[Extraction of temperate vegetation phenology thresholds in North America based on flux tower observation data].

Flux tower method can effectively monitor the vegetation seasonal and phenological variation processes. At present, the differences in the detection and quantitative evaluation of various phenology extraction methods were not well validated and quantified. Based on the gross primary productivity (GPP) and net ecosystem productivity (NEP) data of temperate forests from 9 forest FLUXNET sites in North America, and by using the start dates (SOS) and end dates (EOS) of the temperate forest growth seasons extracted by different phenology threshold extraction methods, in combining with the forest ecosystem carbon source/sink functions, this paper analyzed the effects of different threshold standards on the extraction results of the vegetations phenology. The results showed that the effects of different threshold standards on the stability of the extracted results of deciduous broadleaved forest (DBF) phenology were smaller than those on the stability of the extracted results of evergreen needleleaved forest (ENF) phenology. Among the extracted absolute and relative thresholds of the forests GPP, the extracted threshold of the DBF daily GPP= 2 g C.m-2.d-1 had the best agreement with the DBF daily GPP = 20% maximum GPP (GPPmax) , the phenological metrics with a threshold of daily GPP = 4 g C.m-2.d-1 was close to that between daily GPP = 20% GPPmax and daily GPP = 50% GPPmax, and the start date of ecosystem carbon sink function was close to the SOS metrics between daily GPP = 4 g C.m-2.d-1 and daily GPP= 20% GPPmax. For ENF, the phenological metrics with a threshold of daily GPP = 2 g C.m-2.d-1 and daily GPP = 4 g C.m-2.d-1 had the best agreement with the daily GPP = 20% GPPmax and daily GPP = 50% GPPmax, respectively, and the start date of the ecosystem carbon sink function was close to the SOS metrics between daily GPP = 2 g C.m-2.d-1 and daily GPP= 10% GPPmax.