Study On Restoration Of Saline-Alkali Patchy Degraded Grassland Based On Spatial Expansion Of Dominant Clonal Grass-Leymus Chinensis In Songnen Grassland Of Northeast China

Grassland is the largest terrestrial ecosystem in China.Most of the grassland area is now facing the problem of degradation rapidly due to overgrazing,human activities,grassland reclamation,and unsustainable conservative measures.Hence,it becomes a serious concern to carry out long term planning and widespread management or conservative measures for degraded grasslands.Clonal plant species Leymus chinensis?Trin.?Tzvel.,a dominant clonal grass of Songnen meadow steppe,Northeast China,can expand quickly and provide a large network by producing rhizomes and ramets,which could contribute to the vegetation regeneration of degraded patches under heterogeneous conditions,thus the population expansion has been considered to have high potential utilization for degraded grassland restoration.The biotic factors?e.g.interspecific competition?,abiotic factors?e.g.saline-alkali stress?and common management practices?e.g.mowing,N addition and litter removal?could impact on spatial expansion and growth of L.chinensis.However,whether and how these factors affect the expansion of L.chinensis into saline-alkali soil patches of degraded grassland is largely unknown.In order to comprehensively evaluate the effects of these environmental factors and management practices on the spatial expansion of L.chinensis from healthy soil patches to saline-alkali soil patches,two field experiments and one pot experiment were conducted to examine the effects of mowing,litter removal,interspecific competition,nitrogen application,and intensity of saline-alkali soil stress on the L.chinensis spatial expansion,growth and biomass allocation patterns,and also on the morphological and physiological traits of L.chinensis.The main results were as follows:Results from the field experiment showed that both moderate mowing?40%cut off the plant height?and N addition(10 g N m^-2 y^-1)can greatly promote the expansion of L.chinensis from healthy soil patches to saline-alkali soil patches.Specifically,moderate mowing increased the ramet number and total rhizome length of L.chinensis in saline-alkali patches by 29%and 55%cross the years,respectively,and that N addition increased them by70%and 88%,respectively.L.chinensis developed different strategies to promote the expansion to saline-alkali soil patches in response to mowing and N addition.Mowing stimulated L.chinensis to allocate more biomass into belowground part thereby increased the potential to biomass allocation in saline-alkali patches.However,N addition did not change the biomass allocation of L.chinensis between belowground and aboveground and also between saline-alkali patches and healthy patches,instead increased the total biomass both in saline-alkali and healthy soil patches,i.e.the overall growing ability,thereby promoted the expansion ability to saline-alkali patches.Our study suggested that mowing and N addition may be adopted as the important management measures to help restore the degraded soil patches,and N addition had a stronger effect than mowing management practice.The results from field litter removal experiment showed that litter removal significantly improved the spatial expansion of L.chinensis ramets by increasing the total rhizome length and ramet number in saline-alkali soil patches.Litter removal treatment also increased bud densities and total biomass of ramets in healthy soil patches and saline-alkali patches as well.Our results suggested that long-term enclosure management of patchy degraded grasslands could prohibit the growth and spatial expansion of L.chinensis due to excessive litter accumulation.Our study also further indicated that moderate utilization of grassland such as mowing or grazing may contribute to speed up the restoration of saline-alkali soil patches by reducing the negative effects of litter accumulation.To further explore the effects of interspecific competition and mowing on the spatial expansion of L.chinensis,and its'underlying mechanisms,one pot experiment was conducted.Hierochloe glabra used as competitor specie.Results showed that mowing or competition alone encouraged the spatial expansion of L.chinensis in healthy soil patches towards saline-alkali soil patches with longer rhizomes under moderate saline-alkali soil stress,though their combination treatment showed non-significant effect.In contrast,all of the mowing and competition treatments declined the expansion of L.chinensis into the saline-alkali soil patches producing shorter rhizomes under severe saline-alkali stress.Moreover,mowing or competition alone reduced aboveground biomass of ramets in healthy patches,but increased their above-and below-ground biomass in saline-alkali patches under a moderate stress,indicating that mowing and competition promoted L.chinensis to allocate more biomass to improve expansion abilities.Additionally,mowing treatment also efficiently induced compensatory growth from a physiological perspective through increasing 23%and16%photosynthetic rate than no mowing.In response to mowing and competition,L.chinensis employed diverse adaptive strategies throughout the spatial expansion process.In conclusion,in slightly patchy degraded grasslands,moderate mowing utilization,N addition as well as litter removal can improve L.chinensis spatial expansion to saline-alkali soil patches and increased their ramet recruitment and total biomass production in saline-alkali soil patches,thus help to speed the restoration of patchy degraded grassland in this region,and also L.chinensis community with rich species could have larger potential to restore the degraded patches.Our study indicated that long-term enclosure and grazing prohibition may not be any more effective measure to help restore the patchy degraded grasslands in L.chinensis meadow steppe of northern China.