The water-holding capacity (WHC) of the understory in the headwater regions of major rivers plays an important role in both the capacity of the forest water reservoir and water quality and quantity in the butted rivers. Although forest gaps could regulate water-holding patterns in the understory by redistributing coarse woody debris (CWD), fine woody debris (FWD), non-woody debris (NWD) and understory vegetation, little information is available on the effects of forest gaps on understory WHC. Therefore, we investigated the WHCs of CWD, FWD, NWD, herbaceous vegetation, mosses, epiphytes (including fern and lichen growing on the surface of logs) and soils from the gap centre to the adjacent closed canopy in an alpine forest at the upper reaches of the Yangtze River. The total WHC of the alpine forest understory components was approximately 300 mm. Soil layer had the largest contribution to the total understory WHC (90%), and among the aboveground components, CWD and mosses contributed 5% and 4% to the aboveground WHC, respectively. With the exception of that of the herbaceous layer, the WHC of the forest floor increased from the gap centre to the closed canopy. Although mosses had the lowest biomass allocation on the alpine forest floor, the water-holding ratio (k) of mosses reached 485%. In conclusion, biomass is the parameter that most strongly and positively correlated with the WHC of the alpine forest understory, and forest gap formation decreases the understory WHC of alpine forest resulting from a decrease in organic soils, CWDs and mosses. Copyright © 2015 John Wiley & Sons, Ltd.