Fine roots play key roles in soil resource acquisition (Johnson et al. 2003; Guo et al. 2008). They are plastic and their functional traits vary widely across plant species and environmental conditions (Guo et al. 2008; Holdaway et al. 2011). The morphology of fine roots is often directly related to physiological functions (Eissenstat 2000; Pregitzer et al. 2002) and the development of an efficient fine root system is crucial for trees to ensure sufficient nutrient uptake in various conditions (Ostonen et al. 2013).
Fine roots are composed of absorptive fine roots (usually referred to the first and second order roots) and transport fine roots (higher orders in the branching hierarchy), with the uptaking capacity of nutrients and water being declined with increasing root order (McCormack et al. 2015). Root tips (first order root) and their mycorrhizal symbioses (the association between the fungi and the roots) are the most metabolically active parts of absorptive fine roots. Their traits are good indicators of the adaptability of root systems to environmental conditions (Ostonen et al. 2017). The changes of root tip traits are expressions of the plastic responses to their environmental conditions (Ostonen et al. 2013; Kong et al. 2014a; Eissenstat et al. 2015; Soudzilovskaia et al. 2015).
Absorptive fine root traits such as diameter, surface area, branching ratio, specific root length (SRL), tissue density and mycorrhizal colonization are crucial for resource acquisition of trees (Eissenstat et al. 2015; Wang et al. 2016; Yan et al. 2019). However, recent studies showed that root traits can be divided into two major dimensions: 1) mycorrhizal colonization which is related to root construction, maintenance and persistence, and 2) branching architecture which is related to root plastic responses to environment (Kong et al. 2014b). Plants may choose to allocate resources to root tips or mycorrhizal fungi in different environmental conditions (Hodge and Storer 2015).
Plant root branching is a critical root trait (Liese et al. 2017) and is paramount for the acquisition of adequate soil water and nutrients (Duque et al. 2019). The plasticity of root systems in terms of the variations in the size, shape and surface area of the roots (Xie and Yu 2003; Motte and Beeckman 2019) was largely associated with root branching (Nibau et al. 2008). The capacity of root branching can improve the adaptation of plants to environmental conditions by anchoring and foraging in the soil (Motte and Beeckman, 2019). Usually, proliferating a predominance of absorptive roots was considered as an extensive fine root foraging strategy, which requires greater C allocation of trees to root formation (Ostonen et al. 2017). Contrary to prolific root branching, mycorrhizal colonization enhances the nutrient absorption of plants through the large surface area of mycelium (Smith and Read 2008), which was considered as an intensive fine root foraging strategy with less investment to fine roots (Ostonen et al. 2017). This inoculation of fungi of roots assists plants in obtaining nitrogen (N), phosphorus (P) and many other nutrients, as well as water from the soil (Clark and Zeto 2000; Jakobsen et al. 2005; Liese et al. 2017; Frew et al. 2018) to promote the growth of plants (Muthukumar and Udaiyan 2006; Kong et al. 2014b).
Due to significant influences to belowground and aboveground, fine root traits related to resource investment and acquisition—branching ratio and the rate of mycorrhizal colonization were considered as two important root traits in association with resources foraging strategies (Kong et al. 2014b; Zemunik et al. 2015; Liese et al. 2017; Chen et al. 2018). Thus it is important to understand the relationship between these two fine root traits and environmental conditions, which is essential to understand belowground resource acquisition of trees. In the past several decades, studies have found that branching architecture and mycorrhizal colonization showed large variations within species growing in different environmental conditions (Johnson et al. 2003; Loudet et al. 2005; Cudlin et al. 2007; Osmont et al. 2007). For example, plants allocate more biomass for root proliferation when growing together in interspecific plants competition (Hodge et al. 1999; Liao et al. 2019), which resulted in the increase of absorptive root (the first and second order roots) biomass (Guo et al. 2004; McCormack and Guo 2014).
Since Chinese fir (Cunninghamia lanceolata) is widely distributed in Subtropical China (N21º41´- N34º03´, E101º30´- E121º53´ and 70-2900m in altitude) (Bian et al. 2014) and is the most important commercial plantation tree species in China, we chose Chinese fir as our study object. According to previous study (Piao and Liu, 2011; Li et al., 2019), Chinese fir is colonized with arbuscular mycorrhizal (AM) fungi and not colonized with ectomycorrhizas (EM). The main research objective of the study was to determine the relationship between these two mechanisms (root branching and root tip AM colonization) and environmental conditions (MAP, MAT, soil C, soil N, soil P, and soil pH) using Chinese fir plantation forests located in Subtropical China. We hypothesized that: 1) environmental variables were the main factors causing the variations of these two mechanisms, and 2) environmental conditions with limited resources (e.g. low soil N and soil P) resulted in the increase of AM colonization, as observed by Jakobsen et al. (2005) and Liu et al. (2015).