Recently, a number of publications have reported that many physiological properties of vascular epiphytes are a function of plant size. This short review will summarize what is known to date about this phenomenon, describe the possible mechanism and will discuss the consequences for the present understanding of epiphyte biology. Size-related changes are also known from other plant groups and it is argued that close attention should be paid to the size of the organisms under study in order to understand the performance and survival of a species in the field. In the light of these findings, the results of many earlier studies on epiphyte ecophysiology are now difficult to interpret because essential information on the size of the specimens used is missing.

In relation to the drought-prone and nutrient-poor habitat, vascular epiphytes are routinely referred to as inherently slow-growing plants, although actual evidence is rare. To test this notion we measured in situ growth of the understorey orchid Aspasia principissa and the tank bromeliad Vriesea sanguinolenta, and, for the latter species, also the growth under favourable conditions in the greenhouse. Using growth analysis we show: (1) that in an intraspecific comparison, small to intermediate individuals yield the highest relative growth rates (RGR) in situ: A. principissa: 1.6 10 -3 d -1 ; V. sanguinolenta: 3.3 10 -3 d -1 ; (2) that the bromeliad reaches maximum size after ca. 15 yr, while the orchid needs at least 20 yr; and (3) small V. sanguinolenta plants exhibit a highly plastic growth response to favourable conditions in the greenhouse, reaching an almost 10-fold increase in RGR. In spite of a substantial increase in growth under more favourable conditions, our results are consistent with the notion that epiphytes are inherently slow growing organisms. Nomenclature: D'Arcy (1987). Abbreviations: DM = Dry mass; LL = Length of the longest leaf of a plant; PPFD = Photosynthetic photon flux density; PsbL = Length of the most recent pseudobulb; RGR = Relative growth rate; SC = Size category.

G. 2005. Long-term population dynamics of the epiphytic bromeliad, Werauhia sanguinolenta . Á/ Ecography 28: 000 Á/000.The population dynamics of the epiphytic bromeliad, Werauhia sanguinolenta , growing in the moist tropical forest of Barro Colorado Island, Panama, was studied for seven years from 1997 through 2004. In contrast to the generally held notion of the great importance of moisture availability for growth and survival of vascular epiphytes, no demographic process showed a significant correlation with the amount of annual precipitation or the varying number of rainy days per year. Most deaths, for example, were rather related to substrate instability (tree falls, branch breakage, or flaking bark) in all but the smallest size classes. We found evidence for both competition and facilitation. Elasticity analysis revealed that the finite rate of population increase, which invariably exceeded unity, was mostly influenced by survival (stasis) and to a lesser extent by growth, and very little by fecundity. In contrast to earlier reports on disastrous outbreaks of herbivores in this epiphyte species, the long-term impact of herbivory on the population dynamics of W. sanguinolenta was negligible. Being at least facultatively autogamous, reproduction seems to be controlled by resource availability alone: this is suggested by long intervals between reproductive events, and a decrease in size and an increased mortality after reproduction. We conclude that the demography of this epiphytic bromeliad is probably influenced at least as much by biotic factors (i.e. the dynamics of the substratum) as by abiotic limitations.G. Zotz (gerhard.zotz@unibas.ch), Botanisches Inst. der Univ. Basel, Schö nbeinstrasse 6, CH-4056 Basel, Switzerland. Á/ S. Laube, Fachber. Biologie, Abt. Allgem.

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This field study with the C 3 bromeliad Vriesea sanguinolenta (Cogn. & Marchal 1874) was initiated to explore the importance of size-related ecophysiological changes in vascular epiphytes in a natural tropical setting. In this species, a step change from atmospheric to tank-forming life form occurs during early ontogeny, followed by a continuous size increase of individuals with water-impounding tanks. Although our study focused on the water-impounding phase, this growth pattern also allowed us to compare ecophysiological consequences of a step change in life form with those associated with size increments among plants of identical life form. The shift in life form was accompanied by relatively minor changes, for example in leaf morphology (decrease in leaf thickness and trichome density) and leaf physiology (decrease in photosynthetic capacity), while there were more substantial changes during the tankforming 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 water supply in larger plants may be the proximate cause for the observed size-related differences in leaf anatomy (relative reduction of water storage tissue, and relative and absolute increase in chlorenchyma thickness), leaf morphology (increase in stomatal density, decrease in trichome density), and leaf physiology (increase in net rates of CO 2 uptake, more conservative stomatal behaviour, higher residual transpiration). The results are compared with previous studies on heteroblasty in bromeliads, but are also discussed in the context of a gradual shift from a droughttolerance to a drought-avoidance strategy. Key