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Leaf support biomechanics of neotropical understory herbs¹

²Organization for Tropical Studies, La Selva Biological Station, Interlink 341, P.O. Box 02-5635, Miami, Florida 33102-5635 USA; ³Department of Organismal Biology, Ecology and Evolution, University of California, Los Angeles, California 90095 USA

Plants in light-limited tropical rainforest understories face an important carbon allocation trade-off: investment of available carbon into photosynthetic tissue should be advantageous, while risk of damage and mortality from falling debris favors investment into nonphotosynthetic structural tissue. We examined the modulus of rupture (_max), Young's modulus of elasticity (E), and flexural stiffness (F) of stems and petioles in 14 monocot species from six families. These biomechanical properties were evaluated with respect to habitat, rates of leaf production, clonality, and growth form. Species with higher E and _max, indicating greater resistance per unit area to bending and breaking, respectively, tended to be shade-tolerant, slow growing, and nonclonal. This result is consistent with an increase in carbon allocation to structural tissue in shade-tolerant species at the expense of photosynthetic tissue and growth. Forest- edge species were weaker per unit area (had a lower E), but had higher flexural stiffness due to increases in stem and petiole diameter. While this is inefficient in requiring more carbon per unit of structural support, it may enable forest-edge species to support larger and heavier leaves. Our results emphasize the degree to which biomechanical traits vary with ecological niche and illustrate suites of characteristics associated with different carbon allocation strategies.

Key Words: biomechanics • herb • modulus of elasticity • modulus of rupture • monocot • petiole • stem • understory

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