1 Introduction
During the past 40 years, large areas of primary forests in China have been rapidly logged and converted to secondary and plantation forests due to economic development (Jian et al., 2011). Forest conversion could directly or indirectly affect species distributions and the composition of biological communities (Meiners & Pickett, 1999; Suchomel et al., 2014; Tinya et al., 2021), resulting in significant changes in the structure of forestry biotic communities and associated ecosystem services (Hughes, 2017; Zhang et al., 2020). Primary forests can be climax or subclimax forests, where dominant trees are close to or older than their age at physiological maturity (Hilbert & Wiensczyk, 2007; Morales-Hidalgo et al., 2015). Secondary forests can be understood as a degradation of the primary forests and differ from primary forests in many respects, such as vegetation structure, primary productivity and ecological function (Grau et al., 1997). Plantation forests usually consist of one or several indigenous or exotic tree species that grow rapidly, with a simple vegetation structure and high tree abundance (Felton et al., 2008). Numerous studies have demonstrated that primary forests play an indispensable role in the conservation of birds (Lindenmayer et al., 2015; Manuwal & Huff, 1987), plants (Kosulic et al., 2020), carabids (Elek et al., 2001; Magura et al., 2015), moths (Luque et al., 2007) and small mammals (Umetsu et al., 2007; Bogdziewicz et al., 2014; Rebelo et al, 2019) especially those that are forest-dependent or/and endemic. However, some studies have shown that the diversity of plants (Graham et al., 2019), small mammals (Suchomel et al 2012; Suchomel et al., 2014; Heer et al., 2015), and birds (Zhang et al., 2020) in secondary and plantation forests were equal to or even higher than that in primary forests, especially in heterogeneous landscape regions where secondary and plantation forests are distributed adjacent to primary forests (Bernard et al., 2009; Yang et al., 2015; Zhang et al., 2020).
Small mammals play an important role in food web dynamics (Ernest & Brown, 2001; Wu et al., 2019). Insectivores that prey on invertebrates (e.g., insects) can significantly affect the functioning of forest ecosystems, and herbivorous species (e.g., Tamiops swinhoei ) (including granivores and frugivores) serve as seed dispersers (Urgoiti et al., 2018). On the other hand, they are prey for birds, reptiles and large mammals (Chubbs & Trimper, 1998; Dawson & Bortolotti, 2000; Zhao et al., 2020). In addition, small mammals are easy to investigate and characterized by low dispersal ability and strong ecological adaptability. Consequently, small mammals are usually treated as ecological indicators of environmental change (Carey & Johnson, 1995; Ernest & Brown, 2001), making them ideal models for examining the effects of forest compositional change on biodiversity and community structures (Carey & Johnson, 1995).
Although taxonomic diversity (TD) is the most commonly used metric for assessing the conservation values of different regions, limitations exist in its ability to locate the conservation priority area because it implicitly treats all species equally (Davis et al. 2018). In nature, species perform different ecological functions in an ecosystem and have different degrees of phylogenetic relatedness. Therefore, functional diversity (FD) and phylogenetic diversity (PD), which take into account the functional traits and species-specific evolutionary history respectively, have been suggested as important diversity metrics in conservation biology (Diaz & Cabido, 2001; Diaz et al., 2013). In addition, understanding the processes of community assembly and the underlying mechanisms community assembly are important for the development of conservation rules. With the development of statistical methodology, it has become possible to quantify the relative importance of habitat filtering, interspecific competition and neutral assembly in community assembly based on the phylogenetic and functional information of species (Montano-Centellas et al., 2020; Webb et al., 2002; Zhang et al., 2020).
For small mammals, most previous studies comparing diversity and community structures between different forest types (e.g., primary, secondary, and plantation forests) did not consider the effect of seasonality (Rebelo et al., 2019; Wu et al., 2019). Because resource availability (e.g., leaves, fruits and seeds) and habitat structures (e.g., crown density, herb coverage and shrub coverage; Williams et al., 2010) in a forest vary among seasons, seasonality should be taken as an important covariate in elucidating species diversity pattern and community assembly (Hurlbert & Haskell, 2003). Therefore, integrated studies comparing the TD, FD, PD and community structures of small mammals among seasons are valuable to replenish our understanding how forest conversion affects small mammal diversity and community structures. Unfortunately, such studies are extremely rare (but see Pedersen et al., 2010).
Mt. Liangshan, located in the southwestern part of Sichuan Province, have large areas of primary forests. However, primary forests in Mt. Liangshan were logged heavily and converted to secondary and plantation forests due to the increasing demand for timber during the 1960 s-1970 s. With the establishment of nature reserves and the implementation of policy of prohibiting logging (e.g. the Natural Forest Protection Program), all forests in the nature reserves, protected by law then and therefore, were exposed to little human disturbance. Zhang et al. (2020) assessed the differences in three dimensions of avian diversity (TD, FD and PD) and community structures among the primary, secondary and plantation forests in the Meigu Dafengding Nature Reserve in Mt. Liangshan. They found that TD, FD and PD in the secondary forests were significantly greater than those in the primary forests. In addition, habitat filtering was the dominant process in the primary forests. However, these results cannot be simply extrapolated to other taxa because the taxa in an ecosystem generally differ in their evolutionary histories and ecological service functions.
In this study, using a robust dataset obtained from four-years field survey, we studied small mammal diversity (TD, FD and PD) and community structures in the primary, secondary and plantation forests in Mt. Liangshan. We aimed to answer the following two questions:
Are small mammal taxonomic, functional and phylogenetic diversity affected by forest conversion?
Do the ecological processes structuring small mammal communities differ among the three types of forests?