The relationships between fiber dimensions and growth rate in Populus nigra

Background and objectives: In most cases the goal of silvicultural practices is to increase diameter and height of the trees. In turn, any change in growth rate and growth pattern of trees may result in wood variation. On the other hand, the technical properties of wood and items made from the wood are strongly depended to the wood anatomical characteristics particularly fibers length. In this paper the effect of growth rate of poplar trees was studied on the fibers biometric attributes.
Materials and methods: Young Poplar trees (Populus nigra spp. betulifolia) were selected from a trial cultivation near Karaj city, Iran. Fifteen 10-year old trees were cut and a disc with 20 mm thick was taken at breast height, then selected discs were transferred to the laboratory. The cut trees were classified into the 3 classes based on the growth rate- slow growing, moderate growing, and fast growing - with 5 trees in each class. Thereafter, diameter growth, circumference growth, and fiber biometric parameters were measured for each separated annual ring. The results were analyzed using different statistical methods and the correlation between different items and their variation evaluated from pith to bark. The relationships between different parameters were studied in the annual rings with same cambial age in order to eliminate possible effects of age.
Results: In young poplar trees correlations were insignificant between fiber dimensions and growth rate in most of the rings. However, there were some positive and significant correlations between growth rate and fiber length. The correlation coefficient between circumference growth and fiber length were higher than those obtained between diameter growth and fiber length. Despite of insignificant effect of growth rate on fiber biometric attributes, whole-disk fiber dimensions which were weighted based on the growth area of each ring increased as measured by stem diameter. This is so because of fast-growing trees have more of their growth area concentrated in the rings further from the pith than do slower growing trees, and these rings have larger fibers compared with the rings closer to the pith.
Conclusion: The circumference growth is a better parameter than diameter growth for the study of relationships between the growth rate and the fibers biometric attributes. Unexpectedly, the correlations between growth rate and fiber length not only was not negative but also was positive in some of the rings. Whole disc weighted fiber dimensions increased as tree diameter increased which shows that increase in growth rate of young poplar trees has no decreasing effect on fiber dimensions. It looks that results obtained here with young poplar trees is extensible for all the silvicultural practices that may affects growth rate.
Keywords: Fiber dimensions, Diameter growth, Circumference growth, Cambial divisions, Poplar tree.