Effect of the different microenvironments generated by wind within individuals over stem´s mechanical properties of patagonian shrubs.

IOGNA PA; SCHOLZ FG; BUCCI SJ; GOLDSTEIN G; ASKENAZZI J

Congreso; Brazilian Congress of Plant Physiology; 2011

Patagonian wind´s are strong, dry and blow predominantly from the west all year. In this situation the wind created patches in the vegetation (Soriano 1981). This patch structures create well differentiated microenvironments at leeward and windward sides of the patch with different biotic and abiotic conditions (Aguiar and Sala, 1997). Leeward areas of the patchs have on average wind velocities of only 20% of the exposed areas (Soriano y Sala 1986). The bark has the potential to bring rigidity and stiffness to cope with the external conditions (e.g. wind). 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 (Silk et al., 1982; Niklas, 1992). Been the wind one of the factors that have more influence over stem´s mechanical properties, plants face this factor by increasing or decreasing his flexural rigidity and Young´s modulus (Heuchert et al., 1983; Telewski y Jaffe, 1986). This study was carried out in Patagonian steppe (45 57’ S; 67 31’ W) in two dominant shrubs of the zone: Colliguaja integerrima and Berberis heterophylla. The objetive was to evaluate the effect of the microenvironments generate by the strong patagonian winds over the mechanical relationships of the stems within individuals, and the contribution that the bark have to stem rigidity in this scenario. The mechanical variables measure were flexural rigidity (EI), Young´s modulus (E) and second moment of axial area (I) of intact stems that then were stripped (remove their bark) and remeasure taking note of the reductions in the transverse area resulting from the removal of tissues. Also bark and xylem density, bark thickness and proportion of bark in the stem were record. All variables were measured on 10 samples per specie and wind orientation (windward and leeward). The results show that for both species and wind orientations, the stems were significantly less rigids without their barks and for both species there was a tendency of the stems to be more rigids at windward. In Berberis heterophylla the bark was significantly thicker on windward stems (p< 0.05), with a significantly higher contribution of the bark to EI in this orientation (p< 0.1). Also the proportion of bark was significantly higher at windward (p< 0.05). Bark density show a tendency of been higher at windward than leeward and xylem density have an opposite tendency. At windward the bark confer a higher contribution to stem rigidity for the mayor thickness, proportion of bark and bark density show at this orientation, having an important role in stem mechanics. Colliguaja integerrima show a tendency of have a thicker bark and higher proportion of bark at windward but this bark don´t have a significant contribution to EI. Xylem density was significantly higher in windward stems (p< 0.05). The bark don´t contribute much to stem rigidity, been the mayor contribution to the tendency of more rigid stems at windward made by the xylem, through the significantly higher xylem density shown in windward stems.