Hydrological effects of forest litter of Larix principis-rupprechtii plantations with varying densities in Liupan Mountains of Ningxia, China.

To explore the impacts of stand density on the hydrological functions of litter layer,we chose fifteen thinned stands of 31-year-old Larix principis-rupprechtii forest in May of 2015, in Liupan Mountains of Ningxia, China. The thickness and storage of litter of these stands were surveyed, and the water-holding capacity and water absorption rate of litter were measured by soaking method. It showed that there was an obvious difference among stand densities for all the parameters describing the hydrological functions of litter, including the thickness, the storage and the water-holding capacity of litter. According to the stand density gradient of 800, 1 000, 1 200, 1 600, and 1 800 trees/ha, the litter storage was 12.53, 14.91, 17.67, 19.71, and 18.42 t/ha, showing a maximum of litter thickness and storage in the stand with density of 1 600 trees/ha; the water-holding ratio of litter was 176.78%, 171.89%, 182.52%, 184.58% and 169.60%, and the corresponding water-holding capacity was 22.15, 25.63, 32.25, 36.38 and 31.24 t/ha respectively, showing the maximum water-holding ratio and water-holding capacity in the stand with density of 1 600 trees/ha. Water-holding capacity of litter had a logarithmic relationship with soaking time (R20.89), and water absorption rate showed a power function with soaking time (R20.88). The modified water-interception capacity was 12.50-21.33 t/ha, showing a maximum of water-interception capacity in the stand with density of 1 600 trees/ha. The results showed that, under the given conditions of tree growth and tree age in this study, the hydrological functions of litter reached their maximums at the stand density of 1 600 trees/ha. Moreover, the stand density should not be lower than 1 200 trees/ha, in order to avoid a sharp reduction of the hydrological functions of litter. Taking into account the variation of other ecological services with stand density of middle-aged plantation of L. principis-rupprechtii in the same area, it is recommended to maintain the rational stand density range of 1 200-1 600 trees/ha.