1 Introduction
The dimensions of rooting systems, indicated by root depth and lateral
extent, represent the amount of soil space occupied, which determines
the uptake of resources by fine roots
(Schenk and
Jackson, 2002;
Zhou et al., 2020; Freschet et
al., 2021). Adjustment of root distribution in soil profiles is crucial
for plant resource acquisition and can thus affect plant resilience to
environmental stress (Fan et al., 2017; Zhou et al., 2020). For example,
deep roots are a key strategy for xerophytes to survive in arid
ecosystems (Schenk and Jackson, 2002; Zhou et al., 2020; Fan et al.,
2017). Numerous studies have found that fine roots of woody plants are
concentrated in shallow soil (Huang et al., 2008; Zewdie et al., 2008;
Gwenzi et al., 2011; Gao et al., 2020;
Li et al., 2020; Yang et al.,
2021), whereas deep roots occur in seasonally dry areas (Schenk and
Jackson, 2005). The fine root vertical pattern of many plants exhibits
high plasticity under soil heterogeneity (Padilla and Pugnaire, 2007; Li
et al., 2020; Luo et al., 2021). Therefore, understanding the pattern of
root distribution across environmental gradients and plant ages may
provide insight into the
adaptive
processes of plant under environmental stress.
Soil heterogeneity is caused by the patchy distribution of coarse
particles and nutrients in soil
profiles (Huang et al., 2023a, 2023b). Coarse parts of soil (particle
size > 2 mm), usually called rock fragments, are widespread
in terrestrial ecosystems and their content plays an important role in
soil hydrological conditions such as infiltration (Poesen and Lavee,
1994; van Wesemael et al., 2000; Zhou et al., 2009;
Zhang et al., 2011; Zhang et al.,
2016). Many soils contain rock fragments as a result of processes of
soil genesis and human activities such as tillage and resource
extraction (Hu et al., 2021). Rock fragment content (RFC) is critical
for other soil properties, such as soil physical structure (Xu et al.,
2012; Gargiulo et al., 2015,
2016), water and nutrient availability (Rytter, 2012; Qin et al., 2015;
Zhang et al., 2016; Ceacero et al., 2020), and microbial composition
(Certini et al., 2004; Hong et al., 2021; Huang et al., 2023a). In soils
with high RFC (Rytter, 2012; Ceacero et al., 2020; Huang et al., 2023b),
growth and biomass accumulation decreases in plants
(Mi et al., 2016; Hu et al.,
2021), but the allocation ratio of the below-ground parts increases (Hu
et al., 2021). The rocky soil, water stress, and infertile environment
limits agroforest development of the arid area. However, little is known
about the vertical distribution pattern of fine root profiles in
response to changes in RFC.
Plants adjust the soil space colonized by roots to obtain sufficient
resources to cope with variations in the soil particle composition
(Schenk and Jackson, 2002, 2005; Bengough, 2003). A strong sensitivity
of rooting depth to local soil hydrological conditions has been
identified (Schenk and Jackson, 2005; Fan et al., 2017; Zhou et al.,
2020), whereby roots remain shallow in waterlogged land, but usually
deepen in arid regions (Schenk and Jackson, 2002; Fan et al., 2017).
Soil hydrology is determined by soil texture (Sperry and Hacke, 2002;
Fan et al., 2017); coarse-grained (sandy or gravel) soils with low
water-holding capacity allow deep infiltration profiles that encourage
deep roots (Schenk and Jackson, 2005; Fan et al., 2017; Zhou et al.,
2020). The coarse particles present in the soil also increase macropores
and reduce mechanical resistance (Xu et al., 2012; Gargiulo et al.,
2015, 2016), which is conducive to the penetration of the root system
into the deep layer (Clark et al., 2002;
Bengough, 2003). However, whether
the rooting depth increases when the content of coarse soil particles
increases and water and nutrient levels decrease remains to be explored.
The vertical profile of fine roots is also regulated by genotype,
presenting as an inherent characteristic of species with wide
interspecific variation (Peek et al., 2005; Gao et al., 2020; Li et al.,
2020; Zhou et al., 2020; Luo et al., 2021). Simultaneously, stand age
determines the spatial distribution of roots (Peek et al., 2005; Zhang
et al., 2018). Previous studies have focused on the seasonal dynamics of
fine root distribution in the short term, mainly explaining the rooting
strategies of plants in response to the dry-wet season (Cheng et al.,
2002; O’Grady et al., 2005; Wang et al., 2016). These observations have
not yet provided a deep understanding of the dynamic patterns in fine
root distribution and resource acquisition of woody plants across
different ages and their time cumulative responses to soil
heterogeneity.
In the present study, we observed the fine root vertical profiles of
four xerophytic species along an
RFC gradient (0%, 25%, 50%, and 75%, v v-1) and
during three years of growth (the second, third, and fourth years) in
the arid valley environment of western China (Hu et al., 2021). These
four species are important native species in the arid valleys of the
Hengduan Mountain region, with irreplaceable ecological value in
vegetation restoration and soil and water conservation
(Li et al., 2008, 2009; Wu et al.,
2008; Bao et al., 2012). We examined fine root biomass and morphology
across the soil layers along the RFC gradient for three years of growth.
Our objectives were to investigate: 1) how the vertical pattern of fine
root depth and traits varied with the RFC gradient, 2) the temporal
pattern of fine root depth and traits along soil profiles and RFC, and
3) interspecific differences in the dynamic response of fine root depth
and vertical distribution to the RFC gradient. We hypothesized that: 1)
root distribution deepened with the increase in RFC, based on the
characteristics of poor water-holding capacity and low strength of
coarse soils (Bengough, 2003; Schenk and Jackson, 2002, 2005; Fan et
al., 2017); 2) with the increase in age, the response of root
distribution to changes in RFC showed a cumulative decreasing effect of
time, because younger plants (seedlings) are more sensitive to soil
heterogeneity than older ones (Padilla and Pugnaire, 2007).