conditions and about the same amount in cloudy conditions. It is emphasized that the amount and quality of light reaching the ground vegetation is strongly influenced by the duration and distribution of sunny periods through the day. The interpretation of the differences in the light climate between the forests in terms of forest structure and the significance of the differences for the ground vegetation are discussed. The difficulties of extrapolating to longer periods and possible improvements in methodology for further work are discussed. It is suggested that future studies should seek to embrace all regenerative phases and not only the high forest phase in any one area and that Evans's area survey method in conjunction with hemisphere photographs has great potential as a means of studying light as an ecological factor.

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