Wind effects on bark and wood contribution to biomechanical traits of Patagonian shrubs

IOGNA P.A; SCHOLZ FG; BUCCI SJ; GOLDSTEIN G.

Congreso; Brazilian Congress of Plant Physiology; 2011

The bark is that portion of the plant body consisting of all tissues external to the vascular cambium and it has the potential to bring rigidity and stiffness to cope with the external conditions. Engineering theory shows that the location of the bark is ideally suited for mechanical support because bending stresses reach their maximum intensities at the surface of any support member.  As the wind is one of the factors that have more influence on stem´s mechanical properties, plants face this factor by increasing or decreasing his flexural rigidity and Young´s modulus. Patagonian steppe is a wide region of South-Argentina where winds are strong, dry and blow predominantly from the west all year originating different microenvironment conditions at leeward and windward sides of the plants. We evaluated wind effects on stem biomechanical traits and determined the contribution that the bark have to rigidity of shrubby species (Colliguaja integerrima and Berberis heterophylla) in Patagonia (45 57’S; 67 31’W). Flexural rigidity (EI), Young´s modulus (E) and second moment of axial area (I) were measured in intact stems and in the same stems without their bark. Bark and xylem density and bark proportion and thickness were also recorded. Variables were measured on 10 samples per species and at windward and leeward of each plant.  For both species and wind orientations, the stems were less rigid without their bark (15-20%) and for both species there was a tendency to become more rigid at windward (25%). Windward stems had thicker and higher proportion of bark with higher contribution of bark to EI than leeward stems (only significant for B. heterophylla). Changes in bark and wood density were only observed in C. integerrima (lower bark density and higher wood density at windward than leeward). We concluded that bark has an important role in stem mechanics conferring higher EI to Patagonian shrubs. Windward stem´s EI is increased by different mechanisms in these species; for B. heterophylla is a consequence of changes in bark traits whereas for C. integerrima is due to variations in wood density.