A comparison of montane and lowland rain forest in Ecuador: II. The climate and its effects on the distribution and physiognomy of the forests

  • Grubb P
  • Whitmore T
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1. The climates of two rain forest sites, one montane (altitude 1710 m) and one lowland (altitude 380 m) are described. There is a cooler, wetter season at both, lasting approximately from April to July. 2. The weather at the montane site is analysed; two types of day, `fog-bound' and `fog-free', are distinguished. On typical fog-bound days the forest is shrouded in stratus cloud for much of the time. On typical fog-free days there are prolonged sunny periods. Groups of fog-bound and fog-free days probably alternate throughout the year, the fog-free periods lasting up to 1 or 2 weeks in the dry season. No such division into extreme weather types is possible for the lowland forest; no fog was encountered here. 3. Detailed observations have been made on the temperature and relative humidity during 3-week periods in the undergrowth of the two forests and also in large clearings where the conditions are believed to have approximated to those in the upper part of the forest canopy. 4. On fog-bound days the temperature change in the montane forest undergrowth is very small and the relative humidity rarely falls below 95%. On fog-free days the diurnal range in temperature and the minimum relative humidity are about the same as on average days in the lowland forest. A similar parallel is found for conditions in the clearings. 5. The daily march of atmospheric conditions is followed in detail. On fog-free days in the montane forest and on average days in the lowland it approaches that already described for lowland forests elsewhere during the dry season; on fog-bound days in the montane forest it approaches that described for lowland forests in the wet season. The severely desiccating periods, taken arbitrarily as those with relative humidity $\eqslantless 80%$ , are of approximately equal duration and frequency on fog-free days in the montane forest and on average days in the lowland. 6. It is shown that the results probably represent a generally valid comparison of montane and lowland forest climates in the part of Ecuador visited. The importance and general occurrence of dry periods as well as frequent fog, even in Upper Montane Forest, is stressed. 7. The factors controlling the distribution of the three rain forest formation-types on tropical mountains (Lowland, Lower Montane and Upper Montane) are discussed. A consideration of the Massenerhebung effect leads to the conclusion that the forest types are primarily correlated with the frequency of fog (stratus or cumulus cloud close to the ground) rather than with any aspect of the temperature regime. The Massenerhebung effect arises in the tropics because clouds form at lower altitudes where the air masses are moister before lifting and cooling, i.e. when over the sea or approaching the foothills of a large massif. Exposure to high winds and soil factors are important in producing stunted facies of each of the three types, including so-called `Elfin woodland'. 8. The greater abundance of epiphytes in the montane forest than in the lowland is related to the more frequent wetting with liquid water from fog rather than to a constantly higher humidity. CR - Copyright © 1966 British Ecological Society

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  • P J Grubb

  • T C Whitmore

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