Abstract:
The effects of an increasing moisture on trees of the tropical species-rich mountain rain forest in the South
Ecuadorian Andes was investigated, using the daily total water consumption (TWC) and the instantaneous
water use ef?ciency (WUE, ratio of photosynthetic CO2 uptake per water loss by transpiration) as eco-
physiological indicators. Two canopy and one sub-canopy tree species, (Vismia tomentosa, Clusiaceae, an
as of yet unknown Lauracee, and Spirotheca rosea, Bombacaceae) were the experimental objects. Seasonal
changes as well as a long-term (18 months) trend of increasing precipitation caused an inverse reaction
of the TWC of the trees. Because of a rather unlimited water supply to the trees from a permanently high
water content of the soil, transpiration followed mainly the atmospheric demand of water vapor, and
increasing moisture hence reduced water loss by transpiration. It was hypothesized that in spite of the
reduction in transpiratory water loss photosynthetic carbon acquisition would be not or less affected
due to an increase in water use ef?ciency. Concomitant measurements of photosynthetic net CO2 uptake
showed the expected increase of WUE in V. tomentosa and S. rosea, but no clear reaction of the Lauracee.
Accompanying measurements of stem extension growth con?rmed an undiminished growth of V. tomen-
tosa and S. rosea but showed also suspended growth of the Lauracee during the wettest months. While
TWC can be continuously monitored with the heat dissipation technique, WUE is determined by leaf
porometry in campaigns for which access to the canopy is required. Simultaneous recordings of the gas
exchange of leaves at 4 different positions in the crown of one of the experimental trees (V. tomentosa)
showed the usability of the trait WUE in combination with the total daily water consumption as indicator
set for assessing the response of trees to a subtly changing climate. However, not all tree species appear
as likewise useful indicator trees
