Abstract:
Grassland degradation can affect
plant
eco-physiological properties and
thus influence ecosystem productivity and ecosystem function. However,
how land degradation affects the relationship between plant biomass and
eco-physiological properties of active restoration grasslands and native
grasslands in alpine meadow is less understood. A series of degraded
grasslands (non-degraded, slightly degraded, moderately degraded,
heavily degraded and extremely degraded grasslands) and a series of
active
restoration
grasslands (active restored grasslands with different growth time, 5
years, 9 years, 11 years, 14 years and 17 years) were selected to
investigate the relationship between eco-physiological properties and
aboveground biomass (AGB) of grasses alongside degradation. Results
showed that the AGB, net photosynthetic rate (Pn) and plant nitrogen (N)
concentration decreased significantly with increased levels of
degradation in native grasslands. Plant photosynthetic capacity, plant N
and phosphorus (P) concentration significantly decreased at 9th year or
longer than 9 years of replanting time in active restoration grasslands.
Plant eco-physiological properties in active restoration grasslands
shows stronger association with its AGB than native grasslands. In
native grasslands, degradation affect AGB directly and through Pn
indirectly. In active restoration grasslands, degradation affect AGB
directly and through Pn, plant N and P indirectly. Our results indicated
that through improving plant nutrients to restore degraded active
grasslands may be more effective than to restore degraded native
grasslands.
Key words: Active restoration; Grassland degradation;
Photosynthesis; Plant nitrogen concentration; SEM modeling
Introduction
Alpine grasslands cover more than 60% area of the Qinghai-Tibetan
Plateau (QTP) (Li et al., 2014; Shen et al., 2015; Dong et al., 2020)
and play an integral role as livestock production, biodiversity
maintenance and water conservation in this region (Qiu et al., 2008;
Wang et al., 2009; Miehe et al., 2019; Shen et al., 2021). However, due
to climate change and human activity, these grasslands have been
degraded in recent decades (Shang et al., 2008; Dong et al., 2020; Peng
et al., 2020). This not only influenced local economic development but
also altered the relationship between plant productivity and
eco-physiological properties, leading to ecosystem instability (Andrade
et al., 2015; Guo et al., 2019; Bai et al., 2020). To prevent grasslands
degradation and to restore these degraded grasslands, the active
restoration means has been applied widely in this region (Shang et al.,
2008; Feng et al., 2010; Li et al., 2018; Bai et al., 2020). However,
the active restoration grasslands degraded again and have led to a
continuous undermine soil texture, plant productivity and plant
eco-physiological properties (Shang et al., 2016; Li et al., 2018; Gao
et al., 2019; Shen et al., 2019).
Therefore, understanding the difference effect of land degradation on
the relationship between plant biomass and eco-physiological properties
alongside degradation is important for guiding the rebuilt of degraded
grasslands.
Plant
photosynthetic
capacity is very sensitive to varying environmental conditions and can
reflect ecological strategies of plants to its survival habitat (Skelton
et al., 2017; Wang et al., 2018; Trueba et al., 2019; Shen et al.,
2021). Nitrogen (N) and phosphorus
(P) are important component of photosynthesis pigments and
photosynthetic enzyme that may be affected by grassland degradation, and
thus to impact on plant photosynthetic capacity and biomass accumulation
(Li et al., 2018; Guo et al., 2019; Shen et al., 2021). It has been
suggested that plant
photosynthetic capacity, plant N
and P concentration could be used to identify the level of grassland
degradation (Angassa, 2014;
Giangiacomo et al., 2014). However, litter is known on how
plant eco-physiological properties
(including plant photosynthetic capacity, plant N and P concentration)
can act as an indicator of degradation level, and how the differential
relationship between plant biomass and the eco-physiological properties
of native and active restoration grasslands of alpine grasslands.
Many studies have reported that
grasses in native grasslands typically have higher belowground biomass
and higher foliar N concentrations compared with active restoration
grasslands (Shang et al., 2008, Li et al., 2014, 2018), this leading to
plants with a competitive advantage in native grasslands. However, other
research has also demonstrated that higher soil nutrient availability or
higher degradation level could eliminate this advantage and negatively
impact on photosynthetic activity, especially in N-richer ecosystems or
in heavily degraded grasslands (Xu et al., 2015, 2018; Ye et al., 2018;
Shen et al., 2021). Based on the above background, we hypothesized that:
1) plant eco-physiological properties of grasses would show differential
responses between native grasslands and active restoration grasslands,
and 2) aboveground biomass of grasses in active restoration grasslands
shows stronger association with its eco-physiological properties than
native grasslands alongside degradation, and 3) the different declining
mechanisms of aboveground biomass exist between native grasslands and
active restoration grasslands.
Materials and methods
Study site
The field experiment was carried out in an alpine meadow located the
Dawu village, Maqu county of Qinghai Province, China. The geographic
site is 100º 12´E, 34º
28´N and 4200 m ASL. The mean
annual precipitation and mean annual temperature is 510 mm and -0.6℃,
respectively. The soil is clay soil and the vegetation is alpine
meadow.
We selected a series of degraded grasslands (non-degraded (ND),
slightly-degraded (SLD), moderately degraded (MD), heavily degraded (HD)
and extremely degraded (ED) grasslands) of native grasslands (Guo et
al., 2019; Peng et al., 2020) and a series of active restoration
grasslands (active restored grasslands with different growth time, five
years (5Ys), nine years (9Ys), eleven years (11Ys), fourteen years
(14Ys) and seventeen years (17Ys)). The 17Ys, 14Ys, 11Ys, 9Ys and 5Ys
grasslands site were the severely degraded grasslands and were replanted
with E. mutans in 2000, 2003, 2006, 2008 and 2012, respectively.
Each site area were more than 10000 m2.
Plant photosynthetic capacity measurements
Plant photosynthetic capacity (net
photosynthetic rate (Pn), intercellular CO2concentration (Ci ),
transpiration
rate (Tr ) and stomatal conductance (gs )) of all key plants
of grasses were measured using a portable photosynthesis system
(Li-6400, Lincoln, NE, USA) between 10:00 am-11:00 am in July, 2018. Six
leaves of each grass species at each site were chosen for replication
measurement. Instantaneous water
use efficiency (WUEi )
=Pn/Tr .
Field sampling and laboratory analysis
In early August of 2018, a field survey was conducted and six
sampling quadrats (0.5 m×0.5 m)
were randomly selected at each site. In each quadrat, the coverage and
species richness of community and grasses were recorded and plant
aboveground biomass (AGB) of community and grasses were clipped and
oven-dried at 70℃ to constant
weight to get the AGB. Meanwhile, we collected the aboveground parts of
grasses in each site for nutrient analysis.
Plant nitrogen (N) and
phosphorus (P) concentration were
measured by a flow injection auto-analyzer (FIAstar 5000 Analyzer) (Bao,
2000; Guo et al., 2019).
Statistical analysis
One-way ANOVA was used to test the differences in plants
eco-physiological properties and AGB in native grasslands and active
restoration grasslands, respectively. A multi-comparisons of a least
significant difference (LSD) test in analysis of ANOVA was used to test
the effect of grasslands degradation of native grasslands and the effect
of replanting time in active restoration grasslands of plant AGB,
photosynthetic capacity and nutrient concentration. Principal component
analysis (PCA) was used to assess the relationships between plant AGB
and eco-physiological properties of grasses and two structural equation
model (SEM) were used to identify the direct and indirect pathways that
determined grasses’ AGB in the two types of grasslands.
Results