**1. Introduction**

Forest litter refers to the general term for all organic matter in the forest ecosystem that is produced by aboveground plants and other biological components and returned

**Citation:** Liu, X.; Wang, Z.; Liu, X.; Lu, Z.; Li, D.; Gong, H. Dynamic Change Characteristics of Litter and Nutrient Return in Subtropical Evergreen Broad-Leaved Forest in Different Extreme Weather Disturbance Years in Ailao Mountain, Yunnan Province. *Forests* **2022**, *13*, 1660. https://doi.org/10.3390/ f13101660

Academic Editors: Fuzhong Wu, Zhenfeng Xu and Wanqin Yang

Received: 25 August 2022 Accepted: 4 October 2022 Published: 10 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to the forest surface as a source of material and energy for decomposers to maintain ecosystem functions, including litter leaves, litter branches, flower, and fruit reproductive organs and debris [1,2]. Leaf litter is considered to be an important survival strategy for plants to cope with adverse growth conditions, such as soil drying due to temperature reduction or drought [3,4]. Forest litter is an important structural and functional unit of material circulation and energy flow in the forest ecosystem. Its litter, accumulation, and decomposition are basic ecosystem processes. Important functions have irreplaceable ecological roles, and a large amount of organic matter and mineral elements are transported from the canopy of plants to the soil surface through the litter. Therefore, the collection and measurement of forest litter are important means of studying the structure and function of forest ecosystems [5–8]. To a certain extent, litter yield and nutrient return are suitable indicators of the overall function of forest ecosystems and play a key role in forest ecosystem dynamics, nutrient cycling, and forest productivity [9–12].

The forest types in subtropical regions are important forest ecosystems unique to the same latitude in the world. They have high primary productivity and are biodiversity hotspots, which play an important role in carbon storage in global terrestrial ecosystems [13]. The montane moist evergreen broad-leaved forest in the Ailao Mountain Nature Reserve in Yunnan is currently the largest and most well-preserved subtropical evergreen broad-leaved forest in my country. It is one of the valuable zonal vegetation. It is of grea<sup>t</sup> significance to study the dynamic change law of yield, nutrient cycle law and composition characteristics of the litter of the evergreen broad-leaved forest in Ailao Mountain to understand the nutrient availability and productivity of the forest system [14–17].

Forest litter has a distinct seasonal pattern, mainly in a unimodal, bimodal or irregular pattern [18–21]. The temporal variability of forest litter is an important source of uncertainty in the forest carbon cycle, and biological factors such as forest type, origin, forest age, tree species richness, phenological rhythm and genetic characteristics are important factors affecting the seasonal pattern of litter [22–25]. Different forest communities in different climatic zones around the world are not the same. According to research, the annual litter of the main forest types in each climatic zone can be specifically expressed as rainforest > evergreen broad-leaved forest > mixed coniferous and broad-leaved forest > deciduous broad-leaved forest > coniferous forest [26–28]. Although in the past few decades, litter yield, structure and composition, decomposition rate, and its influencing factors have been extensively studied around the world, the existing dynamic changes of litter are mostly related to litter yield, nutrient The results of a single study on the return of elements or nutrients for one year are mostly planted forests, and there are relatively few studies on primitive natural forests [29–31]. In addition, the research on the evergreen broad-leaved forest in this region mainly includes the vegetation type and diversity research, and the multi-year comparative comprehensive research combining the dynamic law of litter yield and nutrient return is rarely reported [7,10,32–36].

Forest litter is an important carbon pool for forest ecosystems. Its nutrient cycle and nutrient balance are strongly affected by climate, and play a complex source-sink effect in the process of global change. It is one of the basic parameters of carbon exchange with the atmosphere and is closely related to global change and the circulation of materials in the global ecosystem [37]. Climatic factors (including temperature, precipitation, long-term extreme weather, etc.) are important factors affecting forest productivity, in addition, trees take a long time to regenerate and increase biomass, so forest ecosystems are sensitive to extreme weather disturbances. In recent years, forest litter has begun to be studied in the context of the global environment, focusing on its important role in carbon and nutrient cycling [38,39]. Climate change will not only directly affect the community structure and vegetation composition of forest ecosystems by changing climatic factors such as temperature and precipitation, but also indirectly affect the primary productivity level of forest ecosystems and the nutrients of forest ecosystems by changing the area and intensity of natural disturbances. The distribution and material circulation process have an impact, which in turn affects the amount, composition and dynamic changes of litter

in forest ecosystems [40–42]. Specifically, with global warming since the 20th century, the scope, frequency, and intensity of extreme weather events around the world have increased significantly. Compared with conventional weather change, extreme weather will have a greater impact on forest ecosystems, increasing the The survival pressure of forest communities and the potential risk of local extinction have caused a series of crises to the security of forest ecosystems [43]. In particular, extreme weather disasters such as drought, ice and snow have become one of the most important climate disasters due to their high frequency and large scope, and they have always been widely concerned [44,45].

From a geographical point of view, Yunnan, as one of the areas with the most frequent drought disasters in Southwest China, has attracted extensive attention of many scholars. However, most of the current related researches focus on the analysis of the spatial and temporal distribution characteristics and causes of drought and flood disasters in Yunnan. The research results on the combination of litter dynamics and drought in typical subtropical evergreen broad-leaved forests have not ye<sup>t</sup> been reported [46,47]. In 2010, parts of Yunnan suffered continuous severe drought, which was the most severe drought event ever recorded in Southwest China, which caused serious damage to the forest ecosystem in the region [48,49]. Subtropical evergreen broad-leaved forests have become the main victims of ice and snow disasters due to their evergreen and relatively wide canopies [50], and mechanical damage is particularly serious. Therefore, the disturbance of extreme freezing rain and snow weather disturbances to subtropical forest ecosystems is also very serious [51,52]. As far as ice and snow disasters are concerned, because they mainly occur in Europe and eastern North America, the research on the impact of snow and ice disasters on forest vegetation is also mainly concentrated in this region, resulting in the response model of subtropical forests to snow and ice disasters is still not completely clear [53–55]. However, in January 2015, Ailao Mountain encountered a catastrophic ice and snow weather disturbance, resulting in a certain degree of fragmentation of the evergreen broad-leaved forest in the area, and overall changes in the woodland habitat and forest structure. The forest ecosystem and diversity have caused serious damage [56]. Once the dynamics of litter changes, the material cycle of the forest ecosystem will be affected, and the productivity and service functions of the forest will change [42,57]. Therefore, it is very important to study the dynamics of litter in the study area. In addition, human beings and forest ecosystems are closely related, and these changes will inevitably have a direct impact on human beings. How to promote the flow of material circulation and energy in post-disaster ecosystems, strengthen the protection of biodiversity in damaged forest ecosystems, and restore ecological functions is an urgen<sup>t</sup> problem to be solved. However, the solution to these problems is inseparable from the research on the changes of litter and its nutrient cycle after disasters [37,58].

However, the current research on whether extreme drought will change, how it affects, and the degree of impact on the litter dynamics of subtropical forest ecosystems and the development process of forest ecosystems is relatively scarce, and the comparison of the degree of damage to the same forest ecosystem caused by different extreme weather disturbances not clear [37,42]. To this end, this study selects three special years with equal intervals of growth in the virgin forest of Ailao Mountain National Nature Reserve, including 2005 (a normal year not disturbed by extreme weather) and 2010 (a year disturbed by extreme arid weather), 2015 (the year disturbed by extreme ice and snow weather), the amount of litter in different parts and the nutrient element content and return amount of each component were measured to explore the components and total amount of litter in normal years and before and after being disturbed by different extreme weathers. The interannual variation, monthly dynamic variation, seasonal dynamic variation characteristics of litter volume and its significant differences with each component and nutrient element content and return amount. On the basis of a comprehensive analysis of the impact of extreme weather change on litter dynamics, this paper attempts to summarize the variation laws and nutrient cycling laws of litter under different extreme weather disturbances, in order to enrich the ecological structure, ecological structure, and nutrient cycle of the montane humid evergreen broad-leaved virgin forest. The research on the function provides a theoretical basis for the material cycle, nutrient balance, and ecological restoration of the disturbed forest ecosystem. This paper discusses how to better apply the ecological function of litter to forest ecosystem management, so as to improve the managemen<sup>t</sup> level of forest ecosystem and give full play to the self-sustaining mechanism of forest ecosystem.

According to previous studies, we know that extreme weather interference can form a large number of abnormal litters, which refers to the fresh residues and litters of individual plants or plant organs caused by external forces under extreme weather, fire, or geological disasters, such as a large number of fallen trees, twigs and litter leaves on the forest land due to the impact of natural disasters such as low temperature, snow, freezing, or typhoon, to affect the amount of litter and its nutrient content [44–47,59]. Based on this, we propose the following main research questions: (1) Can extreme drought and ice and snow weather disturbance affect litter yield and nutrient return? (2) Will extreme arid weather disturbance hinder or destroy the normal succession process, seasonal dynamic changes, and decomposition rate of forest ecosystems? (3) If there is an impact, is the impact of different extreme weather disturbances on the components of litter consistent?

#### **2. Materials and Methods**

#### *2.1. Overview of the Study Area*

The study area (Figure 1) is located in the Xujiaba area (24◦32 N, 101◦01 E) in the core area of Ailao Mountain National Nature Reserve, with an altitude of 2400–2600 m, and the soil is fertile acid yellow-brown soil [14]. According to the long-term monitoring data of the Ailao Mountain Forest Ecosystem Research Station, the average annual temperature in this area is 11.3 ◦C, the average annual precipitation is 1931 mm, and the average annual evaporation is 1192 mm. The climate belongs to the southwest monsoon region, with distinct dry and rainy seasons. The precipitation in the rainy season (May to October) accounts for about 85% of the annual precipitation, and the average annual relative humidity is 85% [15,16]. The zonal vegetation is the mid-mountain humid evergreen broad-leaved forest, the canopy is highly closed, the community tree, shrub, and grass layers are clearly layered, the interlayer plants are common with woody vines, and epiphytes are abundant, mainly mosses and ferns. The dominant species are *Castanopsis rufescens* Hook. f. et Thoms., *Schima noronhae* Reinw. ex Bl. Bijdr., *Lithocarpus xylocarpus* (Kurz) Markgraf, *Camellia forrestii* (Diels) Coh. St., *Machilus bombycina* King ex J. D. Hooker and other large trees, as well as shrubs such as *Sinarundinaria nitida* Franch. widely distributed in the forest and Plagiogyria communis and other herbs.

#### *2.2. Sample Plot Setting and Litter Collection*

The evergreen broad-leaved forest community in this area was selected, and a square fixed plot with a plot area of 1 hm<sup>2</sup> (100 m × 100 m) was established, and the established fixed plot was divided into 100 small plots of 10 m × 10 m. A total of 25 of them were randomly selected for long-term litter observation. A box-type litter collector with an area of 1 m<sup>2</sup> was placed horizontally on the forest floor or at a certain distance from the ground. The bottom of the collector was 0.5 m from the ground, and nylon mesh screens were used, and the surrounding areas were fixed with PVC pipes. Then, all the fresh litter in the collector is collected at the end of every five years at equal intervals, that is, at the end of each month in 2005, 2010, and 2015. Stones and other sundries are numbered and placed in sterile plastic bags, and all litter is used to record the amount of litter recovered [2,60,61]. We separated the collected litter by branches, leaves, flowers, and fruits (reproductive organs), other components (bark, moss, lichen, debris), and cleaned up the attached impurities. Then the litter were dried to constant weight in the oven at 65 ◦C and weighed and recorded for calculating the litter amount of each component of the litter [62,63]. The litter amount of each litter component was converted from the average value of the corresponding components in 25 collection frames. The monthly litter amount

was the sum of the litter components of the month, and the annual litter amount is the sum of the litter amount of 12 months [25,33,64–66].

**Figure 1.** Study Area Map.

#### *2.3. Determination of Litter Nutrients*

The dried and weighed litter analysis samples were ground with a plant crusher and sieved through a 60 mesh to determine the nutrient contents. Carbon (C) and nitrogen (N) were measured with a carbon element analyzer (EA3000, EuroVector, Milan, Italy) [33–35]. For the determination of other elements, the sample was digested by the H2O2–H2SO4 digestion method and then prepared into the solution to be measured. The content of total phosphorus (P) is determined by the Mo-Sb colorimetry method, and the content of total potassium (K), total sulfur (S), total calcium (Ca), and total magnesium (Mg) were determined by the flame photometer and spectrophotometer [6,36]. The amount of nutrients returned by litter is closely related to the amount of litter and nutrient content in the litter.

#### *2.4. Calculation of Nutrient Return of Litter*

The amount of nutrient return of litter is closely related to the amount of litter and the nutrient content in the litter. The amount of nutrient return is equal to the product of the amount of litter and the nutrient content in the litter. The specific calculation is that the monthly nutrient return amount of each component of the litter is the product of the nutrient content of the component of the litter in the current month and the litter amount of the component in the current month. The total annual nutrient return amount of the litter is the total return amount of each component of the litter in 12 months [25,33,37,38].

## *2.5. Data Analysis*

After using the Shapiro–Wilk test to test the normality of the data, one-way ANOVA and LSD were used to compare the difference in the amount of litter in different parts of different years and its components, the content of nutrient elements, and the amount of return. For seasonal dynamic changes, the coefficient of variation was used to characterize

the annual variation of litter volume, which was obtained by dividing the standard error of litter volume in different years by its mean. The annual variation of litter was obtained by dividing the difference between the maximum monthly litter volume and the minimum monthly litter volume by the monthly average litter volume during the observation period. All data statistical analysis and chart production were completed on Excel 2010 and SPSS 25.
