Anatomical Variations in the Woody Plants of Arid Areas

Abstract

Variations in secondary xylem characteristics in 13 xerophytic species growing in an arid sandy region in China are described and compared in detail from an ecological perspective. All species showed similar wood structure (except Haloxylon ammodendron - rayless), obvious growth ring boundaries (except in H. ammodendron, Tamarix mongolica and Zygophyllum xanthoxylon - sometimes discontinuous), ring to semi-ring-porosity, a simple perforation plate, alternate intervessel pitting, non-septate fibres, paratracheal confluent axial parenchyma, helical thickenings and heterocellular rays. However, some quantitative differences in rays and vessels between species were observed. Rays are uniseriate in Salix psammophila, one- to two-seriate in Tamarix mongolica and Hippophae rhamnoides, two- to five-seriate in Ammopiptanthus mongolicus, Lespedeza bicolor, Z. xanthoxylon, Nitraria tangutorum, Elaeagnus angustifolia, and Calligonum mongolicum, and generally four- to ten-seriate in the other three species. This quantitative study of anatomical characteristics revealed that secondary xylem cells have high adaptability to desert conditions. Vessels with small diameter, either solitary or grouped in multiples, very short elements and minute pits are among the responses to demand for greater water transport capacity, and the appearance of such features in the xylem of arid zone species is interpreted as a strategy for conductive safety. T. mongolica and A. mongolicus had narrower vessels and higher vessel frequency than the other 11 species, which could lead to lowered vulnerability and mesomorphy value. Thus it was considered that the anatomical features of T. mongolica and A. mongolicus were more likely to suffer from water stress than those of other species. Fibre length and vessel element length were measured and analysed, with their horizontal variations showing either decreasing, increasing, considerable fluctuation or constant tendency with age. Fibre length is less than 900 µm, which is defined as “short range” according to IAWA Committee classification. There was significant difference both in among-tree and within-tree fibre length and vessel element length except in A. mongolicus.