ABSTRACT

It is known that Haloxylon has successive cambia and its radial growth rate is not clear as indicated by the fact that the annual boundary cannot be determined by dendrochronological approach. Radial increment through successive cambia (RISC) layers possess similar morphological feature as compared to annual rings by megascopic cross sectional observations. Previous studies have demonstrated that the dendrochronological methods cannot be applied to some species that possess successive cambia. The objective of the present study is to clarify the growth rate of RISC layer in Haloxylon ammodendron by a series of radiocarbon dating. Two H. ammodendron specimens (S1 and S2) were collected from the southern edge of Gurbantünggüt Desert in Northwestern China. Radiocarbon measurements revealed the numbers of annual RISC layer to be 2.98 ± 0.51 (S1) and 3.65 ± 0.53 (S2). Linear regression shows a strong relationship between growth period (x) and radial increment accumulation (y): y = 1.52x + 0.76 (R² = 0.96, P < 0.001) for S1 and y = 1.70x + 4.49 (R² = 0.94, P < 0.001) for S2, respectively. Anatomical observations on the RISC layer using scanning electron microscope (SEM) image showed that: (1) the early wood and late wood were not present or differentiated; (2) axial vessel groups (about 2-10 vessels) are often distributed in the bark-side; and (3) sclerenchyma cells as junctions tightly adhered vessel groups at outside in each RISC layer. These observed features are completely different from those in annual ring morphologies. We tentatively hypothesize that these features are due to the formation of multiple RISC layers within one growing season in order to aid in water storage in the stem to survive in harsh habitat conditions. This research will help further the knowledge in the physiological and ecophysiological researches on H. ammodendron and other successive cambia species.