1 INTRODUCTION
Ecological projects are essential for the adaptation and restoration of
ecosystems in response to environmental changes and human disturbances
(Cai et al., 2015; Dong et al., 2020). Revegetation not only contribute
to the naturalization of the landscape, but also reduce soil erosion,
and increase carbon capture and recreational functions of the territory
(Lasanta et al., 2015). Degradation of alpine meadow atop
Qinghai-Tibetan Plateau (QTP) has been one of great concerns of academy
community, pastoralists, and government officials over past four decades
(Qin, 2014; Dong et al., 2020). One of characteristics of degraded
alpine meadow is that the sedge-dominant vegetation is replaced by
unpalatable or poisonous forb-dominated vegetation (Santonja et al.,
2019). The severely degraded alpine meadows are characterized by
fragmented turf with denudated patches. Many trials have been conducted
to select suitable forages that can be used in the alpine meadow region
and turned out only some gramineous perennial grass can adapt to the
local natural condition (Zhang et al., 2014; Zhang et al., 2017). No
suitable perennial legumes have been selected to apply to the ecological
restoration of degraded meadow (Shang et al., 2017).
Due to the impossibility to restore degraded alpine meadows with native
dominant Kobresia plants, mix-seeding(mixture) gramineous
perennial grass has been regarded as an ideal option for revegetation of
degraded alpine meadow on QTP because it increases the diversity and
stability of the planted community(Ma et al., 2002; Shi et al., 2009),
though the pristine alpine meadow vegetation communities are mainly
composed plant species of Kobresia family, such as Koresia
pygmaea , Koresia humilis etc.( Qiao & Duan, 2016). Planting
perennial grasslands on degraded alpine meadow can not only increase the
land utilization rate and restore the degraded grassland vegetation as
soon as possible, but also ease the grazing pressure on natural
grasslands and prevent grassland degradation and desertification.
The primary grasses being planted for revegetating severely degraded
alpine meadow are limited to few graminoid varieties, such as drooping
wildryegrass (Elymus nutans ), crymophila bluegrass (Poa
crymophila ), Kentucky bluegrass
(Poa pratensis ) and Chinese fescue (Festuca sinensis ).
Among these germplasms, drooping wildryegrass is the major seed material
for ecological restoration and the ratio of seeding rate of drooping
wildryegrass in mixture accounts for 50%. This reflects drooping
wildryegrass’ relative ease of establishment, high forage production and
sufficient seed supply.
However, the established grassland with perennial grasses begin to
decline within 2 to 4 years and causing economic and ecological loss
(Dong et al., 2007). Some sown grassland becomes more severe degraded
land (bare land), challenging the sustainable use of revegetated
grassland in alpine regions (Shang et al., 2006). It is recorded that
most monocropped perennial grasses can be utilized for around ten years
in low altitude areas, but the yield of monoculture of perennial grasses
in plateau area begin to decline 3 years after planting and much shorter
than expected (Dong et al., 2010).
It is undeniable that re-degradation of sown pasture on the plateau may
be caused by a combination of biotic and abiotic stresses. The
interspecies (intraspecies) competition maybe an important cause (Lin et
al., 2018). The authors argue that the decline or re-degradation of
vegetated grassland resulted from interspecies competition between
mix-seeding (co-seeding) plants, and between grasses and unpalatable
forbs. The degradation of drooping wildryegrass monoculture may resulted
from autotoxicity.
Given the fact that a lot of research on plant allelopathy focused on
farmland crops and turfgrasses, and little information about allelopathy
of perennial gramineous forage grass is available, we focus our
attention to allelopathic effects of drooping wildryegrass on seed
germination and seedling growth of mix-seeding plants and itself. We
suppose that the concentration of allelopathicals is below the threshold
value of inhibiting in the early stage of the establishment of community
or monoculture and are conducive to plants that are invaded or
co-planted. With the growth year, the allelopathicals accumulate in the
soils, drooping wildryegrass not only outcompete co-seeding grasses but
also release allelopathicals which enter the soil by leaching and
inhibit the germination and the growth of seedlings of itself. The
competitiveness of Elymus nutans is weakened, which provides
opportunities for invasion of unplatable plants and further accelerates
the degradation of seeded pasture and replacement of unwanted
vegetation.
The objectives of this study were: (1) to evaluate the allelopathical
potential of wildryegrass on highland crops, co-seeding grasses and
wildryegrass itself; and, (2) to reveal the potential cause of sown
grassland degradation and provide reference for sustainable use and
management of revegetated grassland in alpine region.