Carbon and water fluxes are more responsive to water restriction in grassland than in forest ecosystem in the semiarid Loess Plateau, China

Abstract

This study examined the responses of net ecosystem productivity (NEP) and evapotranspiration (ET) to environmental factors in forest and grassland ecosystems based on eddy covariance observations from 2020 to 2023. Wavelet analysis was employed to capture the cyclical fluctuations of carbon and water fluxes and assess the impact of water limitation across daily to annual timescales. Non-water restriction and water restriction periods were detected using Budyko’s aridity index. The results showed photosynthetically active radiation (PAR) was the most synchronous factor affected forest and grassland NEP, with NEP earlier than PAR 1.48 ± 11.27 h and 1.31 ± 9.35 h on daily timescale, respectively. Meanwhile, net radiation (Rn) was the most synchronous factor for forest and grassland ET, with ET earlier than Rn 2.40 ± 8.04 h and 2.28 ± 4.15 h on daily timescale, respectively. On annual timescales, air temperature (T_a) and PAR emerged as the primary synchronous drivers for forest and grassland NEP, respectively. While Rn influenced annual ET in both ecosystems, forest ET exhibited significantly longer lagtimes (−29.32 ± 18.47 d) compared to grassland (−27.55 ± 1.91 d). On daily timescale, the lagtime of NEP and ET in forest and grassland ecosystems with T_a was significantly were shorten on water restriction period compared with the non-water restriction period. On annual timescale, the lagtime of NEP and ET with solar energy and soil water were shorten during water restriction periods in forest and grassland ecosystems. Our finding highlighted NEP and ET in grassland exhibit a greater responsiveness to water restriction conditions than forest across various timescales.