| Global climate change,characterized by warming and precipitation change(including precipitation amount,interval,seasonal distribution,and interannual variation),has become an internationally accepted fact.The influence of precipitation change on the structure,function and process of ecosystem has been a hot topic in global change ecology.The planted Robinia pseudoacacia Linn.forest,as the main vegetation type on the Loess Plateau in China,plays an important role in regulating ecosystem carbon cycle and climate change.The understory vegetation is an important component of Robinia pseudoacacia plantation,which has significant functions such as soil and water conservation,species diversity,contribution to productivity,and promotion of nutrient cycling.At present,it remains unclear how understory vegetation of Robinia pseudoacacia plantation responds to precipitation change on the Loess Plateau.Here,I investigated the effects of precipitation change on soil basic physical and chemical properties,community structure,species diversity,net primary productivity and ecological stoichiometry of the understory vegetation in a planted Robinia pseudoacacia forest on the Loess Plateau via a manipulative throughfall experiment with 9 treatments(80%,60%,40%and 20%throughfall reduction,20%,40%,60%and 80%throughfall addition with control,i.e.,P80-,P60-,P40-,P20-,P20+,P40+,P60+,P80+and CK,respectively).The objective of this research was to reveal the response and adaptation of some key ecological processes and functions of Robinia pseudoacacia plantation to precipitation change,and thus to provide a theoretical basis for the future vegetation construction and ecological restoration of the Loess Plateau.The main results were as follows:(1)The effects of precipitation change on various soil physical and chemical properties were different.The 80%throughfall reduction treatment significantly reduced the soil water content in 0–20 cm,20–40 cm and 40–60 cm soil layers(referring to the average value in April-August,the same below).The 80%throughfall addition treatment significantly increased the soil water content in 0–20 cm and 40–60 cm soil layers.The impact of precipitation treatment on the soil water content decreased with soil depth.The impact of precipitation treatment on the soil p H was not significant.The concentrations of soil carbon,total nitrogen and phosphorus in the study area were 19.2,1.55 and 0.63 g kg-1,respectively.The soil had 2.9±0.43 mg kg-1 available phosphorus on average,accounting for 0.461%of total phosphorus.Just as manipulated throughfall didn't significantly affect the concentrations of soil carbon or nitrogen,so neither did it affect total phosphorus or total organic phosphorus.However,it had a significant effect on soil available phosphorus.(2)The stratification of the shrub and herbaceous layer in the understory of the Robinia pseudoacacia plantation was not obvious.The understory vegetation was mainly composed of Rosaceae,Leguminosae,Compositae,and Gramineae herbaceous species.Precipitation treatments changed the understory species composition and community structure of the Robinia pseudoacacia plantation on the Loess Plateau.Species of different life forms and water ecological types differently responded to precipitation change.Under extreme drought conditions(80%and 60%throughfall reduction),shrubs replaced herbaceous species as the dominant species in the community.Under extreme precipitation conditions(60%and 80%throughfall addition),Compositae species and forbs replaced Gramineae species as the dominant species in the community.Species diversity showed a unimodal trend with the increasing precipitation,with the peak value of 20%throughfall addition treatment,and the Simpson indexes of 60%throughfall reduction and 80%throughfall addition treatments were significantly lower than that in control.Species diversity was positively correlated with the soil water content in the 0–20 cm soil layer.(3)The aboveground net primary productivity(ANPP)of the understory community of Robinia pseudoacacia plantation showed a unimodal trend with the increasing precipitation,with the peak value 230.3 g m-2yr-1(40%throughfall addition treatment in 2016)and 230.3 g m-2yr-1(20%throughfall addition treatment in 2017).Compared to the control,P80-and P60-significantly reduced ANPP by 40.1%and 37.7%(average for 2016-2017).The belowground net primary productivity(BNPP)showed a unimodal trend with the increasing precipitation with the peak value 361.5 g m-2yr-1(20%throughfall addition treatment in 2016)and 340.7 g m-2yr-1(20%throughfall addition treatment in 2017).Compared to the control,P80-and P60-significantly reduced BNPP by 42.2%and 33.0%(average for 2016-2017).ANPP and BNPP were positively correlated with soil water content in 0–20 cm and 20–40 cm soil layers.BNPP decreased with soil depth,and the proportion of 0–20 cm soil layer BNPP in 0–60 cm soil layer decreased with decreasing precipitation.There was no significant difference in the ratio of belowground to aboveground net primary productivity(BNPP/ANPP)among different precipitation treatments.There was a negative correlation between the ratio of BNPP/ANPP and the precipitation after treatments in 2017.The reduced major axis regression analysis showed that there was an isometric growth relationship(exponent b=1.063)between BNPP and ANPP under different precipitation treatments.(4)Both ANPP and BNPP were positively and significantly correlated to various biodiversity indices,suggesting that the projected increase of precipitation in this study region could promote plant diversity that benefits ecosystem productivity.Plant diversity could explain 24.9 and 43.9%of the variation in ANPP and BNPP,respectively.However,the ratio of BNPP/ANPP was not significantly correlated to plant diversity,indicating that the precipitation-induced change in diversity did not exert a strong impact on the carbon alloction between aboveground and belowground.(5)Both soil C/P and N/P were significantly and positively correlated with soil water content.Root C,C/N,and C/P were all significantly and negatively correlated with soil water content.Leaf C,N and N/P of the understory vegetation were all significantly and positively correlated with soil water content.These results suggested that the predicted precipitation in the planted forest on Loess Plateau will likely impact the ecological stoichiometry of soils and plants in this region,thus leading to a shift from nitrogen to phosphorus limitation which might affect the nutrient uptake and utilization efficiency by plants in these ecosystems.The leaf C and N contents had an allometric growth relationship with an allometric growth exponent 0.598.However,other elements in roots and leaves had no allometric growth relationship,suggesting that the underlying mechanisms of various elements in plant organs were not always the same in response to climate change,including precipitation change.In summary,the community structure,species diversity,net primary productivity and ecological stoichiometry of the understory vegetation in the planted Robinia pseudoacacia forest on the Loess Plateau were sensitive to precipitation change,and precipitation increase was beneficial to improve the stability of community structure and carbon sequestration ability.The extremely drought(i.e.,precipitation<159.1mm in April-August)and extreme precipitation(precipitation>494.1mm in April-August)not only significantly reduced species diversity,but also led to the replacement of dominant species.The response of aboveground and belowground productivity to precipitation change was synchronous,and belowground productivity played an important role in the total community productivity.Productivity and species diversity showed a linear positive correlation,and the increase of species diversity would promote the improvement of community productivity.The influence of precipitation change on the ecological stoichiometry varied among different elements,and long-term precipitation change might lead to out of sync of C-N-P cycles(i.e.,decoupling).This study has important implications for vegetation restoration,biodiversity conservation and improving ecosystem stability on the loess Plateau under the scenarios of climate change. |
PDF Full Text Request