| Global climate change brought climate warming and intensified droughts in local areas,which increased the frequency of insect pest outbreaks.The forest insect pest outbreaks and disturbance from human activities would cause the loss of leaves,which would bring a series of adverse effects on the ecosystem.Under the co-disturbance of climate change and human activities,the growth rate of forest renewal layer seedlings has slowed down,the mortality rate has increased,the natural regeneration of forests has been hindered,and the structure and function of ecosystems have been severely affected.The regeneration of woody plant seedlings is an important process to maintain the biodiversity of forest ecosystems,and plays a central role in the dynamic evolution of forests.In different stages of forest development,plants will encounter varying degrees of insect pest interference.In the process of plant community succession and vegetation restoration,the seedling stage is the most vulnerable period of growth and most sensitive to environmental changes.In nature,the growth of plant seedlings is often affected by abiotic factors such as light and moisture,as well as biotic factors such as insect feeding and human disturbance.The combined effects of biotic and abiotic factors on plant individuals and the recovery process of plant individuals after leaf damage are the current hotspots in plant physiology and ecology.Studying the response mechanisms of plant seedlings to complex environments such as defoliation,drought and shading,and understanding the response differences of various species,can better protect the regenerative layer seedlings and predict the changes of vegetation distribution under future climatic conditions,as well as provide theoretical basis for the selection of species in vegetation restoration.Based on the above background,deciduous woody plants in the warm temperate zone,Robinia pseudoacacia,Amorpha fruticosa,Quercus acutissima and Quercus rubra,were studied as the research objects.Simulation of leaf damage by artificial defoliation.The main focus of our research project was the recovery mechanism of plant individuals after defoliation.At the same time,it combined with the intensified drought caused by climate change and the multiple light environments faced by woody plant seedlings during the regeneration process.We set up a greenhouse control experiment.By measuring the plant growth index,leaf morphological and anatomical traits,and leaf gas exchange parameters,stem hydraulic traits,non-structural carbohydrate concentration and other functional traits,we comprehensively explore the response and recovery mechanisms of seedlings to defoliation treatment in different environments.The main results are as follows:In order to study the effect of different defoliation modalities on seedling growth and physiological traits,this experiment selected two Fabaceae woody species,R.pseudoacacia and A.fruticosa,and performed different defoliation treatments:no defoliation,defoliation of top down 50%of the crown height,defoliation of the bottom up 50%of the crown height.The results showed that the growth and physiological traits of seedlings could quickly recover after moderate defoliation.No matter what method of defoliation was applied,the growth indicators of the two species recovered to the control level after 60 days.In the early stage of recovery(days 1-14),defoliation treatment had a significant effect on the carbon allocation of R.pseudoacacia and A.fruticosa seedlings,but the carbon allocation of two plant species basically recovered in the later stage.However,after 60 days of treatment,there were still significant differences in leaf traits in response to different defoliation treatments:the specific leaf area of the lower defoliation treatment was significantly lower than control group,but there was no significant difference in the specific leaf area of the upper defoliation treatment compared with the control group of the two species.After removing the leaves,both R.pseudoacacia and A.fruticosa seedlings adopted positive recovery strategies such as reflushing new leaves and quickly mobilization of carbon reserve.However,R.pseudoacacia responded to defoliation faster and had a shorter recovery time than A.fruticosa.Thus,R.pseudoacacia is more suitable for vegetation restoration than Amorpha fruticosa in areas with frequent insect pests.Based on the above research results,this experiment took R.pseudoacacia and A.fruticosa as the research objects and applied different light treatments on the basis of moderate defoliation,and further studied the response mechanisms of regenerative layer seedlings to leaf damage under different light environments.Two light levels(high light condition and low light condition)and two defoliation levels(no defoliation and defoliation)were set.The experiment was lasted for 70 days and was harvested in four times to determine plant growth index and biomass allocation,leaf gas exchange parameters,specific leaf area,leaf thickness,carbohydrate concentration and other plant traits.The results showed that the defoliation treatment under shading conditions would significantly inhibit the growth of R.pseudoacacia and A.fruticosa seedlings due to insufficient carbohydrate supply.Compared with the control group,shading treatment significantly reduced the leaf thickness and lignin concentration of the two species,but the specific leaf area and total chlorophyll concentration of the shading treatment increased significantly,indicating that under low light conditions,the two species would reduce the leaf structure investment and increase the light capture resources investment,which reflected the resource trade-off strategy between plant functional traits in low light environments.During the experimental treatment,the response sequence of plant traits to light and defoliation was different,and the sensitivity of plant growth to stress was higher than that of photosynthesis,which lead to the accumulation of nonstructural carbohydrate concentration in the early stage of the experiment.In order to study the response mechanism of plants to leaf damage under different soil moisture conditions,this experiment took Q.acutissima and Q.rubra seedlings as the research objects and applied different water treatments on the basis of moderate defoliation.Two water treatment levels(normal irrigation and drought)and two defoliation levels(no defoliation and defoliation)were set.The experimental treatment lasted for 60 days,and the harvest was divided into short term(10 days)and long term(60 days).By measuring the plant growth index and biomass allocation,leaf gas exchange parameters,stem hydraulic characteristics,and nonstructural carbohydrate concentration.The results showed that:defoliation treatment accelerated the consumption of carbon reserve by drought stress and inhibited the growth of Q.acutissima and Q.rubra seedlings.Q.acutissima maintained high hydraulic safety by reducing the stem water potential and stem specific conductivity in the early stage of drought.Moreover,the stem starch concentration was higher than the control treatment after 60 days of defoliation treatment,but the stem biomass was lower than the control in Q.acutissima.The results showed that after defoliation,Q.acutissima adopt a relatively conservative resource acquisition strategy,and more resources are used for storage rather than growth,which would help the seedlings to resist recurrent insect pests.Q.rubra adopt a relatively active resource acquisition strategy during early period after defoliation and drought,and its growth was not affected by defoliation and drought treatment in the early duration.Q.rubra maintained normal growth by increasing the net photosynthetic rate of the remaining leaves,reducing stem water potential,and changing carbon allocation patterns.At last,in order to study the importance of carbon reserve for plants under defoliation and drought conditions,this experiment took different sizes of R.pseudoacacia and A.fruticosa seedlings as the research objects,and applied different water and defoliation treatments.Two plant sizes(1.5-months-old and 3-months-old),two water levels(normal irrigation and drought)and two defoliation levels(no defoliation and defoliation)were set.The results showed that defoliation could alleviate the effect of drought in seedlings and seedlings with large carbon reserve were more effective in the compensatory mechanism of defoliation and drought treatment.Under the drought treatment,the stem water potential of the large R.pseudoacacia seedlings was significantly increased by 24%compared with the treatment without leaves.Compared with the high-water treatment,the stem starch concentration of the large R.pseudoacacia seedlings under drought treatment was significantly reduced by 26%to keep the soluble sugar concentration in the stems stable.The concentration of soluble sugar in the roots of A.fruticosa seedlings increased with the aggravation of drought,while the concentration of leaf soluble sugars decreased,indicating that there was a trade-off between the carbon allocation of roots and leaves under drought treatment.It is inferred that in the context of global climate change,plant individuals with large carbon reserves have stronger resistance to insect pests and human disturbance,which is related to their complex compensatory and trade-off mechanisms.In general,insect pests,human disturbance,shading,and drought all limited the growth of seedlings and the recovery of physiological indicators,but different species had different response strategies to stress.R.pseudoacacia and A.fruticosa are typical fast-growing restoration species,and their growth and physiological indicators could quickly recover after defoliation.However,the growth rate of Q.acutissima and Q.rubra was slower,and the growth could not recover in a short time after defoliation.Under different water and defoliation treatments,we found that defoliation could relieve the effect of drought on hydraulic parameters in R.pseudoacacia seedlings,which would be beneficial to protect plants and reduce xylem embolism.In Q.rubra seedlings,it was found that the defoliation treatment would accelerate the consumption of carbon reserve under drought.In addition,moderate defoliation had little effect on the concentration of non-structural carbohydrates of R.pseudoacacia,while the severe carbon limitation caused by the defoliation treatment under shading conditions,and significantly reduced the carbon reserve and growth of R.pseudoacacia.The size of carbon reserve also played an important role in the response of R.pseudoacacia and A.fruticosa seedlings to drought and defoliation treatments.The plant traits measured in this study covered growth and biomass allocation,leaf gas exchange parameters,chlorophyll concentration,leaf thickness,leaf vein density,specific leaf area,stem hydraulic traits,non-structural carbohydrate concentration,etc.,involving different organs of the whole plant.By combining hydraulic traits and carbon assimilation traits at the individual seedling level,we compared the responses of different species to stress factors from the aspects of tolerance and recovery mechanism of species,which enriched the research results on the coupling of water-carbon process at the whole plant level.Through continuous monitoring of plant traits and multiple harvests at different growth stages,this study revealed the sequential changes of plant traits during the restoration process of species.The results of this study are helpful to understand the response mechanism of plants to insect pests and human disturbance in the context of climate change,and provide scientific basis for predicting the dynamic changes of woody plant seedling regeneration and vegetation distribution,and provide more precise suggestions for guiding forest practice activities such as vegetation restoration and reconstruction,forest management in warm temperate zone. |
PDF Full Text Request