Response Of Seedling Emergence And Establishment To Extreme Drought Events In Alpine Meadow On The Tibet Plateau

Extreme drought events are one of the important manifestations of climate change,which have profound impacts on the structure and function of ecosystems.Alpine meadow is one of the most widely distributed vegetation types on the Tibetan Plateau,which has important ecological service functions and biodiversity values.Seed germination and seedling establishment are key processes for community renewal and maintenance,and water availability is the main factor affecting seed germination and seedling emergence in arid and semi-arid regions.However,there is still a lack of research on the responses and mechanisms of seed germination and seedling establishment of alpine meadow plants to extreme drought events.In this study,we collected mature seeds of 40 common species from a typical alpine meadow community in the Tibetan Plateau,and used rain shelters to simulate extreme drought events with different durations(2,3,7,8,9 months),with unsheltered plots(receiving natural rainfall)as control treatments.We aimed to investigate and analyze the changes of seed germination rate,survival rate and establishment rate of alpine meadow plants at community level and species level after experiencing different durations of extreme drought events,and to explore the relationship between these changes and plant life history,community dominance,germination peak season and seed size.The main results are as follows:(1)After experiencing extreme drought treatments,the emergence rate,survival rate and establishment rate of species at the community level showed an overall upward trend.There were interspecific differences in the responses of species to extreme drought.Some plants(Elymus nutans,Saussurea japonica,Festuca sinensis,Geranium wilfordii,Tibetia himalaica,etc.)maintained a high emergence rate after experiencing extreme drought events of different durations,while some plants(Galium aparine,Euphrasia regelii,Capsella bursapastoris,Oxytropis ochrocephala,etc.)showed a low emergence rate after experiencing extreme drought events of various lengths.The emergence rates of Pedicularis semitorta,Cerastium fontanum,Euphrasia regelii,Thlaspi arvense,Oxytropis kansuensis,Tibetia himalaica,Festuca rubra,Picris hieracioides,and Astragalus laxmannii did not change significantly after experiencing extreme drought events of different durations.(2)Our results show that: community dominance component is the main factor affecting seedling emergence rate,life history type is the main factor affecting seedling survival rate,extreme drought is the main factor affecting seedling establishment rate.In addition,the effect of life history type on emergence rate and establishment rate was relatively weak,and it changed with the change of community dominance component and emergence peak season.In addition,the effect of emergence peak season on seedling survival rate was also modulated by community dominance component.(3)Significant interacting effects between different plant life history types and extreme drought were seen in terms of seedling emergence and establishment rates(emergence rate P<0.05,establishment rate P<0.05).After experiencing extreme drought events of different durations,perennial plants showed significant increases in seed germination rate,seedling establishment rate and seedling survival rate,while annual and biennial plants only improved their regeneration ability significantly after short-term extreme drought events,but not after long-term ones.(4)There were substantial interacting effects on seedling emergence and establishment rates due to the components of community domination and extreme drought(emergence rate P<0.001,establishment rate P<0.01).Species with different dominance components responded differently to extreme drought events.After experiencing extreme drought events,dominant species showed significant increases in seed germination rate,survival rate and establishment rate;common species had no significant change in seed germination rate,but significant increases in seedling survival rate and establishment rate;rare species had no significant change in seed germination rate and seedling survival rate,but significant increase in seedling establishment rate.(5)Significant interaction effects between the peak emergence season and the severe drought on seedling emergence and establishment rates were observed(emergence rate P<0.001,establishment rate P<0.001).After extreme drought events,the emergence rate of spring peak emergence species did not change significantly,but the survival rate and establishment rate increased significantly.summer peak emergence species had significantly increased survival rate and establishment rate after short-term extreme drought events.autumn peak emergence species had significantly increased emergence rate and establishment rate after long-term extreme drought events.(6)Seed size did not affect survival rate and establishment rate,but showed a positive correlation with emergence rate.However,this relationship changed after experiencing extreme drought events of different durations.Specifically,after the control and experiencing short-term extreme drought for 3 months and long-term drought for 8 months,the larger the seed,the higher the emergence rate.This study showed that: after experiencing extreme drought events,the emergence rate,seedling survival rate and establishment rate of alpine meadow plants increased overall.However,different species responded differently to extreme drought,which was related to plant life history type,community dominance component,and seasonal trait of emergence peak.The interspecific difference in response of alpine meadow plants to extreme drought events should be an important mechanism for extreme drought affecting alpine meadow community structure.