4.1 Fine root depth increased with RFC
The potential fine root depth (β ) in woody species increased with increasing RFC gradient over 3 years (Fig. 1 and 2). Fine root biomass below a soil depth of 30 cm increased noticeably at high RFC (Fig. 3); these adjustments of below-ground biomass in vertical profiles caused the β to increase along the RFC gradient. A deep root distribution implies that plants occupy increased vertical space in the soil and have a better chance of accessing adequate resources in soils with high RFC (Jackson et al., 2000; Padilla and Pugnaire, 2007; Zhou et al., 2020; Freschet et al., 2021). The coarse texture of the rocky soil resulted in rapid infiltration, which led to deep water infiltration in the soil profile (Schenk and Jackson, 2002, 2005; Fan et al., 2017). Plants typically extend the vertical distribution of roots to follow deep infiltration in coarse soils (Schenk and Jackson, 2002, 2005; Laio et al., 2016; Fan et al., 2017). In addition, the increase in RFC also led to a decrease in water and nutrient levels (Fig. 3 and S6), which encouraged plants to increase root depth and biomass in the soil profiles to ensure resource access (Fig. 6; Jackson et al., 2000; Padilla and Pugnaire, 2007; Li et al., 2020; Zhou et al., 2020).
Across the soil profiles, both the length density and specific length of fine roots increased in all observed species with increasing RFC (Fig. 2 and 7). Increases in length density and specific length of fine roots in soil profiles can promote the exploration range of roots in soil and facilitate root access to limited soil water and nutrients in high RFC environments (Crain and Dybzinski, 2013; Comas and Eissenstat, 2009; Freschet et al., 2018, 2021). Soils with high RFC also showed an overall decrease in fine root diameter (Fig. 2, 7, and S4). This was because soil macroporosity increases with increasing RFC (Huang et al., 2023b), which reduces the soil mechanical resistance (Xu et al., 2012; Gargiulo et al., 2016) and results in finer root systems (Bengough, 2003; Clark et al., 2003). Finer roots are beneficial for root-soil contact and resource acquisition in barren soil with high gravel content (Bengough, 2003; Ma et al., 2018; Freschet et al., 2020).
4.2 Fine root vertical profile varied with growth
An important finding of this study is that with increasing growth, the changes in soil structures have a cumulative decreasing effect on the fine root vertical distribution. Over the years, the variation ranges in fine root depth, biomass, and length density along the RFC gradient showed a decreasing trend in three woody species (Fig. 2), which was consistent with our second hypothesis. In these three woody species, fine root depth and biomass increased with age (Fig. 1 and 2). These results suggest that seedlings are more sensitive to soil heterogeneity than adults, which is consistent with the results of a previous study (Padilla and Pugnaire, 2007). Deepening of the fine roots suggested that older plants were able to access multiple resources and thus adapt to resource constraints with increasing RFC (Schenk and Jackson, 2005; Padilla and Pugnaire, 2007; Li et al., 2020; Zhou et al., 2020).
In addition, soil with 75% RFC aggravated the degree of thinning of fine roots with increasing plant age in all species (Fig. 4). The increase in macropores in soil with 75% RFC reduced mechanical resistance, resulting in finer roots with each passing year (Clark et al., 2002; Bengough, 2003; Gargiulo et al., 2015, 2016). Specific fine root length in the soil profile increased with increasing years of growth, and the largest increase was found under 75% RFC (Fig. 4). In soils with high RFC, poor resources may be an important reason for long roots (Freschet et al., 2015, 2018). Finer and longer roots facilitate older plants to be in contact with the soil and explore larger soil spaces, which is beneficial for obtaining more resources (Crain and Dybzinski, 2013; Chen et al., 2018; Freschet et al., 2020; Lu et al., 2022).