In the past two decades, increasing human activity (i.e.,
overgrazing) in the Tibetan Plateau has strongly influenced plant succession
processes, resulting in the degradation of alpine grasslands. Therefore, it
is necessary to diagnose the degree of degradation to enable implementation
of appropriate management for sustainable exploitation and protection of
alpine grasslands. Here, we investigated environmental factors and plant
functional group (PFG) quantity factors during the alpine grassland
succession processes. Principal component analysis (PCA) was used to
identify the parameters indicative of degradation. We divided the entire
degradation process into six stages. PFG types shifted from rhizome bunchgrasses to rhizome plexus and dense-plexus grasses during the degradation
process. Leguminosae and Gramineae plants were replaced by sedges during the
advanced stages of degradation. The PFGs were classified into two reaction
groups: the grazing-sensitive group, containing
Alpine grasslands are one of the most important grassland types on earth, and they are distributed across the tundra zone of northern Eurasia and North America. More than 48 % of alpine grasslands are distributed on the Tibetan Plateau of China (Sun and Zheng, 1998; Wang et al., 1998; Harmsen et al., 2008). Alpine grasslands represent one of the major natural types of pastures for pastoralists living in alpine regions, especially for those living on the Tibetan Plateau, where livestock grazing is the most important human activity (Zhang et al., 2003).
Livestock mainly affects alpine grasslands through two ways. First, their
grazing can affect the structure and composition of plant community, and the
constitution of plant life forms and ecotypes in alpine grasslands (de la
Paix et al., 2013; Zhao et al., 2013; Mekuria and Aynekulu, 2013). Second,
their trampling can reduce infiltration rates, surface sealing, and physical
crust formation (Cerdà and Lavee, 1999; Angassa, 2014). With increased
grazing, a part of alpine grasslands gradually degrade and become bare soil
due to decreased vegetation protection (Zhang et al., 2003a, b; G. X. Wang et al., 2007, Q. L. Wang et al., 2007;
Foggin, 2008). Consequently, this reduces the
role of alpine grasslands in soil and water protection (Wen et al., 2010;
Brandt et al., 2013; You et al., 2014). Such grazing-induced degradation of
alpine grasslands was observed in the early 2000s (Q. L. Wang et al., 1997;
Liu et al., 2008; Wang et al., 2009; Harris, 2010; Lin et al., 2013a, b),
mainly because livestock number increased from approximately 0.8
In the past decade, degradation in alpine grasslands has been getting more and more serious due to increasing grazing density. This has started to affect the living of pastoralists and the development of local economy. How to restore these degraded grasslands and maintain sustainable development of alpine grasslands is a big challenge. An important prerequisite for this is how to diagnose the degree to which alpine grasslands have degraded (Li et al., 2014). So far, numerous studies have separately used plant community (Han et al., 2008; Lin et al., 2013a, b; Angassa, 2014; Giangiacomo, 2014) or environmental indexes (Lin et al., 2010, 2013a, b) as indicators to diagnose grassland degradation (Li et al., 2014; Wang et al., 2015). However, grassland degradation caused by grazing is a very complicated ecological process, including changes in both vegetation and soil. This emphasizes the importance of the plant–soil system for improving degradation of alpine grasslands.
Among the plant–soil system, plants are the link of the atmosphere, biosphere, hydrosphere, and lithosphere (Brevik et al., 2015). The existence of plants can protect the soil surface against kinetic energy of drops, reduces runoff, and increases infiltration (Groen and Woods, 2008). Therefore, the vegetation cover plays a fundamental role in the soil development and soil erosion (Cerdà, 2002; Keesstra et al., 2014), and soil degradation (Ziadat and Taimeh, 2013), and also in the geomorphological (Nanko et al., 2015) and hydrological behavior of the Earth system (Keesstra, 2007; Gabarrón-Galeote et al., 2013) and their interactions with the biota (Araújo et al., 2014; Bochet et al., 2015). At the same time, plants can shape soil microenvironments through living roots (Bardgett, 2002; Puente et al., 2004; Cerdà, 2002; Dai et al., 2013; Keesstra et al., 2014; Shang et al., 2014; Keesstra, 2014; Gabarrón-Galeote et al., 2013) and affect microbial function (Wang et al., 2015; Pereg and McMillan, 2015). In contrast to vegetation, the soil system provides an important carrier for growth of plants and microorganisms. Almost all nutrient transformation processes operate by microorganisms in the soil. Therefore, the analysis on the soil–plant system must be approached from a multidisciplinary strategy (Brevik et al., 2015).
The locations of experimental sites.
To identify the degradation stages of the Tibetan
The experimental sites were located in the flat ground whose slopes are less
than 5
In this study, we investigated 96 plots (100 m
Total vegetation coverage, the percentage coverage of each plant functional
group, and the aboveground/belowground biomass proportion in all plots were
investigated in August 2009. Aboveground biomass was estimated by harvesting
plants from five 0.25 m
Gramineae and sedge are divided into three major plant life forms (PLFs) in
Tibetan
The degradation succession of Tibetan alpine
On the basis of the stated traits, plants were divided into six plant functional groups (PFGs):
Gramineae, other sedges,
All statistical analyses and construction of graphs were performed by the
Canoco 4.5 software package for Windows. Euclidean cluster analysis (ECA)
was used to divide the 96 plots into 6 stages. Live root biomass, dead root
biomass, soil bulk density, and the thickness of mattic epipedon were used
as the environmental factors in the principal component analysis (PCA).
Pearson's correlation coefficient was calculated to identify any
correlations between variables. Arithmetic means with standard errors were
calculated for all of the data. Plant community importance values were based
on the follow equation:
Detailed information about the six degradation successional stages
of alpine
Plant functional groups and their composition or traits.
The succession process of the alpine
The characteristics of the four plant functional groups in a
degradation successional series of Tibetan alpine grasslands:
Living-root biomass (left) and dead-root biomass (right) at 0–10 and 10–20 cm depths.
The values represent the means
The thickness of mattic epipedon over the course of succession. The
values represent the means
As the grassland became increasingly degraded, the importance values of Leguminosae initially increased and then decreased (Fig. 3e). The importance values of Forbs were low during stages I and VI, but were similarly high during all other stages (Fig. 3a–f).
The quantity of both live and dead roots increased during early succession,
and then decreased with increasing grassland degradation. The highest
live-root biomass in the top 10 cm of soil occurred at stage IV (19.4
Live- and dead-root biomass in the 10–20 cm soil layer increased during the early stages of succession, with a steep decrease in the final stage (Fig. 4). Similar live-root biomass was recorded between stages II and III, but was significantly higher at stage IV compared to stages I and VI. The highest dead-root biomass was recorded at stage V (Fig. 4b).
The thickness of the mattic epipedon increased over the first five stages of
succession; however, the mattic epipedon disappeared at the final stage,
because it was destroyed. The greatest thickness of the mattic epipedon
occurred at stage V (18.4
Surface soil-bulk density over the course of succession. The values
represent the means
The space coverage over the course of succession. The values
represent the means
The plant functional groups and environment PCA ordination bioplot.
Black items denote plant functional groups, red items denote environmental
factors. “V weight” denotes the soil bulk density, “space” denotes the
space in community (bared place), “thickness” denotes the thickness of
mattic epipedon, 0–10 L denotes the live roots in the 0–10 cm soil layer,
10–20 L denotes the live roots in the 10–20 cm soil layer, 0–10 d denotes
the dead roots in the 0–10 cm soil layer, 10–20 d denotes the dead roots
the 10–20 cm soil layer, herb denotes the non-leguminous broad-leaved herb
plant functional group, sedge denotes the sedge plant functional group
(excluding
Soil bulk density in the top 10 cm decreased with the succession process,
due to increased root biomass, with the lowest value being recorded at stage IV, and then increased in the final stage, with the highest value of 1.1
Bare-ground coverage in the plant community increased during community succession, showing three states. The first state was in stage I, in which almost all soil was covered (93 % coverage). The second state included stages II and III, with approximately 20 % bare-ground coverage. The third state encompassed stages IV to VI, with approximately 50 % space coverage (Fig. 7).
The principal component analysis of the PFG and environmental factors
matrices showed that two important principal components explained 82.9 %
of the total variance (Fig. 8). The first axis explained 49.1 % of the
total variance, showing a strong positive correlation with
The environmental factors were divided into two new types: (1) the first
environmental axis was related to mattic epipedon characteristics, whereas
(2) the second environmental axis was related to soil bulk weight. The first
PFG was strongly related with the plexus-type plant group. The second
plant functional group was strongly related with the forage-type plant group
(Fig. 8). The thickness of mattic epipedon had a strong positive correlation
with
As
A clear changing pattern in PFG characteristics and environmental factors
during the degradation process (Fig. 8) is mainly caused by a shift from
sensitive to endurable plants in response to grazing pressure. As livestock
number increases in alpine grasslands, dense-plexus plants (
The thickening of the mattic epipedon represents a reciprocal response between the plant community and associated environmental factors during the succession process. As the mattic epipedon thickens, many environmental factors such as the thickness of mattic epipedon, and soil bulk as soil moisture and temperature have been changed, generating positive feedback to overgrazing that has dual effects on alpine grasslands. Initially, increased root biomass enhances water retention and nutrient uptake in the soil (Li et al., 2012). To a certain extent, this action improves the quality of alpine grassland soils. However, increased biomass leads to higher ratios of roots to soil due to high root volume (G. X. Wang et al., 2007, Wang et al., 2008). Subsequently, the number of dead roots increased due to altered environmental factors. The decomposition of these dead roots was not enhanced for two reasons. First, thick mattic epipedon obstructs the air diffusion and water infiltration, decreasing microbial activity and decomposition. Second, low temperature also leads to slow decomposition of dead roots. Consequently, root activity decreases and causes an imbalance among soil nutrients. At this point, the degradation of alpine meadows is inevitable (Cao et al., 2007).
Therefore, alpine meadow degradation involves two processes. The first
process is passive and is driven by overgrazing (Lin et al., 2008; Wang et
al., 2008). The second process is active and initiated when the mattic
epipedon thickens due to the increasing dominance of
However, the mechanisms causing grassland degradation need to be elucidated to fully understand the factors that contribute to this process. Future studies should integrate new tools, such as molecular and isotope approaches, to clarify these mechanisms.
PFG numerical features and root activity, together with certain
physical properties of soil, could be used as indicators of the degree of
degradation in alpine grasslands. The visible properties such as PFGs and
the thickness of mattic epipedon were correlated with invisible properties
such as root activities. Therefore, the degree of degradation of alpine
grasslands can be predicted by development of mattic epipedon and changes in
PFGs. Alpine grasslands are very fragile to grazing and are easily degraded.
Based on our study above, the degree of degradation in alpine grasslands can
be well predicted using relatively few environmental factors. This approach
can save time and easily help pastoralists to efficiently manage their
grasslands.
This work was supported by the Strategic Priority Research Program Climate Change (grant no. XDA05050404), the National Natural Science Foundation of China (grant no. 41030105 and 31270576 and 31500368), and Science and Technology Department of Qinghai Province (grant no. 2013-N-540). Edited by: A. Cerdà