SEARCH

SEARCH BY CITATION

Keywords:

  • Andes;
  • chlorophyll fluorescence;
  • compensation period;
  • life forms;
  • mosses;
  • PAR responses;
  • relative humidity;
  • saturation deficit;
  • water relations;
  • Venezuela

Abstract

Aim  Analysis of microclimate factors and physiological responses determining survival and growth of epiphytic bryophytes in the lower canopy and trunk space of north-Andean cloud forests.

Location  Two cloud forests at 2000–2400 m in the northern Andes near Mérida, Venezuela.

Methods  Data-logging of dry and wet-season temperature, relative humidity (r.h.) and photosynthetically-active radiation (PAR) for month-long periods, and laboratory measurements of desiccation tolerance and light responses of selected epiphytic bryophytes.

Results  Rainfall averages 20 mm or less in January and February, and 200 mm or more from August to October, but is very variable at all seasons. The proportion of time ‘wet’ (continuous 100% r.h.) in the months sampled ranged from 8.5% to 52.2% or more; a dry/wet-season range between 20% and 40% is probably commoner. The length of ‘wet’ and ‘dry’ periods approximated log-normal distributions, with mid-points for wet periods ranging from 2.8 to 10.7 h, and dry periods from 6.2 to 17.1 h. The longest recorded dry period was 143 h. Humidity typically rose during the night to > 90% r.h., reaching 100% for significant periods (implying cloudwater (fog) deposition) on about one night in two in all seasons. Of six bryophytes of pendulous growth form, all survived periods of at least a few days’ desiccation; most recovered better from high than low humidities. Measured 95% light-saturation values ranged from 110 to 256 μmol m−2 s−1, somewhat but not greatly higher than ambient light levels

Main conclusions  Environmental conditions in the cloud forests are probably near-optimal for epiphytic bryophytes, but in even the wettest forest these plants must tolerate at least short periods of drying at any time of year, and longer periods seasonally. Interception of cloudwater droplets from moving air is likely to be an important source of water for bryophytes of pendant and other diffuse life forms, especially in periods of low rainfall. Absorption of water from near-saturated air is probably of little physiological significance. Bryophytes of these life-forms are notably conspicuous in tropical-montane cloud forests. They remain prominent into humid temperate regions such as southern Chile, New Zealand and Macaronesia, but progressively disappear at higher latitudes with the stresses of increasing seasonality.