Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest

RESEARCH ARTICLE Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest Jie He, Wanqin Yang, Han Li, Liya Xu, Xiangyin Ni, Bo Tan, Yeyi Zhao, Fuzhong Wu* Key laboratory of Ecological Forestry Engineering, Long-term Research Station of Alpine Forest Ecosystem, Institute of Ecology and Forestry, Sichuan Agricultural University, Chengdu 611130, China a11111 * Abstract Aims OPEN ACCESS Citation: He J, Yang W, Li H, Xu L, Ni X, Tan B, et al. (2015) Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest. PLoS ONE 10(6): e0131528. doi:10.1371/journal. pone.0131528 Editor: Dusan Gomory, Technical University in Zvolen, SLOVAKIA The release of heavy metals (such as Pb and Cd) from foliar litter play an important role in element cycling in alpine forest ecosystems. Although natural forest gaps could play important roles in the release of heavy metals from foliar litter by affecting the snow cover during the winter and solar irradiation during the growing season, few studies have examined these potential roles. The objectives of this study were to document changes in Pb and Cd dynamics during litter decomposition in the center of gaps and under closed canopies and to investigate the factors that controlled these changes during the winter and growing seasons. Received: February 12, 2015 Accepted: June 3, 2015 Published: June 26, 2015 Copyright: © 2015 He 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: The project was financially supported by the National Natural Science Foundation of China (31170423, 31270498), the National Key Technologies R & D Program of China (2011BAC09B05), the Post-doctoral Foundation of China (2012T50782) and the Sichuan Youth Science & Technology Foundation (2012JQ0008, 2012JQ0059). Methods Senesced foliar litter from six dominant species, including Kangding willow (Salix paraplesia), Masters larch (Larix mastersiana), Mingjiang fir (Abies faxoniana), Alpine azalea (Rhododendron lapponicum), Red birch (Betula albosinensis) and Mourning cypress (Sabina saltuaria), was placed in litterbags and incubated between the gap center and closed canopy conditions in an alpine forest in the eastern region of the Tibetan Plateau. The litterbags were sampled at the snow formation stage, snow coverage stage, snow melt stage and during the growing season. The Pb and Cd concentrations in the sampled foliar litter were determined by acid digestion (HNO3/HClO4). Important findings Over one year of decomposition, Pb accumulation and Cd release from the foliar litter occurred, regardless of the foliar litter species. However, Pb and Cd were both released from the foliar litter during the winter and accumulated during the growing season. Compared with the gap center and the canopy gap edge, the extended gap edge and the closed canopy showed higher Pb and Cd release rates in winter and higher Pb and Cd PLOS ONE | DOI:10.1371/journal.pone.0131528 June 26, 2015 1 / 17 Forest Gaps Inhibit Foliar Litter Pb and Cd Release and Accumulation Competing Interests: The authors have declared that no competing interests exist. accumulation rates during the growing season, respectively. Statistical analyses indicate that the dynamics of Pb were significantly influenced by frequent freeze–thaw cycles in winter and appropriate hydrothermal conditions during the growing season, the dynamics of Cd were strongly influenced by species and the presence of a forest gap at different decomposition stages. These results show that forest gaps could inhibit Pb and Cd release from foliar litter in the alpine forest of western Sichuan. In addition, a decrease in the snow depth in the winter warming scenario would promote the release of Pb during foliar litter decomposition. There exist some difference that may be influenced by litter quality, microenvironment and microtopography during litter decomposition. Introduction The amount of certain elements delivered to the soil by decomposing litter is an important component of sustainable forest management [1]. Among potentially toxic elements, lead (Pb) and cadmium (Cd) have been considered among the top 20 pollutants on the USEPA priority list [2]. These heavy metals are toxic to most soil organisms when present in excessive concentrations[3], and the toxicity to soil organisms has a negative impact on material circulation and energy transformation of alpine forest ecosystem. Canopy gaps are recognized as important determinants of forest regeneration [4, 5], and gap formation by disturbances can alter the snow thickness, light intensity, and other environmental conditions that may influence the temporal and spatial dynamics of Pb and Cd at different scales. In our previous studies of foliar litter decomposition, the seasonal snow cover had a significant impact on mass loss, nutrient dynamics and microbial biomass in the alpine forest [6–8]. Although heavy metal dynamics have frequently been studied in forest ecosystems [9–11], data regarding the dynamics of Pb and Cd during litter decomposition in alpine regions, because of variation in the radiation and snow cover, particularly regarding the effects of forest gaps on Pb and Cd dynamics, remain relatively scarce. Forest gaps affect the Pb and Cd dynamics during litter decomposition differently during the winter and growing seasons. Seasonal snow cover is one typical feature of high latitude/altitude ecosystems. Deep and moderate snow patches occur in the gap centers and at the canopy gap edges, which can provide relatively stable environments in which soil organisms can thrive [12]. In addition, the insulating effects of snow and snowmelt result in moisture-temperature conditions that are favorable for the decomposition of foliar litter. As the temperature increases, snowmelt occurs during the snowmelt stage, which results in Pb and Cd leaching [13]. The aforementioned factors may induce the release of Pb and Cd during litter decomposition. Compared with gap centers and canopy gap edges, the thin and absent snow patches at the extended gap edge and under the closed canopy display more intense freezing and repeated freeze-thaw cycles, which can damage the physical structure of the litter and improve litter decomposition [14, 15]. Previous studies have indicated that litter quality may influence decomposition [16–18]. Freeze-thaw cycles result in significant changes in litter chemistry by affecting litter physical structure and improving litter decomposability [19, 20] and potentially contribute to Pb and Cd release from foliar litter. (...truncated)