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.