A tale of two plasticities: leaf hydraulic conductances and related traits diverge for two tropical epiphytes from contrasting light environments

We compared the effects of different light environments on leaf hydraulic conductance (K-lear) for two congeneric epiphytes, the tank bromeliads Guzmania lingulata (L.) Mez and Guzmania monostachia (L.) Rusby ex Mez. They occur sympatrically at the study site, although G. monostachia is both wider ranging and typically found in higher light. We collected plants from two levels of irradiance and measured K-leaf as well as related morphological and anatomical traits. Leaf xylem conductance (K-xy) was estimated from tracheid dimensions, and leaf conductance outside the xylem (K-ox) was derived from a leaky cable model. For G. monostachia, but not for G. lingulata, K-leaf and K-xy were significantly higher in high light conditions. Under both light conditions, K-xy and K-ox were co-limiting for the two species, and all conductances were in the low range for angiosperms. With respect to hydraulic conductances and a number of related anatomical traits, G. monostachia exhibited greater plasticity than did G. lingulata, which responded to high light chiefly by reducing leaf size. The positive plasticity of leaf hydraulic traits in varying light environments in G. monostachia contrasted with negative plasticity in leaf size for G. lingulata, suggesting that G. monostachia may be better able to respond to forest conditions that are likely to be wanner and more disturbed in the future.