The effects of diurnal temperature variation on spring phenology in temperate grasslands of China

Changes in spring phenology have profound effects on productivity, carbon cycling, and soil moisture in temperate grassland ecosystems of China. However, the spatial and temporal variation in spring phenology in the last 20 years and its response mechanism to monthly climate change in different grassland vegetation types have not been clarified. Using the 2001–2020 MODIS EVI (Moderate-Resolution Imaging Spectroradiometer Enhanced Vegetation Index (MODIS EVI) data, this study analyzed the changes in the start of season (SOS) of temperate grassland vegetation growth in China, using the Savitzky–Golay (SG) filtering method and dynamic threshold method to extract grassland spring phenology. The partial least squares regression (PLS) method was used to study the effects of maximum monthly temperature (T
max
), mean monthly temperature (T
mean
), minimum monthly temperature (T
min
), and monthly accumulated precipitation (Pre) on different grassland vegetation phenology (SOS). The Chinese temperate grassland SOS did not significantly advance (2.96 days/decade). The effective image elements were distributed as follows: 82.29% were in the regions with early SOS and 17.71% in the regions with delayed SOS. The effects of T
max
and T
min
changes on temperate grassland SOS varied depending on vegetation type and month, with increases in T
max
and T_min advancing the SOS of TS (February and May), TM (September, December, February, April, and May), and TDS (January and April). However, increases in T_max and T_min delayed the SOS of TS (July, August, December, January, and March), TM (June, November, January, and March), and TDS (June, July, August, December, February, and March). The effect of T
max
and T
min
on temperate grassland SOS was asymmetrical, with their increases acting in opposite directions on TS SOS (June, September, October, November, and April), TM SOS (July, August, and October), and TDS SOS (September, October, November, and May). In TS, the effect of the lowest temperature on SOS was more evident, whereas in TM and temperate dry steppes (TM and TDS), the effect of the highest temperature was more evident. In contrast, precipitation has a greater influence than temperature on the SOS in temperate grasslands, and higher levels of precipitation tend to bring about earlier SOS in these regions. This study underscores the considerable influence of monthly climate fluctuations on the SOS across various types of grassland vegetation, emphasizing the significance of such information for the management and preservation of these ecosystems.