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).