Little direct effect of diurnal temperature amplitude on growing seasonal CO₂ fluxes in alpine humid shrubland, Qinghai‐Tibetan Plateau

Abstract

The amplitude of the diurnal temperature (ADT) has been decreasing under climate change, with substantial anticipated effects on alpine grassland carbon budgets. Here, we quantified the temporal response of the growing seasonal CO2 fluxes to ADT over alpine shrubland on Qinghai‐Tibetan Plateau (QTP) from 2003 to 2016. At a daily scale, net ecosystem exchange (NEE) and gross primary production (GPP) quadratically responded to ADT with optimum values of 15.4 and 13.4°C, respectively. Ecosystem respiration (RES) negatively linearly correlated with ADT. Partial correlation, and classification and regression trees (CART) analysis, both showed that the maximal (MaxTa) or minimal air temperature (MinTa), rather than ADT, played much more important role in daily variations of CO2 fluxes. At a monthly scale, GPP and NEE were both positively and negatively controlled by MaxTa while RES was negatively determined by MinTa, respectively. Monthly ADT exerted a negligible influence on monthly CO2 fluxes. At an annual scale, only MaxTa played a significant role in variations of GPP and RES. NEE did not significantly respond to ADT, MaxTa or MinTa. The little direct correlations between NEE and ADT at daily, monthly or annual scales contradicts a previous hypothesis that a larger ADT would enhance carbon sequestration capacity over alpine ecosystems. Given the positive impact of MaxTa on GPP and MinTa on RES, our study would suggest that a decreasing ADT could indirectly stimulate more carbon loss and weaken the carbon sequestration capacity of alpine shrublands under the scenario of further increases in MinTa over QTP.

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