Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest

RESEARCH ARTICLE Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest Han Li1, Liya Xu1,3, Fuzhong Wu1,2, Wanqin Yang1,2*, Xiangyin Ni1, Jie He1, Bo Tan1,2, Yi Hu1 1 Long-term Research Station of Alpine Forest Ecosystems, Institute of Ecology & Forestry, Sichuan Agricultural University, Chengdu, 611130, China, 2 Collaborative Innovation Center of Ecological Security in the Upper Reaches of Yangtze River, Chengdu, 611130, China, 3 Sichuan Technology and Business University. International Business School, No. 9 Min River Eastern Section, Meishan City, China * Abstract OPEN ACCESS Citation: Li H, Xu L, Wu F, Yang W, Ni X, He J, et al. (2016) Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest. PLoS ONE 11(2): e0148426. doi:10.1371/journal. pone.0148426 Editor: Wenju Liang, Chinese Academy of Sciences, CHINA Received: October 11, 2015 Accepted: January 18, 2016 Published: February 5, 2016 Copyright: © 2016 Li et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This project was supported by the National Natural Science Foundation of China (31270498, 31570445, 31500509) and the China Postdoctoral Science Foundation Special Funding (2012T50782). Competing Interests: The authors have declared that no competing interests exist. The total phenol content in decomposing litter not only acts as a crucial litter quality indicator, but is also closely related to litter humification due to its tight absorption to clay particles. However, limited attention has been focused on the total phenol dynamics in foliar litter in relation to forest gaps. Here, the foliar litter of six representative tree species was incubated on the forest floor from the gap center to the closed canopy of an alpine Minjiang fir (Abies faxoniana) forest in the upper reaches of the Yangtze River and eastern Tibetan Plateau. The dynamics of total phenol concentration in the incubated litter was measured from November 2012 to October 2014. Over two-year incubation, 78.22% to 94.06% of total phenols were lost from the foliar litter, but 52.08% to 86.41% of this occurred in the first year. Forest gaps accelerated the loss of total phenols in the foliar litter in the winter, although they inhibited the loss of total phenols during the growing season in the first year. In comparison with the effects of forest gaps, the variations of litter quality among different species were much stronger on the dynamics of total phenols in the second year. Overall, the loss of total phenols in the foliar litter was slightly higher in both the canopy gap and the expanded gap than in the gap center and under the closed canopy. The results suggest that the predicted decline in snow cover resulting from winter warming or vanishing gaps caused by forest regeneration will retard the loss of total phenol content in the foliar litter of alpine forest ecosystems, especially in the first decomposition year. Introduction Foliar litter is an important source of nutrients and energy inputs in forest ecosystems, and litter decomposition enriches the soil nutrient pool and supports ecosystem productivity [1,2]. Total phenols, mainly occurring as secondary metabolites, accounts for approximately 25% of the leaf dry mass [3] and includes monopolymer flavonoids, phenolic acids and polymer tannins [4]. These phenols in foliar litter can enter and tightly absorb to clay particles, which results in the formation of humus and an accumulation of organic matter and nutrients in the PLOS ONE | DOI:10.1371/journal.pone.0148426 February 5, 2016 1 / 16 The Total Phenol Dynamics in Decomposing Litter soil [3,5,6]. Total phenols in litter also act as a crucial indicator of changes in litter quality because they can inhibit microbial activity and precipitate protein in litter [7,8]. Therefore, understanding the dynamics of total phenol during litter decomposition can help identify litter decomposition mechanisms in forest ecosystems. However, the dynamics of the total phenols in foliar litter remains poorly understood. In previous studies, rapid losses of the total phenols in foliar litter have been demonstrated in the early decomposition stages after leaf fall, which tends to be closely related to eluviation because of precipitation or snowmelt [9,10]. The changes following leaf fall in the total phenols of litterlargely occur via two main routes: 1) complexation with leaf protein [11] and plant cellwall polysaccharides [12], and 2) biodegradation and oxidation [8,13,14]. Therefore, the total phenol dynamics in foliar litter are susceptible to environmental factors, litter quality and microbial decomposers that vary with the season. With the cold climate of the winter, the seasonal snowpack which is formed throughout the late autumn, winter and early spring, maintains microbial community abundance and activity, which are beneficial for accelerating biodegradation [15,16]. In addition, dramatic leaching caused by the melting of deep snow patches is expected to significantly alter the loss of litter total phenols. However, information on the effects of seasonal snowpack on the loss of total phenol content in decomposing litter is unavailable. As a basic mechanism of forest regeneration and succession, gaps resulting from various forest disturbances are widely distributed in primary forests worldwide [17,18]. Gap formation not only changes the plant diversity of the forest ecosystem inside and outside of the gap [17] but also alters the decomposition environment by redistributing precipitation and light and altering the decomposer community via changes to the substrate quality and hydrothermal dynamics in the soil [19,20]. Based on surface atmospheric conditions, radiation, snowpack, soil thermal state and soil unfrozen water content [21], four critical periods (snow formation period, snow cover period, snow melt period and growing season) can be identified during one year of litter decomposition in regions with seasonal snow cover. With the cold climate of the winter, increasing snow cover depths have been observed from the closed canopy to the gap center because of canopy interception [22,23], which leads to significant differences in hydrothermal and microbial conditions among gap positions. The deep snow patches and long-term coverage at the center of the gap maintain the abundance and activity of the microbial community and accelerate biodegradation during the snow formation and coverage period, as well as leaching during the snow melt period [15,16]. However, because of the effects of canopy shading, a suitable decomposition environment with mild light is maintained, and suf (...truncated)