Ecophysiological consequences of differences in plant size: in situ carbongain and water relations of the epiphytic bromeliad, Vriesea sanguinolenta

This held study with the C-3 bromeliad Vriesea sanguinolenta (Cogn, & March al 1874) was initiated to explore the importance of size-related ecophysiol ogical changes in vascular epiphytes in a natural tropical setting, In this species, a step change from atmospheric to tank-forming life form occurs d uring early ontogeny, followed by a continuous size increase of individuals with water-impounding tanks. Although our study focused on the wafer-impou nding phase, this growth pattern also allowed us to compare ecophysiologica l consequences of a step change in life form with those associated with siz e increments among plants of identical life form. The shift in life form wa s accompanied by relatively minor changes, for example in leaf morphology ( decrease in leaf thickness and trichome density) and leaf physiology (decre ase in photosynthetic capacity), while there were more substantial changes during the tank-forming phase. A major trend was a decreasing dependence of larger plants on internally stored water due to a more efficient tank. We suggest that the resulting, more reliable wafer supply in larger plants may be the proximate cause for the observed size-related differences in leaf a natomy (relative reduction of wafer storage tissue, and relative and absolu te increase in chlorenchyma thickness), leaf morphology (increase in stomat al density, decrease in trichome density), and leaf physiology (increase in net rates of CO2 uptake, more conservative stomatal behaviour, higher resi dual transpiration). The results are compared with previous studies on hete roblasty in bromeliads, but are also discussed in the context of a gradual shift from a drought-tolerance to a drought-avoidance strategy.