Intertidal “trees”: consequences of aggregation on the mechanical and photosynthetic properties of sea-palms Postelsia palmaeformis Ruprecht
Sea-palms Postelsia palmaeformis Ruprecht are annual brown algae that grow on wave-swept rocky shores, oRen forming dense stands. Unlike most macroalgae, Postelsia stands upright in air- like trees. The stipe flexibility that permits Postelsia to withstand waves is provided by the low elastic modulus (5-10 MPa) of stipe tissue; in spite of the weakness (low breaking stress, ~ 1 MPa) of this tissue, a large amount of energy ( ~ 100 kJ/m 3) is required to break a stipe because they can be extended by 20-25 % before breaking. Although made of such easily deformed tissue, Postelsia can stand upright in air due to the width (high second moment of area) and resilience of their stipes, but the brittleness (low work of fracture, 400-900 J/m 2) that accompanies this resilience renders them susceptible to bre~age if they sustain deep scratches. Although wave-induced stresses experienced by individuals in aggregations are not lower than those experienced by isolated sea-palms, photon flux densities of photosynthetically active radiation within these dense groves are less than 10% of those above Postelsia canopies. A number of morphological features differ between canopy, understory, and isolated individuals. Canopy plants in dense aggregations are taller than isolated individuals and may exceed limiting proportions for elastic stability. Postelsia shows photo- synthetic characteristics of"shade-adapted" plants, understory individuals being especially effective at using low light. Despite this, blade growth rates of understory plants are lower than those of either canopy or isolated individuals.