| Carbon-source activity refers to the processes that provide carbon(C)for trees such as photosynthesis and remobilization of non-structural carbohydrates(NSC),while C-sink activity refers to the physiological processes that are involved in C utilization such as respiration,defence and growth.Shading,defoliation and elevated atmospheric CO2 concentration(eCO2)may affect C-source activities of trees and thus tree survival,growth and distribution,and consequently forest productivity,C sequestration and forest sevices.Meanwhile,trees can modify their structure and function through changes in photosynthesis,respiration,biomass production and allocation in response to environmental changes.Therefore,understanding responses of tree gas exchanges and biomass production to environmental changes and the underlying mechanisms not only is helpful for predicting the effects of environmental changes on the structure,function and dynamics of forest ecosystems,but also provides scientific bases for policy-making of environmental change adaptation and C neutrality.However,uncertainties remain regarding how environmental changes impact C-source activities through their effects on gas exchanges and leaf area,and thus tree biomass production and allocation.In this study,we first manipulated Csource activities on Manchurian ash(Fraxinus mandshurica)seedlings through girdling,artificial defoliation and shading experiments,aiming at elucidating the effects of changes in Csource activities on photosynthesis,respiration,NSC,leaf and seedling morphology,biomass production and allocation,and exploring the underlying mechanisms.Then,we compiled experiments worldwide on defoliation and eCO2,and performed a meta-analysis to detect the effects of defoliation and eCO2 on tree leaf gas exchanges,C assimilation,biomass production and allocation.Such global syntheses are helpful in revealing how global changes impact Csource activities,and thus the general patterns and mechnisms of leaf gas exchanges and tree growth,which would reduce uncertainties in modelling and predicting forest C cycle under future global change scenarios.The main results were shown as follows:(1)A girdling treatment on Manchurian ash seedlings caused an accumulation of NSC in leaves,which down-regulated light-saturated photosynthesis(Asat)via feedback mechanisms;and the down-regulation of Asat was also associated with decreases in stomatal conductance(gs).A 50%defoliation treatment on seedings did not significantly affect NSC concentrations in all the organs investigated,likely owing to the 88.4%upregulation of Asat,which compensated the negative effect of decreases in leaf area on C uptakes;the up-regulation of Asat resulted from increases in gs.Girdling and 50%defoliation treatments had no significant effect on leaf respiration(Rleaf).Girdling and 100%defoliation treatments decreased stem respiration(RStem)at 9:00 and 13:00,while 50%defoliation decreased RStem only at 9:00.Girdling and 100%defoliation treatments decreased temperature sensitivity of Rstem.(2)A shading experiment on Manchurian ash seedlings showed that seedlings may change acclimatization strategies as light availability decreased.Medium-intensity shade treatment increased both specific leaf area and leaf mass fraction,leading to an increase in leaf area ratio,which may enhance the efficiency of light harvesting.Heavy-intensity shade treatment altered biomass allocation towards stem growth,which may be a light-seeking strategy.The negative effect of shading on NSC was component-and organ-specific,with starch and roots being decreased to greater extents.Heavy-intensity shade treatment,rather than medium-intensity shade,inhibited both Rleaf and RStem.These results suggest that the seedlings maximizd C uptake under medium-intensity shade,while minimized C loss under heavy-intensity shade.(3)A global meta-analysis of artifitial defoliation experiments showed that the reduced treelevel leaf area by defoliation outweighed the enhanced leaf-level photosynthesis,leading to a net reduction in tree C assimilation and a concurrent decrease in NSC concentrations.The negative effect of defoliation on NSC concentrations depended on NSC component and specific organ:starch concentration showed a larger decrease than soluble sugars concentration,and NSC concentrations in roots showed the greatest decrease among all the organs investigated.Defoliation increased leaf production and stem biomass allocation,which may help refoliation and enhance the ability of seedlings to tolerate defoliation,but at the expense of root growth.Defoliation decreased tree growth,with the negative effect being more pronounced for diameter growth than for height growth.The structural equation model analysis showed that defoliation had a direct negative effect on tree growth,suggesting that defoliation exerted C-sink limitation on tree growth.Defoliation also had an indirect negative effect on tree growth through decreasing C assimilation,indicating that growth of defoliated trees was C-source limitated.Recovery time following defoliation had a positive effect on tree growth,suggesting that the negative effect of defoliation on tree growth declined over time.Defoliation also affected C storage that,however,had no significant effect on tree growth,suggesting that tree growth may depend on newly assimilated C and the process of C storage did not reduce C availability for tree growth.(4)A global meta-analysis of eCO2 showed that eCO2 stimulated Asat(?-factor=0.68)and decreased gs(?-factor=-0.36),and water-use efficiency(iWUE)increased in proportion to eCO2(?-factor=1.01);these suggest that Asat and gs are well-coupled,which conform with the theory of optimal stomatal behaviour.Meanwhile,conifers and older trees showed greater increases in Asat but less decreases in than angiosperms and young trees,respectively,with a proportional increase in iWUE being independent of phylogeny and tree age.Warming little affected the responses of Asat and gs to eCO2,whereas drought amplified the Asat response but attenuated the gs response.The patterns of Asat and responses to eCO2 between and within species and with changes in water availability further support the theory of optimal stomatal behaviour.However,Asat,rather than gs,acclimated to eCO2,probably due to N reduction,active investment of N,and sink limitation.(5)A global meta-analysis of eCO2 in combination with N availability showed that eCO2 stimulated tree biomass production(+32.0%),while it induced accumulation of NSC in leaves rather than in woods and roots;these suggest that biomass production may be C-limited,but the magnitude of C limitation may depend on sink strength of specific organs.Biomass responses to eCO2 of N-fertilized trees(+39.6%)were 68.4%greater than those of non-fertilized trees(+23.5%),confirming that tree growth is also N-limited.N limitation was alleviated by the eCO2induced increases in N uptake and N-use efficiency(NUE),with the former being more important.Increases in tree N pool arose from the enhanced production of fine roots with a lower specific root length,whereas increases in NUE resulted from the flexibility in tissue C:N ratios instead of N resorption efficiency.The positive responses of tree biomass production to eCO2 were greater for ectomycorrhizal trees and conifers than for arbuscular mycorrhizal trees and angiosperms,respectively,probably because ectomycorrhizal trees and conifers had greater N acquisition and N use efficiency.In summary,this dissertation showed that trees adjusted resource acquisition strategies to adapt to alternations in C-source activities caused by environmental changes.When shading or defoliation restricted C-source activities,trees maximized C uptake or minimize C consumption through changes in their morphorlogical and physiological traits and biomass allocation,so as to maintain tree growth and survival.When eCO2 stimulated C availability,trees reduced water loss,increased N uptake,enhanced water-and N-use efficiency,which would maintain positive responses of tree biomass to eCO2.These findings not only are important theoretically in tree ecophysiology,C cycling and climate changes,but have practical implications in coping with climate changes and achieving C neutrality. |
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