Seasonal differences in relationships between changes in spring phenology and dynamics of carbon cycle in grasslands

Global warming is dramatically altering the plant phenology of terrestrial ecosystem and thus has caused significant effects on the terrestrial carbon cycle. Many studies have investigated the relationships between the spring phenological changes and the dynamics of carbon cycle, but the seasonal differences in these relationships are still unclear. This study used eddy covariance measurements of grassland net ecosystem productivity (NEP) from nine FLUXNET sites (73 site-years of data) to investigate the relationships between the spring phenological changes (i.e., early green-up, EGU; middle green-up, MGU; and late green-up, LGU) and the dynamics of carbon cycle (including NEP; ecosystem respiration, ER; and gross ecosystem production, GEP) at a ten-day scale. Weak relationships were found between the spring phenological changes and the dynamics of ER at all sites. The advance of spring phenology mainly caused significant increases in GEP (or NEP) during the initial 20-50 d after the green-up date at most sites. With an advancement of 1 d in EGU (MGU and LGU), GEP and NEP were increased by 1.5-6.0 (3.5-7.1 and 2.4-4.7) and 0.7-3.6 (1.5-3.9 and 1.9-4.7) g C/m(2) during the early growing season (EGS), respectively. The advance of spring phenology did not have significant impacts on GEP (or NEP) during the middle and late growing season (MGS and LGS) at most sites, but yielded a significant decrease in GEP (or NEP) during MGS at a few sites. As the grasslands turning green, the duration of the impact of the changes in EGU, MGU, and LGU on GEP (or NEP) was reduced, whereas the magnitude of the impact of the changes on the ten-day cumulative GEP (or NEP) was increased at most sites. Thus, there were two possible impact models for the spring phenological changes on the dynamics of carbon cycle for grasslands: The advance of spring phenology only increases the GEP (or NEP) during EGS, and the advance of spring phenology only decreases the GEP (or NEP) during MGS. These results could be helpful in further understanding the influences of spring phenological changes on the dynamics of carbon cycle for grasslands.