Canopy CO2 concentrations and Crassulacean acid metabolism in Hoya carnosa in a subtropical rain forest in Taiwan: consideration of CO2 availability and the evolution of CAM in epiphytes

The potential importance of CO2 derived from host tree respiration at night as a substrate for night time CO2 uptake
during CAM was investigated in the subtropical and tropical epiphytic vine Hoya carnosa in a subtropical rainforest in
north-eastern Taiwan. Individuals were examined within the canopies of host trees in open, exposed situations, as well
as in dense forests. Although night time CO2 concentrations were higher near the epiphytic vines at night, relative to
those measured during the day, presumably the result of CO2 added to the canopy air by the host tree, no evidence for
substantial use of this CO2 was found. In particular, stable carbon isotope ratios of H. carnosa were not substantially
lower than those of many other CAM plants, as would be expected if host-respired CO2 were an important source of CO2
for these CAM epiphytes. Furthermore, laboratory measurements of diel CO2 exchange revealed a substantial contribution
of daytime CO2 uptake in these vines, which should also result in lower carbon isotope values than those characteristic
of a CAM plant lacking daytime CO2 uptake. Overall, we found that host-respired CO2 does not contribute substantially
to the carbon budget of this epiphytic CAM plant. This finding does not support the hypothesis that CAM may
have evolved in tropical epiphytes in response to diel changes in the CO2 concentrations within the host tree canopy.