Abstract: Two representative desert plants, Tamarix ramosissima and Haloxylon ammodendron, both utilize green assimilating branches to conduct photosynthesis instead of leaves which greatly reduced. There are few studies on the relationship between twigs and the assimilating branch for these two dominant species in the central Asia up to now. In this study, regression equations of twig mass and assimilating branch area within terminal twigs were given, and biomass allocation as well as the relationship between twig size and assimilating branch area within terminal twigs of T.ramosissima and H.ammodendron have been investigated in detail with the aim of investigating the two plant’s difference in relationship of twigsize to assimilating branch size. Taking stem diameter as independent variable, the regression equations on twig mass and assimilating branch area using power function have high correlation, and the power exponent of T.ramosissima is bigger than that of H.ammodendron, indicating that the twig mass and assimilating branch area increase of T.ramosissima is bigger than that of H.ammodendron with increasing twig size. Isometric relationships were found between twig mass and stem mass, twig mass and assimilating branch mass, as well as between stem mass and assimilating branch mass of T.ramosissima, suggesting that its biomass allocation to assimilating branch or stem is independent on twig size. On the other hand, significant allometric relationships were established for twig mass and assimilating branch mass, twig mass and stem mass, as well as stem mass and assimilating branch mass of H.ammodendron, and its stem has preference to biomass allocation with increasing twig size, indicating that this species has greater demand for stem that play a role in support and water transportation. In addition, significant allometric relationships were also obtained for assimilating branch area and twig cross-sectional area (slope>1.0), and assimilating branch area with stem mass (slope<1.0) in terms of T.ramosissima, indicating that there are two different variation tendencies of the proportion of assimilating branch area with twig size which is represented by twig cross-sectional area and stem mass, respectively. As a comparison, both allometric equations for H.ammodendron have slopes significantly smaller than 1.0, indicating that the proportion of assimilating branch area decrease with increasing twig size for this species.

Key words: leaf size, twig size, biomass allocation, terminal twigs, allometry, desert plants