Opposite effects of temperature and precipitation on vegetation growth onset in Africa

Abstract

Understanding the responses of vegetation phenology to climate change is of great importance in predicting land-atmosphere carbon and water exchange. Previous studies have revealed a delayed start of the growing season (SOS) in most African regions over the past three decades, contrasting with the advancing trend observed in the Northern Hemisphere. However, the climatic drivers of this SOS delay in Africa remain unclear. We investigated the responses of SOS to pre-season precipitation ($P_{ps}$) and temperature ($T_{ps}$) across Africa, and quantified the sensitivity of SOS to temporal variations in $P_{ps}$ and $T_{ps}$ from 1982 to 2022. The results reveal that temporal variations of SOS were strongly correlated with both $P_{ps}$ and $T_{ps}$, but with divergent effects in Africa. Specifically, SOS was negatively correlated with $P_{ps}$, while positively correlated with $T_{ps}$. An increase in $P_{ps}$ of 10 mm corresponded to an average advancement of SOS by 1.5 days (i.e., -0.15 days/mm), while a 1 °C warming in $T_{ps}$ led to a delay of 4.7 days (i.e., 4.7 days/°C) in SOS for Africa. Under climate change, $T_{ps}$ over Africa significantly increased 0.03 °C/yr, while $P_{ps}$ slightly increased 0.10 mm/yr. The delay induced by the increasing $T_{ps}$ was more pronounced than the advance induced by the increasing $P_{ps}$, leading to widespread SOS delays across Africa. Furthermore, we observed that SOS was more sensitive to $P_{ps}$ in drier areas (relatively lower annual precipitation areas), while higher sensitivity to $T_{ps}$ in wetter areas (relatively higher annual precipitation areas). Similarly, along the gradients of spatial annual temperature, higher temperature sensitivity was found in lower annual temperature areas. Our study underscores the intricate responses of SOS to climate variables in Africa, where varying local climate conditions contribute to distinct sensitivities, emphasizing the need for a more detailed investigation of the roles of $P_{ps}$ and $T_{ps}$ in African phenology dynamics.