In mountainous areas of southwestern China, especially Guizhou province,
continuous, broadly distributed karst landscapes with harsh and fragile
habitats often lead to land degradation. Research indicates that vegetation
located in karst terrains has low aboveground biomass and land degradation
that reduces vegetation biomass, but belowground biomass measurements are rarely
reported. Using the soil pit method, we investigated the root biomass of
karst vegetation in five land cover types: grassland, grass–scrub tussock,
thorn–scrub shrubland, scrub–tree forest, and mixed evergreen and deciduous
forest in Maolan, southern Guizhou province, growing in two different
soil-rich and rock-dominated habitats. The results show that roots in karst
vegetation, especially the coarse roots, and roots in rocky habitats are
mostly distributed in the topsoil layers (89 % on the surface up to 20 cm
depth). The total root biomass in all habitats of all vegetation degradation
periods is 18.77 Mg ha
Karst is a highly special geomorphology mainly consisting of limestone and
dolomite. This landscape is scattered in many areas worldwide but
continuously and widely distributed in southern America, the Mediterranean
coasts, and China (Sweeting, 1972). China has approximately 3.44 million km
The fragile karst habitat is vulnerable to human disturbances in
southwestern (SW) China. Therefore, forests on karst terrain are easily
degraded when the habitat is destroyed by human activity. The bare or
less-vegetation-covered karst terrain usually leads to significant rocky
desertification, a landscape exhibiting sand desertification in arid Central
Asia but covered by big rocks. A total area of ca. 0.11 million km
The unique karst morphology produces unique ecosystems in SW China. In subtropical China where the East Asian monsoon brings sufficient rainfall and warm air, forming rich, acidic yellow soils, the representative zonal vegetation is the evergreen broad-leaved forest (ECVC, 1980). In the karst mountainous region of SW China, the typical yet azonal vegetation is the mixed evergreen and deciduous broad-leaved forest growing in alkaline soils. The limestone and dolomite create very harsh habitats and thin but calcic soils. The rainfall in this region is sufficient for plant growth (ca. 1100 to 1300 cm per year), but the leakage of soil water leads to low species growth, low biomass and productivity of community, and vulnerability of the forest to environmental change. The forest becomes thorn shrubland and dry grassland when degraded, like in other karst regions such as in Spain (Cerdà, 1997) and the Dominican Republic (Izzo et al., 2013). This change is typical outside of the core area of Maolan National Natural Reserve in southern Guizhou province. Studies focus more on changes of community composition and structure in each restoration stage (Yu et al., 2002), but AGB has been rarely measured (Yang and Cheng, 1991; Zhu et al., 1995; Yu et al., 2002) and only one measurement of root biomass (RB) has been reported (Luo et al., 2010).
Roots have important functions in ecosystems worldwide. Roots sustain vegetation, especially forests, and connect the ecological processes of belowground and aboveground through the fluxes of materials and energy (Schenk and Jackson, 2002). RB and productivity are key factors of global and regional carbon cycles (Cairns et al., 1997), especially fine roots (Jackson et al., 1996, 1997; Matamala et al., 2003). Roots can protect soil against erosion. Root architectures (root density, root length density, root diameter, root area ratio, and root distribution) effectively decrease the soil detachment rate, reducing the erodibility of root-permeated saturated top soils in karst mountains (De Baets et al., 2006, 2007a, b).
Roots in karst vegetation, especially in places occupied by rock outcrops,
are usually displaced on rock surface and in gaps among rocks where very
little soils exist. The harsh habitat in karst terrain makes biomass
observations very difficult; only a few biomass measurements in Guizhou
province (in most cases, AGB) have been reported thus far (Yang and Cheng,
1991; Tu and Yang, 1995; Zhu et al., 1995; Yu et al., 2002). A mixed
evergreen and deciduous broad-leaved forest with 95 % rock outcrop in
Maolan of southern Guizhou has an AGB of 89.20 Mg ha
Research shows that in karst habitat, the AGB of mixed evergreen and deciduous forest is lower than that of zonal subtropical evergreen forest because the rocky habitat with less soil impedes plant growth (Zhu et al., 1995). Preliminary research with very limited root samples indicated that karst vegetation has higher RB than non-karst vegetation in the same bioclimatic zone (Luo et al., 2010). We hypothesize that more biomass allocates to the roots in karst vegetation, but more systematically established plots and samples are needed to prove this hypothesis. Therefore, we intensively re-sampled and re-estimated the RB of karst vegetation in Maolan. We considered three aspects of the differences of RB: at various land cover types, at different root diameter classes, and in different edaphic and rocky habitats. The aims of this study are to investigate the RB and its distribution in soil layers in the mountainous karst forest of SW China in order to reveal the roles of RB in specific karst vegetation restoration and in potential carbon stock increment of degraded subtropical China.
The study was conducted in Maolan National Natural Reserve (25
The reserve is a typical peak-clump karst depression landscape (a cluster of small peaks with a common base) in a mountainous area covered by limestone and dolomite bedrock and with an altitude ranging from 430 to 1078 m above sea level (Fig. 1). The exposed rocks are distributed everywhere. The black limestone soil (rendzina in FAO and China's soil taxonomy classifications) is very shallow and discontinuous but rich in nutrients and calcium. However, soil water easily leaks out through the rock lacunas, resulting in a specific drought if sufficient rainfall is absent.
The karst topography, humid and warm monsoon climate, and specific edaphic and rocky microhabitats make the vegetation in this area different from other non-karst subtropical regions. Evergreen trees (accounting for ca. 65 % of total species) mixed with a proportion of deciduous trees (35 %) in the canopy and sub-canopy layers comprise the typical karst forest, a non-zonal soil climax that is widely distributed in subtropical SW China. Abundant biodiversity and rich rare species can be found in this karst forest (Zhang et al., 2012).
Location of study area in the distribution map of karst terrain in China, and the less-human-disturbed karst peak-clump depression landscape and well-protected mixed evergreen and deciduous broad-leaved forest with roots distributed in rock outcrops in Maolan National Natural Reserve, southern Guizhou province.
Key features of 10 plots for root biomass sampling in Maolan National Natural Reserve, Guizhou province, SW China.
DBH: diameter at breast height; BD: basal diameter.
Artificial fire for agriculture and fern picking, goat and cow grazing, and firewood are three major human disturbances in this region, although vegetation within the natural reserve is well protected. The restoration of degraded karst forest can be naturally performed. Such restoration usually passes through five periods, starting from the grassland, moving to grass–scrub tussock, thorn–scrub shrubland, scrub–tree forest, and finally to the mixed evergreen and deciduous broad-leaved forest (Yu et al., 2002).
Five typical vegetation types representing five land cover types mentioned
above were chosen around the Laqiao and Yaogu villages found outside but
close to the core area of the Maolan Reserve. Each vegetation type grows in
two different soil-rich and rock-dominated habitats. Vegetation was
intensively investigated in 10 plots (no replicates for vegetation types),
each with a total area of
The roots were sampled from July to September 2009. Measuring RB is time
consuming and prone to sampling error, especially in rocky soil (Park et
al., 2007). By comparing three different methods of measuring RB (root
coring, soil pit, and allometric equation) in six forest stands, Park et al. (2007) found that if the depth and diameter distribution of roots are
required, soil pits are the only method allowing characterization of root
distribution by depth in rocky soil and can accurately characterize roots up
to about 2 cm. Many roots extend for long distances into rock lacuna where
few soils exist in the karst terrain (Fig. 1). Some roots can be seen in
deep caves, and their lengths are unknown. The huge rock outcrops and steep
slopes (normally 30–45
Root biomass of karst vegetation in Maolan, southern Guizhou province, SW China.
G: grassland; GS: grass–scrub tussock; S: thorn–scrub shrubland; SF: scrub–tree forest; F: mixed evergreen and deciduous forest.
Small rocks were removed from the soil samples, and then the soil samples
were carefully washed, avoiding the loss of fine roots. All washed roots
were air-dried in the room overnight. Roots were classified into three
categories based on the root diameter: fine root (< 2 mm), medium
root (2 to 10 mm), and coarse root (> 10 mm). The roots were
stored in envelopes, oven-dried at 80
The AGB of all vegetation was estimated (Yuan, 2008) using the allometric
functions established by harvesting 30 individuals of 21 standard trees in
the same region (Zhu et al., 1995). Such allometric functions took the
relationships between biomass of each component (leaf, branch, stem, and all
above components) and tree height and diameter at breast height into
account, with the correlation coefficients (
Statistical test is performed using the
Roots in vegetation on karst terrain are significantly distributed in the top soil layers (Fig. 2). The first top layer (surface to 10 cm depth) accounts for 65 % and the second layer (10 to 20 cm) for 24 % roots. Roots in rocky habitats are more distributed in top 20 cm (90.7 to 95 %) than those in earthy habitat (86.8 to 90 %), but this is not significant. Coarse roots occur more slightly (not significantly) in the two top layers (92.5 %) than medium roots (89.2 %) and fine roots (88.8 %). This result is true in both rocky and earthy habitats. However, in rocky habitats, the roots of the three diameter classes in soil depth below 10 cm are all less than those in earthy habitat. No roots in soil depth between 50 and 60 cm in rocky habitat were detected and no coarse roots in soil depth between 40 and 60 cm were observed in both rocky and earthy habitats. The distribution of roots exhibits a similar pattern in different land cover types as the general aforementioned pattern, but all roots in the types of grassland, shrubby grassland, and scrub shrubland are only distributed in soil depth less than 30 cm (< 20 cm in shrubby grassland). The scrub–tree forest has fine and medium roots in all layers from soil surface to 60 cm and coarse roots in soil depths less than 40 cm. The mixed evergreen–deciduous broad-leaved forest only has roots at depths less than 40 cm.
Frequency of root appearance in soil layers. Note that the total number of plots for root biomass sampling is 10 in each vegetation type. A value of 10 indicates that roots occur in this soil layer in all 10 plots. A value of 0 indicates that no roots occurred in this soil layer in all 10 plots. RG: rock-dominated grassland; SG: soil-rich grassland; RGS: rock-dominated grass–scrub tussock; SGS: soil-rich grass–scrub tussock; RS: rock-dominated thorn–scrub shrubland; SS: soil-rich thorn–scrub shrubland; RSF: rock-dominated scrub–tree forest; SSF: soil-rich scrub–tree forest; RF: rock-dominated mixed evergreen and deciduous forest; SF: soil-rich mixed evergreen and deciduous forest.
The total RB in all habitats of all vegetation types is 18.77 Mg ha
Root biomass in different habitats:
Root biomass in different land cover types:
Consistent with the root distribution (Fig. 2), the RB distribution has a similar pattern in different soil layers (Fig. 3). RB mostly concentrates on the surface up to 10 cm soil depth, accounting for 85.6 % of total RB in all habitats. In 10 to 20 cm soil depth, only 12.6 % of total RB is distributed; very few total RB occurs in soil layer > 20 cm. The coarse roots have the highest biomass in each soil layer above 40 cm, following the medium and fine roots (Fig. 3a). This pattern also exists in rocky (Fig. 3b) and earthy habitats (Fig. 3c), except that coarse roots at 20 to 40 cm depth in rocky habitat have less biomass than medium roots (Fig. 3b). The biomass of coarse and medium roots in rocky habitat is higher than that in earthy habitat at soil depth < 10 cm, but the fine roots have slightly less biomass in rocky habitats than in earthy habitats (Fig. 3b and c). At 10 cm to 20 cm soil depth, coarse roots have less biomass in rocky habitats than in earthy habitats, but other roots have the opposite trend. At depth of 20 to 40 cm, all roots (except for fine roots at 30 to 40 cm) in rocky habitat have less biomass than in earthy habitat (Fig. 3b and c).
On average, the total RB at soil surface to 10 cm increases from grassland to forest climax, but the difference between grass–scrub and scrub shrublands is not significant (Fig. 4a). At 10 to 20 cm, the total RB in grassland is higher than in grass–scrub shrubland and slightly higher than in scrub shrubland. At other depths, the scrub–tree forest always has higher total RB than the mixed forest (Fig. 4a). However, roots in different diameter classes have very different biomasses in each land cover type (Fig. 4b–d). Forest climax has the highest RB in any diameter class from the surface up to 20 cm soil depth compared with other land cover types, except for medium root in grassland at 10 to 20 cm depth. Scrub–tree forest, scrub, and grass–scrub shrublands have intermediate RB at each layer, but their trends are changeable for different diameter classes. However, grassland shows higher fine RB at soil depth < 10 cm than scrub–tree forest and shrubland (Fig. 4b). Grassland has higher medium RB at the first soil layer than grass–scrub shrubland and scrub–tree forest, and the highest medium RB at depth of 10 to 20 cm among all types (Fig. 4c). Coarse RB in grassland is the lowest (Fig. 4d). The scrub–tree forest has the highest RB (in all diameters) in other soil layers less than 20 cm (Fig . 4b–d).
Root biomass (RB) and the ratio of RB to aboveground biomass (AGB) of typical evergreen broad-leaved forests in subtropical China.
As mentioned above, accessing to a right place to sample roots in karst terrain is quite difficult because of the very steep slope, sharp and large extended outcrops, and extremely high heterogeneity of soil distribution (very few in some places and often distributed among outcrop gaps). The commonly used root sampling methods, namely root coring and soil pit sampling, have to be modified in rocky soils (Park et al., 2007). The random selection of sampling sites is impossible on karst terrain. A site with general characteristics of the common habitat in a plot has to be subjectively chosen.
Roots on outcrop surfaces are easily cut, but obtaining all of the roots on the surface is difficult because some roots extend to long distances or to very deep soils or both. Roots distributed in very deep soil and rock gaps are very hard to obtain, possibly underestimating the RB in karst vegetation. Distinguishing the root and root sprout of several shrubs and small trees is also difficult. Both roots and root sprouts were harvested in this study, likely resulting in overestimated RB. Furthermore, coarser roots are encountered too rarely to be estimated by the method of soil pits (Park et al., 2007), resulting in an underestimate of coarse roots. In addition, the difficulty of sampling leads to insufficient root plot areas and inadequate duplicates for statistics. Although RB in this region was sampled using a similar method (Luo et al., 2010), the number of root samples was highly limited (only four soil columns in each vegetation type). Thus, this study is the first to comprehensively measure RB in karst vegetation of SW China.
Soils in karst terrain have extraordinary heterogenetic distribution, both vertically and horizontally (e.g., Wang et al., 2007). This distribution is ascribed to rock outcrops that are distributed everywhere and huge rocks that are embedded in deep underground. Soils are very shallow and tiny on the surface ground and unevenly, discontinuously distributed in deep underground. Therefore, root distribution is vertically and horizontally chaotic. Some roots are distributed on the surface ground and rock surface, some in long-distance rock gaps, and some in very deep rock gaps, even chiseling in deep belowground caves. Such heterogeneities of soil and root distributions result in a need of more soil and root samples to represent sufficiently the root distribution and to fulfill the statistical requirements. Ten soil pits with highly limited volume are insufficient, i.e., the RB in Maolan in this study is possibly underestimated. A greater number and larger-sized supplementary soil pits obtained via sampling using soil drilling equipment are needed. A better way is to set up grid points in 2–5 m intervals among grids.
The peak-clump depression is only one of the eight karst morphological types in SW China. The RB measurement is only a representative of vegetation in this kind of karst landscape. Other karst morphological types, such as the plateau surface type in central Guizhou province, the gorge type in the west, the trough valley type in the north, the fault basin in northeastern Yunnan province, and the peak-forest and plain type in northeastern Guangxi autonomous region, all need their own RB measurements to characterize their belowground features of local and regional vegetation.
The globally averaged root distribution for temperate and tropical forest ecosystems was approximately 60 % of roots in the top 30 cm, estimated based on a soil depth of 2 m (Jackson et al., 1996). However, in karst ecosystems the soil depth is mostly 50–60 cm only and ca. 95 % of roots are distributed in the top 30 cm. A key reason is that most of roots, especially the coarse roots, must be allocated in the top soils to support the shoot to be firmly fixed in the harsh karst habitat. Similarly, ca. 98 % of RB is in the top 20 cm soils in karst ecosystem, whereas ca. 50–70 % of RB is allocated in the top 30 cm soils in world's temperate and tropical forests (Jackson et al., 1996).
Studies of mixed evergreen–deciduous forests at different habitats in Maolan (Yang and Cheng, 1991; Zhu et al., 1995) and of scrubs in central Guizhou province (Tu and Yang, 1995) show that karst vegetation has lower AGB than other typical evergreen broad-leaved forests in subtropical China. A preliminary study indicated that RB in karst vegetation is not lower than that of either broad-leaved or needle-leaved forests in subtropical China (Luo et al., 2010). The present study with more number of root samples further confirmed this finding, showing that our hypothesis of more biomass in karst vegetation being allocated to roots is true. However, it is not the RB in karst forest always significantly greater than that of typical evergreen forests, whereas it is the ratio of RB to AGB.
The RB of mixed evergreen and deciduous broad-leaved forest in Maolan is
only greater than half of measurements of typical evergreen broad-leaved
forests in subtropical regions (Table 3). The average RB in eastern (49.09 Mg ha
The harsh karst habitat is fragile. The root is the only physical support of woody plants in the tiny soil, rocky, and heterogenetic habitat and is the only tissue that can absorb deep underground (river) water. The harsh karst landscape makes the degraded forest difficult to be restored to its original condition. After the disturbances of firewood cutting, fire, and grazing in Maolan area, karst vegetation can be more rapidly restored to scrub shrubland and woodland in 5 to 10 years because of its more rainfall and soils and less interference of human activities. However, in Guanling area of central Guizhou province, the cutting of the original karst forest in the late 1950s completely modified the local habitat, making the restoration of the original vegetation impossible for at least 50 years. This effect is ascribed to the extremely strong human–land contradiction in the plateau surface region of central Guizhou province. Before the 1950s, the karst region of the entire Guizhou province was mostly covered by forests, but in the 1980s to the 1990s, rocky desertification accelerated after karst forest degradation and heavy soil erosion (Zhang et al., 2006). In the 1990s to the 2000s, large-area and high-intensity afforestation slowly minimized rocky desertification, but complete control of the rocky desertification and improvement of local economy of the poor areas are still very difficult tasks. Therefore, an urgent need exists to protect the current karst forests, to restore the degraded vegetation, and to improve the quality of life of locals.
The area of scrub land (ca.
Root biomass of vegetation is reduced in different land cover types from forest to scrub shrubland and tussock grassland. The ratio of root to aboveground biomass in karst forest is greater than that of evergreen forests in the subtropical China. This confirmed that plants growing in tiny, shallow soils and rocky habitat of karst terrain tend to allocate more resources to roots in order to maintain and physically support the ecosystem. However, due to the extraordinary heterogenetic distributions of soils and roots in karst terrain and the difficulty in access to every root in deep underground, root biomass might be underestimated in this study. More and larger-area soil and root samples are needed in future study.
The study was supported by the National Basic Research Program of China (2013CB956704), the Hundred Talents Program of the Chinese Academy of Sciences (2011031), and the Science and Technique Foundation of Guizhou province (GKH-J-2012-2332). We thank Daigui Zhang, Jiedong Zhu, Jingcheng Ran, Luming Wei, Huanbai Xia, and Weilian Yu for their support in the field studies. Edited by: A. Cerdà