The Sensitivity Of Productivity To Extreme Drought In Six Inner Mongolian Grasslands:Patterns And Mechanisms

Natural climate variability is a key determinant of the structure and functions of natural grasslands worldwide.Climate change is currently causing increased variability in precipitation patterns,and statistically unprecedented extreme drought(ED)events are predicted to occur towards the end of this century.An extreme drought event could extremely alter aboveground net primary productivity(ANPP),trigger unprecedented biodiversity loss,and drive shifts in species composition in natural grasslands.The management,conservation,or restoration of grasslands following an ED event requires robust prior knowledge of grasslands' response to ED.Yet,the pattern of ANPP sensitivity(i.e.negative response of ANPP)to any ED event,how an ED event could weaken the temporal stability of grasslands,and what structural mechanism(s)could mitigate ANPP loss under ED remain poorly understood.Here,these knowledge gaps were addressed through a 4-year ED(66%reduction in growing season precipitation,GSP)experiment uniformly conducted across six different arid and semi-arid grasslands spanning a broad precipitation gradient in Northern China.This multi-site experiment with uniform treatments and metrics allowed a comparison of responses and mechanisms across the six grasslands.The following three broad objectives related to the aforementioned knowledge gaps were successively studied in this thesis:1)The sensitivity pattern of different grasslands along spatial precipitation gradient,2)Impacts of ED on temporal stability of ANPP and the relative importance of the underlying mechanisms of stability under ED,and 3)The relative importance of species richness(SR)and species abundance as drivers of ANPP and in mitigating ANPP sensitivity under ED.The first study details investigations on the first objective and data were specifically analyzed to test whether and how dry/xeric sites could exhibit higher sensitivity than the wet/mesic sites or vice versa.A bivariate regression of ANPP on GSP revealed a strong positive GSP-ANPP relationship across the six grasslands under ambient condition.However,the relationship became muted under ED because all the grassland sites,except the wettest(most productive)site,experienced strikingly similar sensitivity.The repeat of the regressions without the wettest site produced similar positive GSP-ANPP relationship slopes for both ambient and drought conditions.The ED-induced reduction of precipitation-use efficiency(PUE)in all sites,except the wettest site,suggests PUE as an underlying mechanism for the observed sensitivity pattern.The study concluded that mesic grasslands maintained higher ANPP than the xeric ones under ED,but the strikingly identical ANPP sensitivity in five out of the six grasslands suggests that precipitation differences may not drive variations in ANPP sensitivity among natural grasslands under ED.In other words,mesic sites may have higher ANPP than xeric sites under ED,but mesic sites may not necessarily experience lesser ANPP sensitivity than the xeric.Next,the second objective of this research was addressed in the second study,and results showed that ED decreased temporal ANPP stability and its underlying mechanisms(species richness,species asynchrony,and species stability)across the six grasslands.Extreme drought decreased species richness for both the two wettest sites and the two driest sites,but not for the two intermediate precipitation sites.Yet,ED only reduced stability of ANPP at the two intermediate sites because they experienced decline in species asynchrony and species stability.Moreover,both bivariate regressions and structural equation modeling consistently indicated that species asynchrony,not species richness or species stability,was the most important mechanism promoting stability of ANPP across the six grasslands,regardless of drought.Further assessment of the relationship of species asynchrony with the components of stability(temporal mean and standard deviation of ANPP)showed asynchrony promoted stability by reducing the temporal variation of ANPP.Thus,the second study concluded that species asynchrony,not species richness or species stability,may be a more general mechanism that promotes stability of ANPP in the face of ED.In the last study,the third objective was addressed by comparing the roles of SR and abundance on ANPP,and in mitigating ANPP sensitivity under ED condition.Under ambient condition,species abundance positively related with ANPP across the six grasslands,but SR only related with the species abundance.Although ED weakened the effect of species abundance on ANPP,species abundance had six-fold stronger positive relationship with ANPP than SR did under ED.It was also found that increase in species abundance,not SR,decreased the sensitivity of ANPP,and species abundance might have mitigated ANPP loss via unaltered abundance of subordinate species and increase in the abundance of transient species.In conclusion,the third study highlighted the importance of species abundance over species richness in promoting productivity,particularly under drought,across the six grasslands.Collectively,the findings of the three studies in this thesis suggest that the grasslands in Northern China could converge in their sensitivity to ED,while high species asynchronous responses to ED and high plant abundance could stabilize or alleviate ANPP loss under ED.Given that the magnitude and duration of the forecasted ED is uncertain,further long-term,multi-site studies that link plant traits to potential mechanisms underlying similar or differential grasslands' sensitivity and stability under ED are recommended.This information would guide the selection of appropriate management measures peculiar to different grasslands under similar ED events.