Tundra soils are one of the world's largest organic carbon stores, yet this carbon is vulnerable to accelerated decomposition as climate warming progresses. The landscape-scale controls of litter decomposition are poorly understood in tundra ecosystems, which hinders our understanding of the global carbon cycle. We examined the extent to which the thermal sum of surface air temperature, soil moisture and permafrost thaw depth influenced litter mass loss and decomposition rates (k), and at which spatial thresholds an environmental variable becomes a reliable predictor of decomposition, using the Tea Bag Index protocol across a heterogeneous tundra landscape on Qikiqtaruk-Herschel Island, Yukon, Canada. We found greater green tea litter mass loss and faster decomposition rates (k) in wetter areas within the landscape, and to a lesser extent in areas with deeper permafrost active layer thickness and higher surface thermal sums. We also found higher decomposition rates (k) on north-facing relative to south-facing aspects at microsites that were wetter rather than warmer. Spatially heterogeneous belowground conditions (soil moisture and active layer depth) explained variation in decomposition metrics at local scales (< 50 m(2)) better than thermal sum. Surprisingly, there was no strong control of elevation or slope on litter decomposition. Our results reveal that there is considerable scale dependency in the environmental controls of tundra litter decomposition, with moisture playing a greater role than the thermal sum at < 50 m(2) scales. Our findings highlight the importance and complexity of microenvironmental controls on litter decomposition in estimates of carbon cycling in a rapidly warming tundra biome.

The Three-River Headwater Region (TRHR) is the source of the Yangtze River, Yellow River, and Lancang River, which is significant to fresh water resources in China and Asia. The ecosystem in the TRHR has undergone great changes in recent decades owing to dramatic climate change and tremendous human pressure. This study focused on assessing the ecosystem change in the TRHR from 2005 to 2012, which was indicated by ecosystem pattern, quality, and service. Based on the actual observation records and widely used biophysical models including Revised Universal Soil Loss Equation (RUSLE), Revised Wind Erosion Equation (RWSQ), and Carnegie-Ames-Stanford Approach (CASA) models, this study assessed the ecosystem services including soil conservation, water conservation, carbon sequestration, and species conservation. The climate variability and ecological rehabilitation promoted ecological restoration, which was indicated by vegetation cover, productivity (carbon sequestration), streamflow, and habitat area increase. However, the increasing precipitation intensified water erosion by enhancing rainfall erosivity, and increasing temperature induced glacier melting and permafrost degradation, which posed a threat to the sustainable development of regional environment. The ecosystem change is the combined result of ecological rehabilitation and climate variability, the effectiveness of ecological conservation efforts is uneven, indicated by coexistence of restoration and degradation, and is likely a temporary improvement rather than fundamental change. The experience of ecological rehabilitation and ecosystem change in the TRHR exemplified the ecological conservation should take climate variability into account, and facilitate synergies on multiple ecosystem services in order to maximize human well-being and preserve its natural ecosystems. (C) 2016 Elsevier B.V. All rights reserved.