Phenological Shifts To Warming And Its Mechanism Over Alpine Permafrost In Tibetan Plateau

Plant phenology is the study of the timing of recurrent biological events,as one of the most sensitive indicators for tracking climate change,it is known as the "pulse of the earth".Plant phenology has a long research history,in recent years,plant phenology has re-fuel interest in the context of climate change by the ecologist.Previous studies on plant phenology have focused on temperate regions in Europe and North America which neglected the response of plant phenology to climate change in other typical ecosystems.Among them,available phenological record on the alpine permafrost is extremely limited due to harsh environment conditions and frequent extreme meteorological events,leaving our knowledge of how native species phenology responds to climate change still largely blank as a result.As an important terrestrial carbon sink in China,Tibetan Plateau harbor two-thirds of global alpine permafrost and more than 60% of this area is covered with permafrost or seasonal frozen ground.The lack of data on how acceleration of climate change couples with carbon cycle-related process by affecting plant phenology in permafrost through manipulative experiment limits our ability to predict changes of carbon sink potential of Tibetan Plateau in a warmer world.Meanwhile,lack of knowledge of species interaction across community in the context of climate change limit our ability of understanding for predicting the future species composition,distribution and diversity.Therefore,carry out ecological observation in the permafrost of Tibetan Plateau based on manipulating climate control experiment are of great significance.For this purpose,we established a permafrost ecological observation station in a typical permafrost-affected region in Zaren Town,Amdo County,Nagqu City,Tibet Autonomous Region,open top chamber(passive warming: 1.6℃ and 3.5℃)and numerically controlled infrared radiator(active warming: 1℃、2℃ and 4℃)were used to simulative ecosystem warming and a 3-years in situ phenological observation was conducted in this station.We use quadrat method to monitor phenology at community level and eight common native species(four early spring species,two middle spring species and two late spring species)was chosen to monitor phenology at species level,the correspond phenological temperature sensitivity and the phenological duration length was calculated using these data.We evaluated the accuracy of pheno-cam by comparison with manual phenological observation and applied extent growing degree model to predict cumulative temperature requirement under warming conditions.The main results are shown in the following:(1)Year-round warming advanced spring phenology and delayed autumn phenology at both community and species level,but the degree of respond varied with species.Growing season length was significantly extended by 9.4 and 20.5 days and accounts for 6% and 13% of the total length of the growing season in two warming treatments,respectively.Spring warming advanced both spring and autumn phenology,due to large advance in spring phenology,growing season was significantly extended by 6.6 days(accounts for 4% of the total length of the growing season)under 3.5℃spring warming,there is no statistical significance change of growing season length under 1.6℃ spring warming.Fruit relevant phenological stage show irresponsibility to all kinds of warming treatments.Both community and species phenology show irresponsibility to winter warming.(2)In contrast to phenological record in Europe,there is no slowdown of spring phenological shifts with warming of native species in permafrost of Tibetan Plateau.With the air temperature increases from 1.6℃ to 3.5℃,the temperature sensitivity of phenology increased by 29% to 46%(depending on species)at both community and species level.The sensitivity of early and middle spring species was higher than that of late spring species,following meta-analysis confirmed the relative universality in the Tibetan Plateau.One of the reasons can be explained by the fact that winter-only warming has no appreciable effect on dates of spring phenophases,which means the chilling requirement for dormancy release might be still fulfilled.(3)Under year-round warming,reproductive and post-fruit vegetation(seed dispersal)phase were allocated more resources,while the vegetative phase was allocated relatively less resources,reflecting the contrast phenological strategies adopted by perennial herbs in alpine meadow from annual species in grasslands.The late spring species tend to allocate more resources to reproductive and post-fruit vegetation phase by shorten its vegetative phase,reflecting a more conservative phenological strategy of late successional species.The presence of large amounts of standing litter duo to fence and low decomposition rates strongly disturbance the phenological observation when using pheno-cam,near-surface remote sensing retrieval fail to capture the initiation of spring phenology in time and overestimate the onset of autumn phenology,results in greatly underestimating the plant growing season length.Most of the existing phenology model fail to correct simulate the negative relationship between forcing and chilling requirement due to insufficient consideration of the effective temperature accumulation under sub-freezing point.In summary,we predict a lengthened growing season for carbon sequestration in the midst of ongoing seasonal asymmetric warming due to continued spring phenology advancement and autumn phenology delay.The extension of effective photosynthetic length associated with the change in phenology and the intercommunity species interactions will have profound implications for regional biological matter cycle and energy flow in a warmer world.