title: Unusual twig "twistiness" in pawpaw (Asimina triloba) provides biomechanical protection for distal foliage in high winds.
abstract: OBJECTIVE
  Deciduous woody species invest considerable resources in the growth of new foliage and distal stems. This new growth is at risk for mechanical damage from high winds and storms. Pawpaw has large leaves borne distally on thin twigs. Following a storm, pawpaw branches sometimes exhibit a persistent "flipped" orientation, slowly returning upright over 24 h. We investigated biomechanical properties of pawpaw twigs, comparing them to co-occurring species with similarly high leaf areas and loads, which do not exhibit this "flipping". Our goal was to determine biomechanical and structural properties in these species and how variation in form might relate to functional differences.
  
  
  METHODS
  We measured flexural stiffness, torsional stiffness, and viscoelastic creep in pawpaw and co-occurring trees Liriodendron tulipifera and Carya cordiformis. We also recorded twig/foliage reconfiguration in high winds. We stained thin cross sections of distal twigs and recorded images using fluorescent light microscopy.
  
  
  RESULTS
  Flexural and torsional stiffness increased with twig radius in pawpaw and tulip tree, although torsional stiffness increased more slowly in pawpaw. Pawpaw had a high ratio of flexural to torsional stiffness (EI/GJ) across a range of twig radii and significant viscoelastic creep compared with the other species.
  
  
  CONCLUSIONS
  Biomechanical data showed that pawpaw twigs were "twistier" than the comparison species, which were shown previously to alleviate drag-induced damage by reorienting petioles and leaves. Pawpaw has an unusual strategy of low torsional stiffness in twigs, allowing for reorientation of the entire distal appendage, likely minimizing drag-induced damage in storms.
authors.affiliation: Widener University, 1 University Place, Chester, Pennsylvania 19013 USA kgoodrich@widener.edu.
authors.forename: Katherine R
authors.initials: KR
authors.lastname: Goodrich
authors.affiliation: Widener University, 1 University Place, Chester, Pennsylvania 19013 USA.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Ortiz
authors.affiliation: Widener University, 1 University Place, Chester, Pennsylvania 19013 USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Coughlin
mesh_terms: D031042:Asimina / Q000033:anatomy & histology / Q000254:growth & development* / Q000502:physiology; D001696:Biomechanical Phenomena; D018515:Plant Leaves / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology; D018547:Plant Stems / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology; D013045:Species Specificity; D014197:Trees; D014919:Wind; D014934:Wood
keywords: Annonaceae; Asimina triloba; flexural stiffness; pawpaw; torsional stiffness; twig biomechanics; twistiness-to-bendiness ratio; viscoelastic creep