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Understanding the mechanisms of moss affecting the understory microclimates may further shed lights on forest recruitment, which is closely associated with hydrological and ecological processes of forest ecosystems. We conducted a one-year field experiment to examine the effects of moss thickness on understory microclimates including ground surface temperature (GST), ground surface moisture (GSM), soil temperature (ST), and soil moisture (SM) in a Picea crassifolia forest of Qilian Mountains, northwest China. We found that moss coverage substantially reduced the fluctuation amplitudes of microclimates (p<0.01), as evidenced by the slower diurnal changes in GST, GSM, ST, and SM of moss-covered soil than those of bare soil. Moreover, our results also showed that moss coverage obviously increased GST and ST in winter (December 2014), whereas significantly decreased GST and ST in summer (June 2015). Comparing with bare soil, thick-moss coverage decreased the average annual GST and ST by 0.55 degrees C and 0.62 degrees C, respectively. However, moss coverage significantly increased the annual GSM and SM (p<0.01), especially the SM of thin-moss coverage was increased much higher than that of thick-moss coverage and bare soil. In addition, moss coverage may also prevent heat flux from air to soil surface because the light radiation was exponentially declined from the upper moss canopy to soil with the increase of moss depths. These results suggested that moss coverage may promote the microclimates of the forest understory with heat insulation and water holding, and thus facilitate the P. crassifolia germination and recruitment through changing the hydrological and ecological processes of subalpine forests.