The Effects And Mechanisms Of Unbalanced Winter-spring Warming On Phenology Of Temperate Trees

Vegetation phenology is highly sensitive to climate change,and is known as the "diagnostic fingerprint" of global climate change.It is also an important biological indicator of understanding ecosystem behavior and response to environmental cues.Phenological changes have an important impact on the ecological process of terrestrial ecosystem.With global climate change,winter warming is more marked than spring warming at high latitudes and high altitudes,and such asymmetric warming could affect chilling and forcing processes,thus delay or advance the spring phenology.Changes in phenology could affect plant photosynthetic carbon production,growth and carbon fixation,which in turn have feedback effects on the climate system.To better understand the effects of unbalanced winter-spring warming on the spring phenology of trees at high latitudes and altitudes and the underlying mechanisms,this thesis systematically studied the following three aspects:(1)First,we analyzed the spring phenology in relation to both winter and spring temperatures across 31 years(1980-2012),based on long-term phenological observation data(Chapter 3).Using the Dynamic Model and the Growing Degree Hour(GDH)model,I analyzed the effects of winter chilling and spring forcing accumulation on the spring phenology of three deciduous trees(Ulmus pumila,Populus simonii and Syringa oblata)in Heilongjiang,and the results were compared with those gained from Xi’an as a reference where the winter and spring warming is relatively balanced.The analysis showed that forcing temperature played a crucial role in determining spring phenology in Heilongjiang,whereas chilling and forcing jointly determined spring phenology in Xi’an.The temperatures during the chilling and forcing periods were lower in Heilongjiang than in Xi’an.The accumulation period started earlier and ended later in Heilongjiang than in Xi’an.Moreover,the chilling accumulation was splitted into two periods by the relatively low winter temperature in Heilongjiang,but similar phenomenon was not found for that in Xi’an.This result implies that future strong winter warming in cold regions will lead to disappearance of that interruption.Future asymmetric winter-spring warming,with a stronger warming in winter than in spring,could decrease the importance of forcing accumulation effects and increase that of the chilling effects on the spring phenology of plants in colder regions.(2)In order to test the results obtained from the long-term phenological observation data in Chapter 3,I designed a controlled experiment to further study the effects of unbalanced winter-spring warming on the spring phenology of young Oak(Quercus petraea)and Pine(Pinus sylvestris)trees(Chapter 4).Four temperature combinations between winter and spring temperature were used: no warming in winter(November22 – February 8)and spring(February 8 – March 15);both warming in winter and spring;only warming in spring and only warming in winter.The results showed that the spring phenology of both the Oak and Pine responded to temperature treatment significantly.Compared to the ambient control,winter warming advanced the budburst3.5 days,spring warming advanced 8.1 days and the balanced winter-spring warming advanced 13.8 days for Oak.For the Pine trees,winter warming delayed 0.8 days,spring warming advanced 9.1 days and balanced winter-spring warming advanced 5.1days.Oak showed 100% advance rate of spring phenology to warming,while Pine showed 88%(winter warming),90%(spring warming)and 90%(balanced winterspring warming)advance rate.The relative importance of chilling and forcing for spring phenology varied with species.Chilling was of greater relative importance to Oak,whereas the Pine phenology was more affected by forcing.(3)Winter and spring warming had significant effects on spring phenology of trees,and the effects varied with species(Chapters 3 and 4).In order to further understand the mechanisms underlying the effects of winter and spring temperatures on spring phenology of trees,defoliation and drought treatments were designed to decrease photosynthetic carbon production and carbon resource of trees.I aimed to test whether winter and spring temperatures alter phenology by altering the availability of carbon resources(i.e.,more carbon resource available,more earlier the timing of phenology is),and whether there is a carbon trade-off between leaf phenology and growth.Defoliation experiment was carried out on Oak and Pine saplings(4-5 years old,about100 cm tall)during the growing season at four levels: control(no defoliation),mild(1/3leaves removed),moderate(2/3 leaves removed),severe(all leaves removed)(Chapter5).I analyzed tissue non-structural carbohydrates(NSC)and the spring phenology next year.Drought experiment was carried out on Oak and Beech(Fagus sylvatica)saplings at four levels in the previous year’s growing season: control(watering once weekly),mild(watering once biweekly),moderate(watering once every four weeks),severe(watering once every eight weeks)(Chapter 6).I measured leaf water potential,photosynthesis,tissue NSC and spring phenology after rewetting.The results showed that mild and moderate defoliation had no significant effect on phenology,but severe carbon limitation(complete defoliation)led to the earliest spring phenology.For example,budburst of completely defoliated Oak trees was 4.7 days earlier than that of undefoliated saplings,and that of the completely defoliated Pine trees was 11.9 days earlier than that of the intact Pine trees.Unbalanced winter-spring warming influenced NSC consumption over winter.After warming in winter and spring,the concentration of NSC decreased in both shoots and roots of Oak,whereas that decreased in shoots but increased in roots of Pine.Winter warming increased NSC consumption during winter dormancy in Oak,especially,mild defoliation significantly increased the NSC consumption by 18.5% over winter.Complete defoliation resulted in a significant reduction in tissue NSC reserves at the end of growing season.Under a severe shortage of carbon reserves,plants changed the carbon allocation strategy in the trade-off between phenology and growth.The stored carbon resources are preferentially used for bud burst and leaf unfolding in early spring to complete the phenology rather than growth.Except for severe carbon limitation,temperature was still the main factor affecting spring phenology.Drought stress decreased net photosynthesis and growth in both Oak and Beech.More NSC was consumed in winter in Beech than in Oak.The post-winter NSC concentration in roots played a more important role in determining the growth and survival of both species.Severe drought resulted in carbon constraints in the roots,especially in beech.The legacy effects of drought on physiology,growth and spring phenology were not observed.Temperature change in winter and spring did not show a legacy effect on spring phenology,but severe defoliation had a legacy effect on spring phenology.This study suggests that the unbalanced winter-spring warming may influence the trade-off between chilling and forcing temperature accumulation at high latitudes and altitudes across species under global climate change.Thus,the changes in tree spring phenology are more complicated.Variations in spring phenology of trees caused by changes in temperature are not a result of the availability of plant carbon resources altered by temperature change.On the contrary,if carbon resources are seriously insufficient,plants will prioritize using the stored NSC for spring phenology and recovering photosynthesis to obtain the new carbon source.Plant growth rhythm may vary with plant phenology,affecting the carbon cycle of ecosystems as well as the ecosystem functions and services.