4.2. The phylogenetic relationship is an underlying factor affecting root system architecture traits
Some functional traits of plants may be influenced by phylogenetic relationships among species, and exhibit a conservatism of phylogeny (Felsenstein, 1985). The closer the genetic relationship, the more similar the traits are (Ackerly and Reich, 1999; Kraft et al., 2010). Root system architecture, as an important functional trait that reflects the resource strategy of underground parts of plants, may also be significantly influenced by phylogenetic relationships among species during their formation and development (Kong et al. 2014; Ma et al. 2018; Zhou et al. 2018). This study examined theK values of phylogenetic signals for six root system architecture traits and found that different root system architecture traits were affected differently by the phylogenetic relationship (Table 2). Among them, significant and weak phylogenetic signals were detected for MRD, indicating that the root diameter of 47 annual ephemerals was affected to the same extent by phylogenetic relationship and environmental factors during their development (Table 2). In addition, in the present study, no significant phylogenetic signals were detected for the other root system architecture traits, which suggests that RD, SRL, SRA, RTD, and TI were not significantly affected by phylogenetic relationship among species (Kramer-Walter et al., 2016).
The results of this study indicate that RD does not exhibit a significant phylogenetic structure, which is consistent with some studies on woody plants (Kramer-Walter et al., 2016; Wang et al., 2019). However, the present result is not similar to the research results of Kong et al. (2014) and Ma et al. (2018) on the root system architecture traits of woody plants, whose research found that phylogenetic relationships have a significant impact on RD. The reasons for these differences may be related to differences in spatial scale, life types of plant, soil conditions, sampling time and methods, and sample size. On the one hand, compared to herbaceous plants, the growth and development of the root systems of woody plants may be subject to stronger directional selection and exhibit strong phylogenetic signals (Valverde‐Barrantes et al., 2017). On the other hand, convergent evolution caused by habitat filtering may cause species with distant genetic relationships to exhibit similar functional traits (Wake, 1991), while divergent evolution caused by biological competition may cause species with closer genetic relationships to exhibit completely opposite functional traits (Blomberg et al., 2003).