**4. Discussion**

#### *4.1. Annual Litter Production*

Forest litterfall is strongly influenced by stand density, plant composition and tree age for a similar climate scenario [17,38,39]. In the present study, we observed that annual total litterfall production ranged from 2617.67 to 4118.01 kg·hm−<sup>2</sup> in different plantations (Figure 1a), which was within the range of tropical and subtropical needle-leaved evergreen forests (531–10523 kg·hm−2) [40]. Meanwhile, we observed that the annual production of total litterfall across the plantations exhibited the following order: pure plantation > plantation with SF > plantation with ET (Figure 1a). In general, for stand level, litterfall production increased with increasing stand density [18,39]. Accordingly, the decreased

stand density in plantations with SF and ET may directly lead to decrease in litterfall. However, the variation pattern in total litter production among the investigated plantations was not consistent with their changes in tree density (Figure 6a). Bray and Gorham [41] first showed that litter production appears to be little affected by differences in plant density within closed-canopy forests, and some other studies exhibiting plantation managemen<sup>t</sup> practices with the main purpose of reducing stand density also concluded that there was no change or significant increase in the production of litterfall [42,43]. Based on these findings the variation pattern may be related to differences in species richness caused by different managemen<sup>t</sup> practices and the time lapse after management. In addition, the plantation with SF (lower stand density but higher species richness) showed higher total litterfall than the plantation with ET (higher stand density but lower species richness) in our study, which was consistent with previous studies suggesting that mixed plantations had higher litterfall production than pure plantations [24]. Although the trend of leaf litterfall was consistent with that of total litterfall, the proportion of broadleaf to total leaf litterfall in the plantation with SF (19.12%) was higher than that in the plantation with ET (4.57%) (Figure S2), which also suggested that the leaf litter produced by artificial replanting of broad-leaved species was more abundant than that produced by natural succession within the same time period after management. This result further indicated that the plantation with SF was more conducive to increasing the species diversity of litter, thus forming different carbon and nutrient cycling patterns. Similarly, the production of twig litterfall in the plantations with SF and ET was higher than that in the pure plantation. A likely explanation is that it was positively correlated with species richness (Figure 6a).

Consistent with many previous studies [17,20,28], we found that the production of litterfall in the three plantations was dominated by leaf litter, which emphasized the key role of leaf litter in material circulation and energy flow in cypress plantations. However, we also found that both managemen<sup>t</sup> practices 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). Our findings therefore sugges<sup>t</sup> that the responses of production to forest managemen<sup>t</sup> practices varied with the litter components. Additionally, although our study lasted one year, the reduction in leaf litterfall proportion provided a new approach to explore the material-related mechanisms of managemen<sup>t</sup> practices, as the stand structure of cypress plantations is relatively stable after 11 years of management.

#### *4.2. Litterfall Monthly Dynamics*

The monthly dynamics of litterfall were unimodal and bimodal, which mainly depended on the climatic conditions and ecological characteristics of the tree species [40,44,45]. Therefore, the differences in canopy density and species composition under different forest managemen<sup>t</sup> practices may result in different monthly variation characteristics of litterfall. However, many previous studies have suggested that these managemen<sup>t</sup> practices (thinning, prescribed burning) did not alter the monthly dynamics of leaf litterfall [42,43]. Consistent with previous findings, we observed that the production of leaf litterfall showed unimodal dynamics, with the peak occurring in May in the three plantations, and reasonable explanations for this result might be as follows: (1) cypress was dominant in the three plantations, and the shedding time of old leaves was relatively concentrated. (2) The climate factors significantly influenced the intra-annual litterfall dynamics [16,46]. The obvious positive correlation between leaf litterfall and wind speed in Figure 6b could also confirm this; therefore, the higher wind speed in May might have led to the high production of leaf litterfall. Meanwhile, the production of total litterfall was controlled by leaf litter; therefore, the monthly dynamics of total litter and leaf litter were similar.

Unlike the leaf litter, we found different monthly dynamic patterns of twig litter, reproductive organ litter and miscellaneous litter in the three plantations. Previous studies showed that the monthly collected twig litterfall that came from dead branches was less directly affected by phenology but was sensitive to climatic factors, such as wind speed and rainfall [45,46]. Therefore, plantations with different managemen<sup>t</sup> practices can vary

in their response to such climate factors and, thus, result in distinct monthly dynamic patterns of twig litter. In general, the shedding time of reproductive organs varies among species, but temperature, wind speed and rainfall could also alter the characteristics of litterfall [46,47]. Therefore, although the production of reproductive organ litterfall showed unimodal dynamics in the three plantations, its peak of production could occur in different months. Nevertheless, there was no uniform pattern in the monthly dynamic changes in miscellaneous litter among different plantations resulting from the relatively complex components, although it was positively correlated with rainfall and air temperature (Figure 6b). Interestingly, we found that the production of leaf litter and reproductive organ litter was significantly higher in the early dry season and lower in the wet dry season than in the pure plantation (Figure 4). Since different plant species have different litterfall production rhythms (e.g., conifers and broadleaved trees) [15], plantations with the highest species richness are more likely to show opposite seasonal trends. Overall, based on the one-year study, we highlight the importance of using monthly or seasonal variations in litter input and subsequent decomposition to explore the effects of plantation managemen<sup>t</sup> practices on material cycling and energy flows in ecosystems.

#### *4.3. Litter Components and Allometric Scaling Relationships of Litterfall*

Variations in species, environmental gradients, human disturbances and other factors would lead to the self-regulation of resource allocation in plants, which would be reflected by different survival strategies [48–50]. Consistent with our expectations, the effects of reconstruction managemen<sup>t</sup> on the relationships between each litter component and the total litterfall, as well as between litter components, were related to the reconstruction treatments (Table 1, Figure 5). Leaf litterfall and total litterfall showed an isometric relationship in the three plantations, which was in agreemen<sup>t</sup> with the results of Ma et al. [27] and Fu et al. [28], and this might be because leaf litter was the major component (more than 80% of the total litterfall production) (Figure 2). The Wald test revealed that the differences between the intercept and coaxial drifts were significant, although there were no significant differences in scaling exponents among the three plantations, and this result indicated that the isometric growth trajectories of leaf litterfall and total litterfall were not completely consistent in the three plantations. Furthermore, the allometric scaling exponents of twig litterfall and total litterfall in the plantation with ET were significantly higher than those in the plantation with ST and the pure plantation, which suggested that within a certain range of total litterfall, the plantation with ET allocated more production to twig litter. Similarly, the plantations with reconstruction allocated more production to reproductive organ litter than the pure plantation. More reproductive organ litterfall is preferred for breeding populations [51]; therefore, this result could explain the high vegetation diversity in plantations with reconstruction treatments.

Leaves are important photosynthetic organs, and the presence of more leaves accelerates the transport of nutrients from aboveground to belowground, thus promoting plant growth [49]. In this study, the allometric scaling exponent between leaf litterfall and twig litterfall in the plantation with ET was significantly higher than that in the plantation with ST and the pure plantation (Figure 5, Table 1), which suggested that within a certain range of twig litterfall, the plantation with ET allocated less production to leaf litterfall. This result further indicated that the plantation with SF might be superior to that with ET only from the perspective of plant growth. In addition, consistent with previous studies [28], we observed that some relationships (leaf litterfall versus reproductive organ litterfall and twig litterfall versus reproductive organ litterfall) could not be expressed by allometric equations in the pure plantation (Table 1), which may be related to reproductive organ litterfall being controlled by complex micrometeorological factors [51]. Unlike in the pure plantation, significant differences in relative growth slopes between the plantations with managemen<sup>t</sup> practices were observed. These disparate responses further emphasized that cypress plantations could modify resource allocation to organs through physiological

integration after intense disturbance and that those allocations were similar to disturbance patterns.
