*2.3. Statistical Analyses*

All statistical analyses were performed with R version 4.1.1 [34], and the accepted significant level was set at α = 0.05. Linear mixed models were used to assess the effects of tree species, organ type, and their interaction on DOM parameters with the 'nlme' package. For each tree species, paired *t*-test was used to examine the differences in DOM parameters between leaves and twigs. For each plant organ, one-way ANOVA with Tukey's HSD comparison was applied to determine the differences in DOM parameters among the tree species with the 'agricolae' package. Spearman's correlation analysis was used to examine the relationships between DOM parameters and initial properties of plant materials with the 'survey' package. A linear regression analysis was used to determine the relationships between DOM biodegradability and initial DOM properties with the 'abline' function.


among the tree leaf treatments (*p* < 0.05), and the lowercase letters indicate the significant differences among the tree twig treatments (*p* < 0.05). For the same tree

species, *p*-value < 0.05 indicate the significant differences between leaf and twig.

**Table 1.** Initial properties of tree leaf and twig in subtropical plantations in southern China.
