Temporal-spatial Changes And Influencing Factors Of The Global Terrestrial Vegetation Seasonality

The growth of terrestrial vegetation varied seasonally with the regular oscillations of temperature,moisture,radiation and the physiological factors in a calendar year.The seasonality of terrestrial vegetation growth is not only sensitive with the climate change but also plays a vital role in regulating the land-gas energy transmission and maintaining the balance of carbon cycle,nitrogen cycle and water cycle.However,previous studies on the vegetation seasonality are mainly focused on the discrete phenological events in the northern hemisphere,lacking generliazed descriptions on the vegetation seasonality at global scale and analyses on key issues at the regional scale.Using the satellite obtained vegetaion indexes,the long-term monitored atmospheric CO₂ concentration,the eddy-covriance measured carbon flux measurements and its upscaled global gross primary productivity?GPP?product along with the outputs of several state-of-art terrestrial biosphere models?TBMs?,this study first quantified the seasonality of vegetation growth and then studied on the key process of its regulation on global carbon cycle separately on the northern high latitudes,the northern mid-latitudes and the tropical evergreen forest regions.The main results are as follows:?1?The trends and IAV of global vegetation seasonality are disparate among latitudes.Based on the Markham Seasonality Index?MSI?,it was found that the vegetation seasonality in the northern high latitudes decreased over 1982-2015 with a rate of-0.027 decade^-1.The MSI in the mid-latitude area increased obviously.The trend of MSI in tropics and the Southern hemisphere are not significant,but the IAV of MSI in evergreen forests are extremely high,approximately 25%?62.1%.?2?Through regulating the growing season length?GSL?,the non-uniform seasonal warming regulates the vegetation growth and the seasonality of the atmospheric CO₂ on the northern high latitudes.The atmospheric CO₂ concentration varies seasonally and its amplitude([CO₂]_amplitude)highly relates with the seasonal dynamics of vegetation growth in the northern high latitudes.We found that the[CO₂]_amplitude had synchronously slowed down with the vegetation growth and GSL since the mid-1990s.Further evidences showed that the[CO₂]_amplitude and the vegetation growth in northern high latitudes are associated with the GSL?r?0.89?.Morover,the dependence of start of growing season?SOS?on spring warming?partial r=0.36?was found much higher than that of the end of the growing season?EOS?on autumn warming?partial r=0.018?.During mid-1990s to mid-2010s,the pause on spring warming is the main cause for the slow-down of the vegetation growth,GSL,[CO₂]_amplitude and the vegetation seasonality.?3?The peak growth dominated the vegetation seasonality in the northern mid-latitude regions?30°N?60°N?.The trend analyses showed that the averaging growth rate of MSI in this region was 0.018 decade^-1 during 1982-2015 and the peak growth controls the trend of MSI in39.3%of the studied areas.Grassland and cropland attributed to 28.9%and 19.6%of the MSI increased areas.The enhancement of the peak growth was the main reasons for the MSI growth in these two PFTs.?4?The minimum growth dominated the IAV of the the vegetation growth and the vegetation seasonality in tropical evergreen ecosystems.We demonstrated a new approach to decommopose the annual gross primary productivity?GPP?into recurrent GPP and basic GPP,which will bring out the seasonal vegetation growth of tropical vegetations.Further study found that the basic GPP dominated the inter-annual variability of GPP in 83.7%of the studied areas.This is caused by the higher rainfall sensitivity of the minimum GPP(GPP_min)than the maximum GPP(GPP_max).This study quantified the seasonality of vegetation growth globally and systemly detected it's contrast trends on the northern high and mid-latitudes,as well as unobvious trend but high inter-annul variablity on the tropics.For regions,the key regulator of vegetation seasonality shifted from the GSL,peak growth to the minimum growth with the migrations of latitudes from north to south.At the same time,the asymmetric response of plant growth to seasonal climate change is stressed in regulating the global carbon cycle.These results are meaningful for predicting the global vegetation dynamics and the parameterization of terrestrial ecosystem models by revealing the variations of the global vegetation seasonality at the temporal-spatial scales.