| Changes in precipitation patterns in the global climate change lead to the increase in frequency and intensity of drought.The climate in the warm temperate zone shows seasonal changes,and the water is highly heterogeneous,that is,the water is not evenly distributed in time and space.Under the influence of global climate change,the following characteristic changes exist in the warm temperate zone:(1)the seasonal change being more prominent;(2)the phenomenon of repeated drought-rewatering becoming more serious;(3)the water spatial distribution heterogeneity increasing.Global climate change poses new challenges to the survival and adaptation of plant species in the warm temperate zone.Seasonal changes will cause plant functional traits to make corresponding periodic responses to maintain normal plant metabolism;while water heterogeneity,especially drought events,will restrict plant growth,reduce plant productivity,in severe cases lead to plant die-back and tree mortality,increase the risk of vegetation degradation,and threaten the stability of the ecosystem.Therefore,under the background of global climate change,understanding the response of plant species to seasonal changes,repeated drought-rewatering,and water spatial distribution heterogeneity is conducive to grasping the response mechanism of plant species to the climate in the warm temperate zone,and has important theoretical significance for guiding afforestation selection,vegetation restoration and reconstruction,and accurate improvement of forest quality.In recent years,the related researches have become a hot spot in plant physiological ecology,vegetation ecology,and restoration ecology.But many problems and disputes still need to be solved,such as the lack of research on the seasonal response mechanism of plant hydraulic traits.The interaction of drought stress and planting methods on plant functional traits is inconclusive,and the combined effects of seasonal changes and planting methods on plant water use strategies need to be studied in depth.Based on the above background,this dissertation takes Robinia pseudoacacia L.and Quercus acutissima Carr.,the two most common afforestation trees in the warm temperate deciduous broad-leaved forest in China,as the main research objects,and explores how R.pseudoacacia and Q.acutissima respond to seasonal changes and water heterogeneity in the warm temperate zone under global climate change through the combination of field monitoring experiment,greenhouse control experiment,and field sampling experiment.First,a field monitoring experiment was set up to elucidate seasonal changes of R.pseudoacacia and Q.acutissima,and their associated species Vitex negundo L.var.heterophylla(Franch.)Rehd.and Rhus typhina L.in hydraulic efficiency-safety tradeoff and segmentation and measured the traits of terminal twigs and perennial shoots for two consecutive years,explored the response mechanism in plant hydraulic traits of the plant species to seasonal change.Then,a greenhouse control experiment was set up to study the response of plant functional traits of R.pseudoacacia and Q.acutissima to repeated drought-rewatering under different planting methods,explored the response mechanism in plant functional traits of the plant species to repeated drought-rewatering.Finally,a field sampling experiment was set up to monitor the water sources of R.pseudoacacia and Q.acutissima in different seasons and planting methods,explored seasonal response mechanism of water use strategies of R.pseudoacacia and Q.acutissima under different planting methods to water spatial heterogeneity.In this dissertation,three studies were conducted to explore the effects of the climate in the warm temperate zone on R.pseudoacacia and Q.acutissima,and how R.pseudoacacia and Q.acutissima responded to the changing environment in the warm temperate zone under the influence of global climate change.In the process of research,this dissertation combined the plant hydraulic model with advanced mathematics,and proposed a novel method to calculate the key points on the hydraulic vulnerability curve,which was verified by experiments.The main results are as follows:(1)R.pseudoacacia and Q.acutissima,and their associated species V negundo var.heterophylla and R.typhina responded to seasonal changes in through weak tradeoff and strong segmentation among hydraulic traits.With the change of seasons,plant hydraulic traits fluctuated regularly.However,changing trends of different hydraulic traits differed,which led to the tradeoff strategy or collaborative change among traits.From the perspective of seasonal change,there is an annual ubiquitous weak tradeoff between plant hydraulic efficiency and safety in the same organization.A ubiquitous weak tradeoff may introduce wide and nearly independent variations in the plant hydraulic traits and make only a limited contribution to species divergence.However,the differences of the same hydraulic traits between the terminal twigs and perennial shoots of the four plant species were significant throughout the year.There were strong hydraulic and vulnerability segmentation strategies between terminal twigs and perennial shoots of R.pseudoacacia and Q.acutissima,while there were only a certain degree of hydraulic segmentation strategy and a strong vulnerability segmentation strategy for hydraulic traits between terminal twigs and perennial shoots of V.negundo var.heterophylla and R.typhina.The relative differences in segmentation strategies of the four plant species reflected their different response mechanisms to seasonal changes.Under the influence of global climate change,different response mechanisms may drive niche differentiation and species coexistence.The results showed that the strong segmentation strategy between annual and perennial branches could ensure the normal function of xylem;various tradeoff and segmentation strategies could be adopted between terminal twigs and perennial shoots of different species to respond to the changing environment and seasonal changes in the warm temperate zone.(2)Mixed planting of R.pseudoacacia and Q.acutissima can enhance the ability of synergistic drought resistance and better deal with the uneven temporal distribution of water in the warm temperate zone.Drought had a significant effect on functional traits of R.pseudoacacia and Q.acutissima under different planting methods.It decreased the stem midday water potential,stem-specific hydraulic conductivity,and the net photosynthetic rate of the two tree species,but had a limited effect on the concentration of non-structural carbohydrates in leaves,stems,and roots,and the ratio of soluble sugar concentration to starch concentration in leaves,stems,and roots,and ultimately reduced the plant height,basal diameter,and biomass.Mixed planting can reduce the competition of light,water,space,and other resources,to make the limited resources more reasonable allocation,so as to weaken the inhibition of drought on R.pseudoacacia and Q.acutissima to a certain extent.After rewatering,plant functional traits were recovered.Repeated drought and rewatering did not make plants produce drought fatigue.On the contrary,it made the two tree species form stress imprint,which could better respond to drought when faced with drought stress again.After rewatering,stem midday water potential quickly recovered,photosynthesis of R.pseudoacacia adopted the equal compensatory strategy,and increased its soluble sugar ratio,so that the plant height and basal diameter increased,and the biomass could be accumulated.Photosynthesis of Q.acutissima adopted the over compensatory strategy,so that the biomass could approach or even reach the level without drought.Therefore,the combination of R.pseudoacacia and Q.acutissima and repeated drought and rewatering can improve the overall drought resistance ability.(3)Mixed planting of R.pseudoacacia and Q.acutissima can help R.pseudoacacia use groundwater in relatively dry season to cope with the uneven spatial distribution of water in the warm temperate zone.Through the study of water sources of R.pseudoacacia and Q.acutissima in different seasons and different planting methods,It was found that the water use strategies of Robinia pseudoacacia and Quercus acacia varied with different seasons and planting modes;under the effect of hydraulic lifting and water redistribution,the water sources of R.pseudoacacia and Q.acutissima could be changed,which made it more convenient for R.pseudoacacia to use groundwater and helped it resist the dry spring.Under the influence of global climate change,the warm temperate zone is gradually arid,and the mixed planting of R.pseudoacacia and Q.acutissima can increase the cooperative drought resistance ability of the two species to deal with the spatial water heterogeneity in the warm temperate zone.(4)Based on the data of the research above,combined with the method of advanced mathematics,this chapter reanalyzed the hydraulic vulnerability curve and found that,differential method can calculate three key points on the hydraulic vulnerability curve more accurately.We determined the lethal point,air entry point,and fastest drop point of R.pseudoacacia by the drought-rewatering experiment.We also verified that the three points can be represented by ?l,?e,and ?m,which can be calculated from the DM,respectively.According to the ? values,we divided the HVC into four periods:?,0?? |
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