**1. Introduction**

Litter decomposition is a crucial component of carbon and nutrient turnover, and determines the carbon balance in forest ecosystems [1,2]. In the past decades, research into this complex ecological process has grown steadily since the development of the litterbag technique [3,4], which reveals that ambient temperature and moisture influence litter decomposition rate in the early stage [5]. Forest gaps are a main natural regeneration method, caused by pests, wildfires, natural stem breakage, human deforestation and other disturbances, and are widely distributed in forest ecosystems [6], which may induce changes in the microenvironment such as: temperature, precipitation, sunlight exposure, snow coverage and further affecting litter mass loss and carbon release during the decomposition

**Citation:** Li, H.; Du, T.; Chen, Y.; Zhang, Y.; Yang, Y.; Yang, J.; Dong, Q.; Zhang, L.; Wu, Q. Effects of Forest Gaps on *Abies faxoniana* Rehd. Leaf Litter Mass Loss and Carbon Release along an Elevation Gradient in a Subalpine Forest. *Forests* **2022**, *13*, 1201. https://doi.org/10.3390/ f13081201

Academic Editor: Francisco B. Navarro

Received: 17 June 2022 Accepted: 26 July 2022 Published: 29 July 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/).

processes [7]. Previous studies suggested forest gaps affected decomposition rates in a different way, which were documented to be greater, weaker or the same compared to that under closed canopies [8–10]. Furthermore, litter decomposition rates and nutrient release were reported to be stimulated, inhibited and insignificantly related to increasing gap sizes across multiple forests ecosystems [11–13]. Such uncertainties may limit our understanding of how gaps affect decomposition-induced carbon cycling in forest ecosystems.

The altitudinal gradient across a relatively smaller spatial scale could mirror the large-scale environmental conditions along climatic gradients, leading to changes in soil microbial biomass, enzyme activity and other driving factors for decomposition [14–17]. In general, the atmospheric and soil temperatures gradually decrease with increasing elevation, accompanied by deeper seasonal snowpack as well as a longer coverage period in higher elevations [18,19]. However, there were divergent results of the variations in soil biological indicators with an increasing elevation, such as promoted or inhibited enzyme activities and soil microbial quotient [20–22], which may directly affect litter decomposition, implying that the decomposition pattern with increasing elevation is still uncertain. Theoretically, getting away from the canopy interception would help the floor in forest gap obtain more precipitation and sunlight [7,23,24], further favoring the abundant biological community [25,26]. Therefore, the effects of forest gap size on litter decomposition along an elevation gradient need to be further assessed to strengthen our uncertain knowledge of this critical carbon-cycling ecological process.

Litter decomposition has obvious seasonal dynamics, which are more remarkable in high-elevation regions with distinct growing and non-growing seasons driven mainly by soil temperature and moisture [27]. Consequently, the strength of forest gap effects on litter mass loss and carbon release in response to an increasing elevation might be highly seasondependent. The non-growing season tends to be a critical period for the decomposition of newly shed litter [28,29]. Snowpack accumulated in forest gaps offers insulated protection against the outer extremely low temperature and frequent freeze–thaw cycles to maintain preferable microenvironmental conditions for decomposers [30], and this induces intensive leaching losses attributed to the strong snowmelt in early spring. Both functions would promote litter decomposition in winter [31,32]. Moreover, enhanced photodegradation and eluviation from sunlight exposure and heavy rain wash would also stimulate the litter decomposition within gaps when compared to that under closed canopy [33,34]. In subalpine regions, relevant information is scarce, especially when discussed in conjunction with different elevation effects.

Subalpine forests in the eastern Qinghai–Tibetan Plateau contain shallow soil thus making litter decomposition a crucial role in ecological processes, such as maintaining carbon balance in the area [35]. In elevations from 3000 to 3700 m, *Abies faxoniana* Rehd. is the dominant tree species and 84% of forest gaps are less than 240 m<sup>2</sup> in these forests according to our previous investigation [36]. A four-year litterbag decomposition experiment was conducted in an *A. faxoniana* subalpine forest of southwestern China, to assess the potential effects of forest gaps (large: ≈250 m2, middle: ≈125 m2, small: ≈40 m<sup>2</sup> vs. closed canopy) on litter mass loss and carbon release in decomposing *A. faxoniana* litter. More specifically, we evaluated the responses of decomposition for three elevations (3000, 3300, 3600 m) to forest gaps at different critical stages for four years. We hypothesized that (i) the *A. faxoniana* leaf litter mass loss and carbon release were lower under closed canopies than in forest gaps; (ii) the gap effects on the mass loss and carbon release would vary with gap size and elevation, and diminish gradually along with the decomposition; (iii) temperature was the key driver of the gap and elevation induced changes in litter decomposition. The objective of this study was to understand the seasonal effects of forest gap on litter decomposition along elevational differences, which could provide crucial ecological data for further understanding how the opening of gaps would induce changes in carbon cycling in subalpine forest ecosystems.

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

#### *2.1. Site Description and Experimental Design*

The study was conducted at the Long-term Research Station of Alpine Forest Ecosystems (31◦14–31◦19 N, 102◦53–102◦57 E, 2458~4619 m *a.s.l.*), located in Li County, southwestern China. It is a transitional area between the Tibetan Plateau and Sichuan Basin with remarkable changes in climate, vegetation and soil types along the elevation gradient [37,38]. The mean annual temperature and precipitation are 2~4 ◦C and 850 mm, respectively [39]. Winter generally starts from late October to the next April, with seasonal snowpack accumulated on the forest floor of up to approximately 50 cm and frequent soil freeze–thaw cycles [9]. *Abies faxoniana* Rehd., *Betula albo-sinensis* and *Sabina saltuaria* are the dominant trees species, and *Salix paraplesia*, *Rhododendron Lapponicum* and *Fargesia spathacea* are the dominant shrubs. The forest soil is classified as Cambisols [40]. The concentrations of total C, N, and P in soil (0~20 cm) were 363, 6.6 and 1.6 <sup>g</sup>·kg−1, respectively [33].

Based on the previous investigation, forest gaps caused by human deforestation and natural stem breakage were widely distributed in the study area [36]. In September 2011, we selected *A. faxoniana* forests with similar slopes (24◦~34◦) and aspects (NE 38◦~45◦) at 3000 m (31◦19 N, 102◦56 E), 3300 m (31◦17 N, 102◦56 E) and 3600 m (31◦15 N, 102◦53 E)as our three study sites. For the 3000 m site, the dominant species are *Betula albosinensi*, *A. faxoniana*, *Berberis diaphana*, *Fragaria orientalis*, *Thalictrum aquilegifolium*, *Polygonum viviparum* etc., the annual precipitation and temperature are 825 mm and 3.6 ◦C, the forest soil is classified as Cambisols, and the organic layer is 15 ± 2 cm. For the 3300 m site, the dominant species are *A. faxoniana*, *Betula albosinensi*, *Sorbus*, *Hippophae rhamnoides*, *Parasenecio forrestii*, *Fragaria orientalis* etc. the annual precipitation and temperature are 825 mm and 2.8 ◦C, the forest soil is classified as Cambisols, and the organic layer is 12 ± 2 cm. For the 3600 m site, the dominant species are *A. faxoniana*, *Prunus tatsienensis*, *Rhododendron lapponicum*, *Salix paraplesia*, *Fargesia spathacea*, *Fargesia spathacea* etc., the annual precipitation and temperature are 825 mm and 1.6 ◦C, the forest soil is classified as Cambisols, and the organic layer is 12 ± 2 cm. At each site, three plots of each gap size were established: three gap sizes were large gaps ( ≈250 m2), middle gaps ( ≈125 m2) and small gaps ( ≈40 m2), with 9 gap plots in total; another three plots (10 m×10 m) under the ambient closed canopy were set up as the control. These forests gaps have all existed for about 25 years, and all the 36 plots were located apart with over 50 m apart from each other. Other detailed information is shown in Table S1.
