**3. Results**

#### *3.1. Annual Litterfall Production*

The annual total litter production and its components showed significant differences among the plantations (*p* < 0.05). The total annual litterfall production in plantations with strip filling (3672.46 kg·ha−2, SF) and ecological thinning (2617.67 kg·ha−2, ET) was significantly decreased by 10.8% and 36.44%, respectively, compared with that in the pure cypress plantation (4118.01 kg·ha−2, PC) (Figure 1a). Similarly, both reconstruction treatments decreased the annual production of leaf and reproductive organ litter, and the greatest declines were observed in the plantation with ET (Figure 1b,d). In contrast, both reconstruction treatments increased the annual production of twig and miscellaneous litter, and a significant difference was observed only between the plantation with SF and the pure plantation (Figure 1c,e).

The proportion of each litter component relative to the annual total litterfall (except the reproductive organ litter) showed significant differences among the three plantations (*p* < 0.05). In general, leaves accounted for more than 80% of the annual total litterfall in the three plantations (Figure 2). Compared with the pure plantation, both reconstruction treatments significantly decreased the proportion of leaf litter relative to total litterfall and increased the proportion of twig and miscellaneous litter relative to total litterfall (Figure 2). Moreover, there were no significant differences in the proportions of leaf, twig and miscellaneous litter between the plantations with SF and ET (*p* > 0.05).

**Figure 1.** Annual production of total litterfall (**a**) and its components ((**b**) leaf litter; (**c**) twig litter; (**d**) reproductive organ litter; (**e**) miscellaneous litter) in different plantations. PC: pure cypress; SF: strip filling; ET: ecological thinning. Different letters denote significant differences in values at the *p* < 0.05 level.

**Figure 2.** The proportions of each litter component in the total litterfall in different plantations. (**a**) PC: pure cypress; (**b**) SF: strip filling; (**c**) ET: ecological thinning.

#### *3.2. Monthly Dynamics of Litterfall*

Litterfall varied monthly, and this variation showed different patterns among plantations and litter components (Table S2, Figure 3). Regardless of the plantation, the production of total litter and leaf litter displayed unimodal dynamics, with the peak occurring in May (Figure 3a,b). Meanwhile, more than 70% of the total litterfall and leaf litterfall occurred during the late dry season (from April to June) and early wet season (from July to September) in each plantation, taking the rank order of pure cypress plantation > plantation with SF > plantation with ET (Figure 4a,b). However, the production of total litter and leaf litter in plantations with SF was obviously higher than that in the other two plantations during the late wet season (from October to December) and early dry season (from January to March).

**Figure 3.** Monthly production of total litterfall (**a**) and its components ((**b**) leaf litter; (**c**) twig litter; (**d**) reproductive organ litter; (**e**) miscellaneous litter) in different plantations. PC: pure cypress; SF: strip filling; ET: ecological thinning. Asterisks denote significant differences among the three plantations within the same month: \* indicates *p* < 0.05; \*\* indicates *p* < 0.01; and \*\*\* indicates *p* < 0.001.

**Figure 4.** Seasonal production of total litterfall (**a**) and its components ((**b**) leaf litter; (**c**) twig litter; (**d**) reproductive organ litter; (**e**) miscellaneous litter) in different plantations. PC: pure cypress; SF: strip filling; ET: ecological thinning. Capital letters indicate differences among seasons within the same plantation, while lowercase letters indicate differences among the three plantations within the same seasons at the *p* < 0.05 level.

The production of twig litter showed a major peak in August and a smaller peak in April in the pure plantation. In contrast, it only peaked in May in the plantation with EF and had no clear pattern in the plantation with SF (Figure 3c). Meanwhile, more than 45% of the twig litter in the pure plantation occurred during the late dry season, while litter in the plantation with EF occurred during the early wet season. However, the production of twig litter in the plantation with ET was significantly higher than that in the pure cypress plantation (Figure 4c).

Moreover, the production of reproductive organ litter in the three plantations was unimodal, with the peaks in the pure, SF and ET plantations occurring in August, June and May, respectively (Figure 3d). In addition, it was highest in the plantation with SF and in the pure plantation during the dry and wet seasons, respectively (Figure 4d).

The production of miscellaneous litter was unimodal in the pure plantation, with the peak occurring in August, while it showed a major peak in August and a smaller peak in May, regardless of whether plantations used SF or ET (Figure 3e). For all seasons, the production of miscellaneous litter in plantations with SF was obviously higher than that in the other two plantations (Figure 4e).

#### *3.3. Allometric Scaling Relationships of Annual Litterfall*

Regardless of the plantation, significant isometric relationships were observed between leaf litterfall and total litterfall, while allometric relationships were observed for annual twig litterfall versus total litterfall and reproductive organ litterfall versus total litterfall (Table 1, Figure 5). The scaling slopes of leaf litterfall versus total litterfall in the different plantations did not differ significantly, with a common slope of 1.020. According to the Wald test, however, the intercept and coaxial drifts showed significant differences among the three plantations (Figure 5). Moreover, the scaling slopes of twig litterfall versus total litterfall in the plantation with ET and reproductive organ litterfall versus total litterfall in the plantations with SF and ET were significantly higher than those in the pure plantation.


**Table 1.** Allometric scaling relationships between different litter components in different plantations (LTo: total litterfall; LL: leaf litterfall; LT: twig litterfall; LR: reproductive organ litterfall).

Notes: PC: pure cypress; SF: strip filling; ET: ecological thinning. A indicates an allometric growth relationship, and I indicates an isometric growth relationship. Different lowercase letters indicate significant differences between the plantations (*p* < 0.05). \* indicates *p* < 0.05; \*\* indicates *p* < 0.01; and \*\*\* indicates *p* < 0.001.

**Figure 5.** Allometric relationships between each litter component and total litterfall (**<sup>a</sup>**–**<sup>c</sup>**) and among litter components (**d**–**f**) in different plantations. PC: pure cypress; SF: strip filling; ET: ecological thinning.

Significant allometric relationships were observed for leaf litterfall versus twig litterfall in different plantations, and the scaling slope in the plantation with ET was significantly higher than that in the pure plantation (Table 1, Figure 5). Meanwhile, significant allometric relationships were observed for leaf litterfall versus reproductive organ litterfall in the plantations with SF and ET. Moreover, for twig litterfall and reproductive organ litterfall, a significant positive allometric relationship was observed in the plantation with SF, while an isometric relationship was observed in the plantation with ET. However, the relationships of leaf litterfall versus reproductive organ litterfall and twig litterfall versus reproductive organ litterfall in the pure plantation could not be expressed by allometric equations.

#### *3.4. Factors Driving the Changes in Litterfall Production*

RDA revealed that vegetation factors explained 62.40% of the variation in annual litterfall production (Figure 6a). Annual total litter, leaf litter and reproductive organ litter all showed high production under conditions of lower diameter at breast height and tree height. Annual twig litter and miscellaneous litter showed high production under conditions of higher species richness and lower stand density and canopy density. Meanwhile, hierarchical partitioning analysis showed that diameter at breast height and tree height were the most important factors (*p* < 0.05), explaining 17.85% and 16.14% of the variation, respectively (Table 2).

**Figure 6.** Relationships of vegetation factors with annual litterfall production (**a**) and of climate factors with monthly litterfall production (**b**) based on redundancy analysis (RDA). The data used in this analysis were the litterfall from the three plantations.



Bold *p* values are significant (*p* < 0.05).

The climate factors explained 46.84% of the variation in monthly litterfall production (Figure 6b). Monthly total litter, leaf litter and twig litter all showed high production under conditions of higher mean wind speed and lower relative humidity. Monthly reproductive organ litter and miscellaneous litter showed high production under conditions of higher precipitation and air temperature. Meanwhile, in terms of the degree of influence on monthly litterfall production (*p* < 0.05), the climate factors ranked as follows: air temperature (16.38%) > relative humidity (15.95%) > mean wind speed (9.09%) > precipitation (5.41%) (Table 2).
