4.3 Fine root vertical profile varied between species
The vertical distribution of fine roots differed significantly between the four species (Fig. 1–3 and 5). Significantly different from the three woody species, in A. vestita , the fine root biomass and length density were highly concentrated in the surface soil, especially under 50–75% of RFC (Fig. 3, S1, and S2). This result suggests thatA. vestita tends to rapidly acquire transient resources in shallow soils (Jobbágy and Jackson, 2001; Valverdes et al., 2007; Li et al., 2020), such as during occasional rainfall events but might be poor at adapting to long-term resource limitations. The other three woody species had more roots in deep soils (Fig. 1); therefore, they were more resistant to environmental pressure (Schenk and Jackson, 2005; Zhou et al., 2020).
We also found interspecific differences in the degree of variation and vertical profiles of fine roots across RFC gradient and age (Fig. 1–4 and S2–S5). The fine root depth of the three woody species showed an increasing trend with an increase in RFC and age (Fig. 2 and 4). Woody plants deepen root distribution to obtain resources in resource-constrained, highly rocky soil, but this trend became weaker with increasing age (Fig. 2). In contrast, A. vestita showed shallow fine root profiles in soils with high RFC and in older plants (Fig. 1 and 2). The results further confirmed that A. vestitadiffers significantly from woody species; it exhibits an opportunistic resource acquisition strategy but has poor resistance to environmental pressure (Jobbágy and Jackson, 2001; Valverdes et al., 2007; Schenk and Jackson, 2005; Zhou et al., 2020). Thus, the fine root biomass ofA. vestita decreased with age, whereas that of woody species increased with each passing year (Fig. 4).