Understanding how rivers respond to changes in land cover, climate, and subsurface conditions is critical for sustainably managing water resources and ecosystems. In this study, long-term hydrologic, climate, and satellite data from the Upper Tahe River watershed (2359 km 2 ) in the Da Hinggan Mountains of northeast China were analysed to quantify the relative hydrologic effects of climate variability (system input) and the combined influences of forest cover change and permafrost thaw (system characteristics) on average annual streamflow (system response) using 2 methods: the sensitivity-based method and the Kendall-Theil robust line method. The study period was subdivided into a forest harvesting period (1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987), a forest stability period (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001), and a forest recovery period (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012). The results indicated that the combined effects of forest harvesting and permafrost thaw on streamflow (+ 47.0 mm) from the forest harvesting period to the forest stability period was approximately twice as large as the effect associated with climate variability (+20.2 mm). Similarly, from the forest stability period to the forest recovery period, the decrease in average annual streamflow attributed to the combined effects of forest recovery and permafrost thaw (−38.0 mm) was much greater than the decrease due to climate variability (−22.2 mm). A simple method was used to separate the distinct impacts of forest cover change and permafrost thaw, but distinguishing these influences is difficult due to changes in surface and subsurface hydrologic connectivity associated with permafrost thaw. The results highlight the need to consider multiple streamflow drivers in future watershed and aquatic ecosystem management. Due to the ecological and hydrological susceptibility to disturbances in the Da Hinggan Mountains, forest harvesting will likely negatively impact ecohydrological processes in this region, and the effects of forest species transition in the forest recovery process should be further investigated.

Abstract:Rapid permafrost thaw and precipitation regime shifts are altering surface and subsurface hydrological processes in arctic and subarctic watersheds. Long-term data (40 years) from two large permafrost watersheds in northeastern China, the Tahe River and Duobukuer River watersheds, indicate that winter baseflows are characterized by significant positive trends of 1.7% and 2.5%·year −1 , respectively. Winter baseflows exhibited statistically significant positive correlations with mean annual air temperature and the thawing index, an indicator of permafrost degradation, for both watersheds, as well as the increasing annual rainfall fraction of precipitation for the Duobukuer River watershed. Winter baseflows were characterized by a breakpoint in 1989, which lagged behind the mean annual air temperature breakpoint by only two years. The statistical analyses suggest that the increases in winter baseflow are likely related to enhanced groundwater storage and winter groundwater discharge caused by permafrost thaw and are potentially also due to an increase in the wet season rainfall. These hydrological trends are first apparent in marginal areas of permafrost distribution and are expected to shift northward towards formerly continuous permafrost regions in the context of future climate warming.

contributed to collection of field data. We thank Liu H.L., affiliated with SITP-CAS and Boston University, for review of fundamental physical equations, codes, and machine learning algorithms used in this study.

Studying carbon and nitrogen stocks in different types of larch forest ecosystems is of great significance for assessing the carbon sink capacity and nitrogen level in larch forests. To evaluate the effects of the differences of forest type on the carbon and nitrogen stock capacity of the larch forest ecosystem, we selected three typical types of larch forest ecosystems in the northern part of Daxing’an Mountains, which were the Rhododendron simsii-Larix gmelinii forest (RL), Ledum palustre-Larix gmelinii forest (LL) and Sphagnum-Bryum-Ledum palustre-Larix gmelinii forest (SLL), to determine the carbon and nitrogen stocks in the vegetation (trees and understories), litter and soil. Results showed that there were significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems, showing a sequence of SLL (288.01 Mg·ha−1 and 25.19 Mg·ha−1) > LL (176.52 Mg·ha−1 and 14.85 Mg·ha−1) > RL (153.93 Mg·ha−1 and 10.00 Mg·ha−1) (P < 0.05). The largest proportions of carbon and nitrogen stocks were found in soils, accounting for 83.20%, 72.89% and 64.61% of carbon stocks and 98.61%, 97.58% and 96.00% of nitrogen stocks in the SLL, LL and RL, respectively. Also, it was found that significant differences among the three types of larch forest ecosystems in terms of soil carbon and nitrogen stocks (SLL > LL > RL) (P < 0.05) were the primary reasons for the differences in the ecosystem carbon and nitrogen stocks. More than 79% of soil carbon and 51% of soil nitrogen at a depth of 0–100 cm were stored in the upper 50 cm of the soil pool. In the vegetation layer, due to the similar tree biomass carbon and nitrogen stocks, there were no significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems. The litter carbon stock in the SLL was significantly higher than that in the LL and RL (P < 0.05), but no significant differences in nitrogen stock were found among them (P > 0.05). These findings suggest that different forest types with the same tree layer and different understory vegetation can greatly affect the carbon and nitrogen stock capacity of the forest ecosystem. This indicates that understory vegetation may have significant effects on the carbon and nitrogen stocks in soil and litter, which highlights the need to consider the effects of understory in future research into the carbon and nitrogen stock capacity of forest ecosystems.

Pinus sylvestris var. mongolica is one of the main species to be afforested in deserts of China. But little work has been carried out on the canopy interception loss of this plant species. For researching the canopy interception loss of a natural P. sylvestris forest, we observed the gross precipitation, gross snowfall, throughfall and stemflow in a sample plot at the Forest Ecosystem Research Station of Mohe in the Great Khingan Mountains of Northeast China from July 2012 to September 2013. Considering the spatial variability of the throughfall, we increased the area rather than the number of collector and randomly relocated them once a week. The results demonstrated that the throughfall, stemflow, and derived estimates of rainfall and snowfall interception loss during the main rainy and snowy seasons were 77.12%±5.70%, 0.80%, 22.08%±5.51% and 21.39%±1.21% of the incident rainfall or snowfall, respectively. The stemflow didn't occur unless the accumulated rainfall reached up to 4.8 mm. And when the gross precipitation became rich enough, the stemflow increased with increasing tree diameters. Our analysis revealed that throughfall was not observed when rainfall was no more than 0.99 mm, indicating that the canopy storage capacity at saturation was 0.99 mm for P. sylvestris forest.