Climate change is predicted to increase both drought frequency and duration, and when coupled with substantial warming, will establish a new hydroclimatological model for many regions. Large-scale, warm droughts have recently occurred in North America, Africa, Europe, Amazonia and Australia, resulting in major effects on terrestrial ecosystems, carbon balance and food security. Here we compare the functional response of above-ground net primary production to contrasting hydroclimatic periods in the late twentieth century (1975-1998), and drier, warmer conditions in the early twenty-first century (2000-2009) in the Northern and Southern Hemispheres. We find a common ecosystem water-use efficiency (WUE(e): above-ground net primary production/evapotranspiration) across biomes ranging from grassland to forest that indicates an intrinsic system sensitivity to water availability across rainfall regimes, regardless of hydroclimatic conditions. We found higher WUE(e) in drier years that increased significantly with drought to a maximum WUE(e) across all biomes; and a minimum native state in wetter years that was common across hydroclimatic periods. This indicates biome-scale resilience to the interannual variability associated with the early twenty-first century drought--that is, the capacity to tolerate low, annual precipitation and to respond to subsequent periods of favourable water balance. These findings provide a conceptual model of ecosystem properties at the decadal scale applicable to the widespread altered hydroclimatic conditions that are predicted for later this century. Understanding the hydroclimatic threshold that will break down ecosystem resilience and alter maximum WUE(e) may allow us to predict land-surface consequences as large regions become more arid, starting with water-limited, low-productivity grasslands.

[1] Precipitation regimes are predicted to shift to more extreme patterns that are characterized by more heavy rainfall events and longer dry intervals, yet their ecological impacts on vegetation production remain uncertain across biomes in natural climatic conditions. This in situ study investigated the effects of these climatic conditions on aboveground net primary production (ANPP) by combining a greenness index from satellite measurements and climatic records during 2000-2009 from 11 long-term experimental sites in multiple biomes and climates. Results showed that extreme precipitation patterns decreased the sensitivity of ANPP to total annual precipitation (P T ) at the regional and decadal scales, leading to decreased rain use efficiency (RUE; by 20% on average) across biomes. Relative decreases in ANPP were greatest for arid grassland (16%) and Mediterranean forest (20%) and less for mesic grassland and temperate forest (3%). The cooccurrence of heavy rainfall events and longer dry intervals caused greater water stress conditions that resulted in reduced vegetation production. A new generalized model was developed using a function of both P T and an index of precipitation extremes and improved predictions of the sensitivity of ANPP to changes in precipitation patterns. Our results suggest that extreme precipitation patterns have substantially negative effects on vegetation production across biomes and are as important as P T . With predictions of more extreme weather events, forecasts of ecosystem production should consider these nonlinear responses to altered extreme precipitation patterns associated with climate change.

The dynamics of sediment transport capacity in gravel-bed rivers is critical to understanding the formation and preservation of fluvial landforms and formulating sediment-routing models in drainage systems. We examine transport-storage relations during cycles of aggradation and degradation by augmenting observations of three events of channel aggradation and degradation in Cuneo Creek, a steep (3%) gravel-bed channel in northern California, with measurements from a series of flume runs modeling those events. An armored, single-thread channel was formed before feed rates were increased in each aggradation run. Output rates increased as the channel became finer and later widened, steepened, and braided. After feed rates were cut, output rates remained high or increased in early stages of degradation as the incising channel remained fine-grained, and later decreased as armoring intensified. If equilibrium was not reached before sediment feed rate was cut, then a rapid transition from a braided channel to a single-thread channel caused output rates for a given storage volume to be higher during degradation than during aggradation. Variations in channel morphology, and surface bed texture during runs that modeled the three cycles of aggradation and degradation were similar to those observed in Cuneo Creek and provide confidence in interpretations of the history of change: Cuneo Creek aggraded rapidly as it widened, shallowed, and braided, then degraded rapidly before armoring stabilized the channel. Such morphology-driven changes in transport capacity may explain the formation of flood terraces in proximal channels. Transport-storage relations can be expected to vary between aggradation and degradation and be influenced by channel conditions at the onset of changes in sediment supply. Published in 2011. This article is a US Government work and is in the public domain in the USA.

Mountain meadow habitats are valued for their ecological importance. They attenuate floods, improve water quality, and support high biodiversity. Many meadow habitats in the western US are degraded, and efforts are increasing to restore these montane meadow ecosystems. Rewatering projects such as pond-and-plug quickly raise the water table by blocking the existing incised stream channel and can result in the rapid recovery of wet meadow habitats. Based on the existing literature, however, it is difficult to determine realistic expectations for outcomes of restoration projects across a range of hydrogeologic conditions. We compared wetland, vegetation, soil carbon, and channel condition variables between ten randomly selected restored and ten paired unrestored montane meadows in California to provide a comparison of habitat conditions. We found that unrestored meadows had a higher proportion of wetland habitat, fewer indicators of channel instability, and greater topsoil carbon stores compared to restored meadows. Restored meadows had more herbaceous biomass within their wetland habitats, but also had more cattle exclosures. The restoration category of the meadow remained important when watershed variables were included in models. While restored meadows were highly degraded prior to project implementation, our results suggest that, in general, conditions do not improve beyond the average conditions of nearby unrestored meadows. Realistic expectations of outcomes and consequences are necessary for managers to make appropriate decisions about restoration options and whether or not to implement rewatering projects that often greatly alter the meadow landscape.