growth rate

Globally, paulownia (Paulownia fortunei (Seem.) Hemsl.) is one of the most important wood species used in forestry. Despite extensive research on this genus, the influence of climatic factors on the growth and anatomical wood properties of paulownia has not been thoroughly investigated. Therefore, this research aims to examine how climatic factors—such as temperature, precipitation, relative humidity, and sunshine hours—affect radial growth and specific wood anatomical traits of cultivated paulownia trees in Gorgan County, Golestan Province, Northwest Iran.

Five 20-year-old Paulownia fortunei trees were selected from Hashemabad in Gorgan County. These trees were felled in late 2012, and a five-centimeter-thick disk was extracted from each stem base. Strips from the bark to the pith were cut radially from each disk, and the surfaces of the specimens were smoothed using a microtome blade before scanning. To enhance the contrast between the vessels and the background tissue, a charcoal blackening technique was employed prior to scanning. Using images obtained from image processing software, several features were measured: growth ring width, average vessel lumen area, porosity, vessel frequency, vessel area greater than 0.06 mm², radial and tangential diameter of vessels, and ray frequency in each growth ring. To eliminate the effect of juvenile wood—which consists of very wide rings in the early years of growth—five rings near the pith were excluded from measurements and analyses. Consequently, growth rings from 1998 to 2012 (15 years) were evaluated. Pearson correlation was used to investigate the relationships between climatic variables (monthly, bimonthly, seasonal, bi-seasonal, and tri-seasonal) and growth variables (growth rate and quantitative anatomical wood characteristics) from three months prior to growth until the end of the growing season. Additionally, internal relationships among the measured variables were assessed similarly.

The radial growth of paulownia exhibited a decreasing trend with age, particularly notable at seven and thirteen years. Other anatomical features (vascular traits and ray frequency) showed an increasing trend; however, vessel frequency did not exhibit a clear age trend. Correlation analysis revealed that all anatomical characteristics had an inverse relationship with growth rate; thus, as growth ring width increased, other characteristics (e.g., vessel area and vessel diameter) decreased. Vascular traits and ray frequency primarily showed positive correlations with each other, while the correlation between vessel frequency and other variables was weaker. Among climatic factors, total monthly rainfall and average sunshine hours in spring (especially May and June) significantly impacted radial growth and anatomical features of paulownia wood. Climatic factors from three months prior to the growth period (the previous winter) did not show significant correlations with anatomical characteristics; furthermore, correlations between anatomical traits and summer climatic factors were much weaker than those observed in spring. Increased spring rainfall enhanced radial growth while reducing anatomical features such as vessel size and ray frequency; average sunshine hours had a similar but less intense effect on these traits. Temperature and air humidity did not significantly affect the studied characteristics.

Overall, it can be concluded that paulownia has high water and light requirements but shows low sensitivity to temperature in the studied region. The months of May and June (mid to late spring) are particularly influential for radial growth and hydraulic architecture (water transport system) in paulownia trees. Increased precipitation and average sunshine hours promote radial growth but reduce vessel size. Additionally, ray frequency can be considered a valuable wood anatomical characteristic for studying the relationship between climate and tree physiology in future research. 

Carbon sequestration through trees is one of the most important strategies to mitigate the carbon emission. The present study was conducted in Khorramabad Agricultural Orchard as randomized complete block design. In each experimental unit, three five-year-old trees were investigated in two groups and 12 poplar colonies or cultivars in three replications. Duncan multiple range test was used to find the mean comparison of the treatments and T test was applied to observe the differences between two groups. The results showed that there is a significant difference in the increment rate and amount of carbon sequestration in the two poplar groups (P<0.01). P. nigra 42.53 revealed the maximum increment in the closed crown group to be 2.6714 m3, and also the highest annual sequestration as 2460.8 kg/ha. In open crown group, the highest increment and carbon sequestration were observed in P.X Pacher to be3.0618 m3 and 1987.909 kg/ha, respectively. Additionally, the open crown group had a higher increment rate compared to the closed crown group. However, there was no significant difference between the two groups in terms of the amount of carbon sequestration. It can be interpreted that the density of open crown group is less than the closed crown one. According to our findings, further studies on carbon sequestration of various tree species and afforestation with fast-growing species with high carbon storage potential is recommended to control the concentration of CO2 in the atmosphere.