Effects of drought acclimation on the mechanical properties of Ochroma pyramidale, Betula pendula and Acacia karroo tree seedling stems

Abstract

Drought acclimation in woody plants tends to be associated with the development of denser vascular tissue. As wood density may be correlated with the mechanical properties of the tissue, we hypothesized that seedlings subjected to drought would develop stems that had a higher density as well as stiffer and stronger tissue. We tested our hypothesis on establishing trees of the species Betula pendula, Acacia karroo and Ochoma pyramidale. The seedlings were grown for 1 year under well-watered as compared with cyclically droughted conditions. The modulus of elasticity in bending, yield stress, density and pith- and bark fractions of the stems at different heights were measured. In all three species, individuals grown under drought conditions had on average stiffer and stronger stems than the well-watered individuals, though non-significantly so in A. karroo. These differences were associated with a higher density in O. Pyramidale but not in the other species. In B. pendula, the differences in mechanical properties appear to be associated with other factors, e.g. modifications of the micro fibril angle in the S2 cell wall layer. Our results indicate that drought acclimation in seedlings affects the mechanical properties of the stem but that the causal acclimational processes may vary between species.