Plant growth pattern indicates divergent responses of gross ecosystem productivity to different periods of drought during growing season in an alpine meadow

Frequent droughts affect the gross ecosystem productivity (GEP) of grassland ecosystems, which in turn affects the global carbon cycle. However, the mechanisms linking the GEP response to drought timings and plant growth rate in alpine grasslands on the Tibetan Plateau are uncertain. We conducted a cumulative drought experiment with early growing season drought (ED) and middle growing season drought (MD) over six years (2016–2021) to investigate the effects of drought timing on the composition, growth rate, and GEP of alpine grassland community. Results showed that ED significantly decreased grass abundance, whereas drought timing did not significantly affect other plant functional groups. ED retarded the start and end of the fast-growth phase for about 13 and 17 days, respectively, whereas it did not change the length of the fast-growth phase. MD did not significantly affect the start, end, or length of fast-growing phase. ED and MD significantly decreased the mean growth rate during the fast-growing phase and increased the seasonal mean living state index. ED and MD significantly decreased the net ecosystem carbon exchange (NEE). ED significantly decreased the growing season mean GEP and ecosystem respiration (ER). However, MD did not significantly affect the growing season mean GEP or ER. ED has a greater negative impact on GEP than MD. Our findings suggest that on the Tibetan Plateau, management practices should be strengthened to increase water availability prior to the fast-growing phase to increase carbon sequestration in the alpine grassland ecosystem and spring forage availability for livestock.