Résumé
In the Neotropics, the genus Hydrangea of the popular ornamental hortensia
family is represented by climbing species that strongly cling to their
support surface by means of adhesive roots closely positioned along
specialized anchoring stems. These root-climbing hortensia species belong
to the nearly exclusive American Hydrangea section Cornidia and generally
are long lianescent climbers that mostly flower and fructify high in the
host tree canopy. The Mexican species Hydrangea seemannii, however,
encompasses not only long lianescent climbers of large vertical rock walls
and coniferous trees, but also short ‘shrub-like’ climbers on small
rounded boulders. To investigate growth form plasticity in root-climbing
hortensia species, we tested the hypothesis that support variability (e.g.
differences in size and shape) promotes plastic responses observable at
the mechanical, structural and anatomical level. Stem bending properties,
architectural axis categorization, tissue organization and wood density
were compared between boulder and long-vertical tree-climbers of H.
seemannii. For comparison, the mechanical patterns of a closely related,
strictly long-vertical tree-climbing species were investigated. Hydrangea
seemannii has fine-tuned morphological, mechanical and anatomical
responses to support variability suggesting the presence of two
alternative root-climbing strategies that are optimized for their
particular environmental conditions. Our results suggest that variation of
some stem anatomical traits provides a buffering effect that regulates the
mechanical and hydraulic demands of two distinct plant architectures. The
adaptive value of observed plastic responses and the importance of
considering growth form plasticity in evolutionary and conservation
studies are discussed.