Ecophysiological Responses Of Four Common Tree Species To Water,Defoliation,Light,and Nitrogen Deposition In The Warm Temperate Zone

Global environmental change,such as regional drought and nitrogen deposition,is becoming more serious and urgent than ever before.Global change also increases the frequency of light environmental heterogeneity and insect damage to plants.The forest vegetation is experiencing great survival stress due to global climate change and disturbances of other factors,leading to the degeneration of forest.The vegetation recovery of degraded forest ecosystem should follow the rules of forest community succession.The succession stage of the target forest community and species,and the local environment should be considered to select species for vegetation recovery.In this study,pioneer species Ailanthus altissima,exotic pioneer species Robinia pseudoacacia,local pioneer species Sophora japonica,which is phylogeneticly close to Robinia pseudoacacia and is considered suitable as a pioneer species,and late-successional species Quercus acutissima were selected as research objects.Prominent ecological factors that greatly affect the growth of each species were selected.Growth morphology,leaf morphological traits,gas exchange,fluorescence characteristics,chlorophyll contents,non-structural carbohydrate content and non-structural carbohydrate allocation,biomass and biomass allocation were measured.The effects of prominent ecological factors on the growth of each species were investigated.This study will help to understand the response mechanisms of tree species of temperate zone to future climate change and to provide theoretical basis and reasonable advices to vegetation recovery.Drought and nitrogen(N)deposition are two important factors affecting plant growth.Previous studies have reported the interaction beteeen drought and nitrogen(N)deposition plants but no consistent conclusion were reached.Ailanthus altissima is a native species in temperate forests of China,this plant is usually used as a pioneer species.We conducted two soil moisture conditions(35%and 75%of the saturated soil moisture content)and three N treatments(0,5,and 15 g N m-2 year-1)in a greenhouse experiment.After 50 d of treatment,plant growth,biomass production and allocation,and leaf physiological parameters were determined.Drought significantly restricted some parameters of growth and biomass accumulation,like height and basal diameter,total biomass,lateral root biomass,leaf biomass,and stem biomass.Some physiological parameters were also affected significantly by drought,leafN content and qP decreased as soil moisture decreased and WUE was highest in drought condition.Drought had no significant effect on biomass allocation,which may due to drought also inhibited the growth of both the above-ground and the under-ground.The high N load increased crown area and total biomass,lateral root biomass,and root biomass.With respect to physiological parameters,net photosynthetic rate,leaf N content,? PSII and ETR were clearly higher in the high N addition group than in the control group.Overall,the interaction between drought and N load had no significant effects on plant growth.Climate change may result in the increase of insect damage to plants and the decrease of rainfall in North China,which greatly affects the growth of forest vegetation and forest regeneration process.Exotic pioneer species Robinia pseudoacacia and local pioneer species Sophora japonica which is phylogeneticly close to Robinia pseudoacacia and is considered suitable as a pioneer species,are vulnerable to insect damage and have tolerances to drought and infertility.We conducted a manipulative experiment in a greenhouse to investigate the growth,biomass accumulation and allocation,leaf morphological traits,gas exchange,non-carbohydrate content and allocation of seedlings.Both species were exposed to three soil moisture treatments(75%(Wl),55%(W2),and 35%(W3)of field capacity),and three defoliation treatments(0%(DO),50%(Dl),and 100%(D2)defoliation).After two weeks of treatment,we found that both species in treatments D1/D2 and W2,and S.japonica in the D1/D2 and W1 treatments,up-regulated the leaf net photosynthetic rate.However,both species in D1/D2 and W3 treatments,and R.pseudoacacia in D1/D2 and W1 treatments had no up-regulation of net photosynthetic rate.After 57 days of treatment,defoliation had no significant effect on photosynthetic capacity and chlorophyll content of either species.In the D2 treatment,growth and total non-structural carbohydrate(TNC)content of both species did not recover to the levels of control seedlings,which means that the growth of 100%defoliated plants did not recover to that of the control group.Plant growth and TNC pools in the D1 treatment recovered only for S.japonica,which suggested that S.japonica had a stronger recovery ability after defoliation than R.pseudoacacia.Under D1/D2 treatments,both species allocated relatively more biomass to their leaves to capture more light energy for carbohydrate assimilation.Defoliation had no significant effect on the allocation of non-structural carbohydrates,which may be due to the offset of non-structural carbohydrates transfer and biomass allocation.The non-structural carbohydrates of leaves of deciduous plants will be transferred to the roots for re-germination in the spring and the autumn.However,defoliated trees allocated relatively more biomass to leaves for restoration of growth.Thus,the fact may be due an offset effect.Drought significantly inhibited the growth of the two species,but most parameters of R.pseudoacacia under any water treatment were significantly higher than that of S.japonica,suggesting that R.pseudoacacia had a better performance than S.japonica under all soil moisture treatments.The two species have different response mechanisms to drought in terms of allocation of biomass and non-structural carbohydrate(TNC)content,indicating different acclimation strategies.In addition,since the R.pseudoacacia is an exotic species,R.pseudoacacia should be used in forest revegetation with caution in the future,although it is widely distributed in China.Nitrogen deposition and light,which is a main ecological factor that affect understory regeneration,were selected as the treatments.Late-successional species Quercus acutissima was selected as research objective.We performed a potted growth experiment with three N addition regime(0?6 and 12 g N m-y-1combined with two illumination levels(8 and 88%of full sunlight)to investigate the interactive responses of Quercus acutissima to light and N addition in terms of seedling growth,leaf traits,biomass accumulation and partitioning patterns,leaf morphological traits,gas exchange,photosynthetic pigment,carbohydrate content and partitioning patterns.After 87 days,we observed that nitrogen deposition had no significant effects on the seedlings regardless of light exposure.Shade significantly reduced plant height,basal diameter,leaf number,total biomass,gas exchange capacity,and carbohydrate content.However,shade significantly increased the amount of photosynthetic pigment,and specific leaf area.The light had a significant effect on the most biomass and carbohydrate allocation.Quercus.acutissima seedlings had higher leaf biomass ratio and stem biomass ratio and lower root biomass ratio and root/shoot in the shade than under high light intensity,which means that the seedlings may invest more photosynthetic resources in aerial shoot growth and less in root growth.In this way,it can produce more leaves and capture more light energy.The carbohydrate ratios also responded differently to the two light regimes possibly because the functions of soluble sugar,starch,and cellulose vary with environment.The soluble sugar ratio was not affected by different light regimes possibly because the plants were well hydrated and therefore not under osmotic stress.In addition,starch is transformed to soluble sugar to maintain high respiration during plant growth in the shade.For these reasons,the soluble sugar ratio did not change and the starch ratio decreased under low light levels.Interestingly,the cellulose ratio increased under low light intensity,which may result from cellulose is the main cell wall component and varies only slightly with environmental change.In addition,the interaction between light and Nitrogen deposition had no effects on Q.acutissima seedlings.In summary,environmental stresses(drought,insect damage and shade)inhibited the growth of plants.All the tree species acclimated to the adverse environments by changing the functional traits and adopting different strategies to obtain and utilize resources.Pioneer species A.altissima acclimated to drought by increasing its water use efficiency.Pioneer species R.pseudoacacia increased root non-structural carbohydrate(TNC)pools ratio to increase the osmotic pressure of root,therefore increasing the ability to obtain water.Pioneer species S.japonica allocated more biomass to root but less biomass to shoot,therefore decreasing the transpiration and increasing the ability to obtain water.Robinia.pseudoacacia and S.japonica increased the net photosynthetic rate and aboveground mass ratio to acclimate to defoliation.Late-successional species Quercus acutissima increased aboveground mass ratio,photosynthetic pigment content and specific leaf area,and decreased leaf thickness in shade compared to high light conditions.Nitrogen addition promoted the growth of pioneer species but had no effects on late-successional species.Therefore,monitoring and management of the growth of the plants at different vegetation recovery satges in the future should be conducted with the focus on the effects of prominent ecological factors to accelerate the vegetation recovery and promote the succession of forest community.