Abstract
Epiphytes that grow in the canopies of tropical and subtropical forests experience different water regimes when compared with terrestrial plants. However, the differences in adaptive strategies between epiphytic and terrestrial plants with respect to plant water relations remain poorly understood. To understand how water-related traits contrast between epiphytic and terrestrial growth forms within the Cymbidium (Orchidaceae), we assessed leaf anatomy, hydraulics, and physiology of seven terrestrial and 13 epiphytic species using a common garden experiment. Compared with terrestrial species, epiphytic species had higher values for leaf mass per unit area (LMA), leaf thickness (LT), epidermal thickness, saturated water content (SWC) and the time required to dry saturated leaves to 70% relative water content (T70). However, vein density (Dvein), stomatal density (SD), and photosynthetic capacity (Amax) did not differ significantly between the two forms. T70 was positively correlated with LT, LMA, and SWC, and negatively correlated with stomatal index (SI). Amax showed positive correlations with SD and SI, but not with Dvein. Vein density was marginally correlated with SD, and significantly correlated with SI. Overall, epiphytic orchids exhibited substantial ecophysiological differentiations from terrestrial species, with the former type showing trait values indicative of greater drought tolerance and increased water storage capacity. The ability to retain water in the leaves plays a key role in maintaining a water balance in those epiphytes. Therefore, the process of transpiration depends less upon the current substrate water supply and enables epiphytic Cymbidium species to adapt more easily to canopy habitats.
Highlights
Epiphytes are an important component of tropical and subtropical floras, and serve vital ecological functions in forest hydrology and nutrient fluxes (Benzing, 1990; Zotz and Bader, 2009)
All of the tested leaf traits varied significantly across species, the magnitude of variation differed for each trait (Table 1)
The coefficients of variation (CVs) were >50% for leaf thickness (LT), epidermal conductance and the time required for drying of saturated leaves to 70% Relative water content (RWC) (T70), but
Summary
Epiphytes are an important component of tropical and subtropical floras, and serve vital ecological functions in forest hydrology and nutrient fluxes (Benzing, 1990; Zotz and Bader, 2009). During the dry season, a decrease in leaf water potential has a stronger impact on the conductance of leaf water vapor by epiphytic Anthurium bredemeyeri Schott than by terrestrial plants of the same species (Rada and Jaimez, 1992). These findings indicate that significant differences in water relations may exist between epiphytic and terrestrial growth forms. There is a lack of comprehensive understanding of the divergences in water-related traits between closely related epiphytic and terrestrial species (Silvera et al, 2009)
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