| Continued global warming has enhanced the atmospheric water holding capacity,which in turn led to dramatic changes in rainfall patterns.As one of the largest ecosystems on earth,grasslands were exceptionally sensitive to rainfall pattern changes.Therefore,understanding response mechanisms of grassland community structure,function and its dominant species to rainfall changes can help to reveal the impact of climate changes on grassland ecological and production service functions.Based on the field experiments carried out in Songnen Grassland in northeast China during 2016 to 2019,we explored the response of species diversity,productivity and its stability,community-level plant traits,and functional diversity to rainfall amount changes.Microcosmic experiment was conducted to investigate the effects of rainfall distribution changes on dominant species(Leymus chinensis)growth.We reported the main results and conclusions as follows:(1)Reduced rainfall significantly enhenced species diversity and exhibited obvious time-accumulation effect(species richness,Shannon-Wiener index,Simpson index and Pielou index increased by 29%,67%,40% and 42%,respectively),while no significant change under increased rainfall treatment.This result was contrary to the "physiological tolerance" theory.Drought weakened the competive advantage of dominant species,but promoted the growth of companion species,finally increased the species diversity.In contrast,increased rainfall enhenced the competitive ability of dominant species and reduced the relative abundance of annual grasses and other forbs,resulting in a slight decrease in species diversity.(2)Reduced and increased rainfall regulated grassland productivity and its stability through different mechanisms,respectively.Although the reduced rainfall suppressed the growth of the dominant perennial grasses by 58%,the annual grasses and forbs replenished the ecological niches(increased 173% and 98% respectively),which ultimately led to no significant change in above-ground productivity.Increased rainfall raised aboveground productivity by remarkably improving the dominant functional group biomass.Grassland maintains productivity stability via species asynchrony and dominant functional group stability under reduced and increased rainfall treatment,respectively.(3)The same amount of rainfall was regulated,however the mechanisms of variation in community level plant traits were different.Reduced rainfall showed a greater impact for community level plant traits than increased rainfall.The former was largely dependent on species turnover(the contributions of species turnover to community level specific leaf area,leaf drought matter content,leaf nitrogen content and leaf carbon content were 43%,81%,51%,99% respectively),while the latter relied on intraspecific variation.With the continuation of the rainfall reduction treatment,the contribution of species turnover to community level traits variation increased gradually.(4)Reduced rainfall treatment triggered "competitive release" between different species,increased species evenness.This result enhanced the diversity of plant functional traits,ultimately improved functional diversity.However,the positive correlation between species diversity and functional diversity disappeared and "functional redundancy" phenomenon emerged at the later stage of the rainfall reduction treatment.Because the trait values of species within the community were restricted to a relatively narrow range of adaptation to drought stress.Long-term rainfall increased treatment slightly enhanced functional diversity through inducing interspecific trait-divergence assembly within community.(5)Leaf and root of L.chinensis adopted different adaptation strategies to cope with drought occurring in different growing season periods.Leaf morphological trait employed water retention strategy(reduced specific leaf area)to cope with drought,but not for leaf mass fraction.Root morphological traits and mass fraction employed water absorption strategy(increased specific root length,specific root area,and root mass fraction).Early drought treatment caused root compensatory growth,resulting in maximum root morphological traits and root mass fraction values.The plant organ biomass allocation and morphology contributed differently to the above-and belowground resource absorption capacity under different drought treatments.Overall,the contribution of organ mass fraction was higher than morphological variation for resource absorption capacity(leaf mass fraction contribution ranged from 68-84% for leaf area ratio;root mass fraction contribution ranged from 65-75% for root longth ratio,and 31-62% for root area ratio).Based on the above findings,our study obtained the influence mechanisms of rainfall pattern varations on the grassland community structural and functional diversity,and the dominant species survival strategies.Reduced rainfall significantly increased species diversity but had no significant influence on above-ground productivity;increased rainfall markedly enhanced above-ground productivity,but had a weak effect on community composition.Productivity stability was maintained by species asynchrony under reduced rainfall;while by dominant functional group stability under increased rainfall.A positive correlation between functional diversity and species diversity was observed at the beginning of reduced rainfall,but persistent drought led to functional redundancy;increased rainfall caused interspecific traitdivergence for co-occurring species,resulting in a weak increase in functional diversity.Dominant species(L.chinensis)leaf and root adopted different adaptation strategies to cope with drought occurring in different periods,overall organ biomass allocation was more important for above-and below-ground resource uptake than morphological traits.This study not only deepened the comprehensive understanding about the impact of rainfall pattern varations on grassland ecosystems,but also provided a theoretical basis for predicting grassland ecosystem processes and functions under future climate change. |
PDF Full Text Request