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Future drought is projected to occur under warmer temperature conditions as climate change progresses, referred to here as global-change-type drought, yet quantitative assessments of the triggers and potential extent of drought-induced vegetation die-off remain pivotal uncertainties in assessing climate-change impacts. Of particular concern is regional-scale mortality of overstory trees, which rapidly alters ecosystem type, associated ecosystem properties, and land surface conditions for decades. Here, we quantify regional-scale vegetation die-off across southwestern North American woodlands in 2002-2003 in response to drought and associated bark beetle infestations. At an intensively studied site within the region, we quantified that after 15 months of depleted soil water content, >90% of the dominant, overstory tree species (Pinus edulis, a piǫn) died. The die-off was reflected in changes in a remotely sensed index of vegetation greenness (Normalized Difference Vegetation Index), not only at the intensively studied site but also across the region, extending over 12,000 km2 or more; aerial and field surveys confirmed the general extent of the die-off. Notably, the recent drought was warmer than the previous subcontinental drought of the 1950s. The limited, available observations suggest that die-off from the recent drought was more extensive than that from the previous drought, extending into wetter sites within the tree species' distribution. Our results quantify a trigger leading to rapid, drought-induced die-off of overstory woody plants at subcontinental scale and highlight the potential for such die-off to be more severe and extensive for future global-change-type drought under warmer conditions.

Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere–atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function. The mechanisms underlying drought-induced tree mortality are not fully resolved. Here, the authors show that, across multiple tree species, loss of xylem conductivity above 60% is associated with mortality, while carbon starvation is not universal.

Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotranspiration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change-such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes.

In coming decades, global climate changes are expected to produce large shifts in vegetation distributions at unprecedented rates. These shifts are expected to be most rapid and extreme at ecotones, the boundaries between ecosystems, particularly those in semiarid landscapes. However, current models do not adequately provide for such rapid effects--particularly those caused by mortality--largely because of the lack of data from field studies. Here we report the most rapid landscape-scale shift of a woody ecotone ever documented: in northern New Mexico in the 1950s, the ecotone between semiarid ponderosa pine forest and pinon-juniper woodland shifted extensively (2 km or more) and rapidly (5 years) through mortality of ponderosa pines in response to a severe drought. This shift has persisted for 40 years. Forest patches within the shift zone became much more fragmented, and soil erosion greatly accelerated. The rapidity and the complex dynamics of the persistent shift point to the need to represent more accurately these dynamics, especially the mortality factor, in assessments of the effects of climate change

Amazon forests are being degraded by myriad anthropogenic disturbances, altering ecosystem and climate function. We analyzed the effects of a range of land-use and climate-change disturbances on fine-scale canopy structure using a large database of profiling canopy lidar collected from disturbed and mature Amazon forest plots. At most of the disturbed sites, surveys were conducted 10–30 years after disturbance, with many exhibiting signs of recovery. Structural impacts differed in magnitude more than in character among disturbance types, producing a gradient of impacts. Structural changes were highly coordinated in a manner consistent across disturbance types, indicating commonalities in regeneration pathways. At the most severely affected site – burned igapó (seasonally flooded forest) – no signs of canopy regeneration were observed, indicating a sustained alteration of microclimates and consequently greater vulnerability to transitioning to a more open-canopy, savanna-like state. Notably, disturbances rarely shifted forests beyond the natural background of structural variation within mature plots, highlighting the similarities between anthropogenic and natural disturbance regimes, and indicating a degree of resilience among Amazon forests. Studying diverse disturbance types within an integrated analytical framework builds capacity to predict the risk of degradation-driven forest transitions.

In the future, Earth will be warmer, precipitation events will be more extreme, global mean sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes will also occur at an accelerated rate as developed areas increase in size and population density. We now have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC) that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do not exist for how these C&LUC will likely impact Earth‐surface processes and hazards. In some cases, we have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models and utilizing these tools will require close collaboration among Earth‐surface scientists and Earth‐system modelers. This paper assesses the state‐of‐the‐art tools and data that are being used or could be used to forecast changes in the state of Earth's surface as a result of likely future C&LUC. We also propose strategies for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration across disciplines are necessary. The main body of the paper addresses cross‐cutting issues, including the importance of nonlinear/threshold‐dominated interactions among topography, vegetation, and sediment transport, as well as the importance of alternate stable states and extreme, rare events for understanding and forecasting Earth‐surface response to C&LUC. Five supplements delve into different scales or process zones (global‐scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in detail. Key Points We review models and data useful for forecasting Earth surface changes We identify key knowledge gaps required to forecast Earth surface changes We strategize how geomorphologists and Earth‐systems modelers can collaborate

Ecological and hydrological processes can interact strongly in landscapes, yet these processes are often studied separately. One particularly important interaction between these processes in patchy semiarid lands is how vegetation patches serve to obstruct runoff and then how this retained water increases patch growth that, in turn, provides feedbacks to the system. Such ecohydrological interactions have been mostly demonstrated for semiarid landscapes with distinctly banded vegetation patterns. In this paper, we use data from our studies and from the literature to evaluate how strongly four ecohydrological interactions apply across other patchy semiarid vegetations, and how these interactions are affected by disturbances. We specifically address four questions concerning ecohydrological interactions: (1) if vegetation patches obstruct runoff flows during rainfall events, how much more soil water is stored in these patches compared to open interpatch areas; (2) if inputs of water are higher in patches, how much stronger is the pulse of plant growth compared to interpatches; (3) if more soil water in patches promotes greater biological activity by organisms such as earthworms that create macropores, how much does this improve soil infiltrability; and (4) if vegetation patches are damaged on a hillslope, how much does this increase runoff and erosion and decrease biomass production? We used the trigger-transfer-reserve-pulse framework developed for Australian semiarid woodlands to put these four questions into a landscape context. For a variety of patchy semiarid vegetation types in Australia, Europe, and North America, we found that patches significantly stored more soil water, produced more growth and had better infiltrability than interpatches, and that runoff and erosion can markedly increase on disturbed hillslopes. However, these differences varied greatly and appeared to depend on factors such as the intensity and amount of input events (rainstorms) and type of topography, soils, and vegetation. Experimental and modeling studies are needed to better quantify how these factors specifically affect ecohydrological interactions. Our current findings do support the conclusion that vegetation patches and runoff-erosion processes do strongly interact in many semiarid landscapes across the globe, not just banded landscapes.

Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part due to a lack of sufficient high-resolution measurements and an incomplete understanding of boundary conditions, soil properties, and other environmental aspects. A continuous and extreme rainfall experiment on an artificial hillslope at Biosphere 2's Landscape Evolution Observatory (LEO) resulted in saturation excess overland flow and gully erosion in the convergent hillslope area. An array of 496 soil moisture sensors revealed a two-step saturation process. First, the downward movement of the wetting front brought soils to a relatively constant but still unsaturated moisture content. Second, soils were brought to saturated conditions from below in response to rising water tables. Convergent areas responded faster than upslope areas, due to contributions from lateral subsurface flow driven by the topography of the bottom boundary, which is comparable to impermeable bedrock in natural environments. This led to the formation of a groundwater ridge in the convergent area, triggering saturation excess runoff generation. This unique experiment demonstrates, at very high spatial and temporal resolution, the role of convergence on subsurface storage and flow dynamics. The results bring into question the representation of saturation excess overland flow in conceptual rainfall-runoff models and land-surface models, since flow is gravity-driven in many of these models and upper layers cannot become saturated from below. The results also provide a baseline to study the role of the co-evolution of ecological and hydrological processes in determining landscape water dynamics during future experiments in LEO.

A large proportion of rainfall in dryland ecosystems is intercepted by plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, woody-plant genus: Juniperus. We observed substantial improvement in plant water status, exceeding 1.0 MPa water potential for drought-stressed plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest.

Ecosystem responses to key climate drivers are reflected in phenological dynamics such as the timing and degree of \"green-up\" that integrate responses over spatial scales from individual plants to ecosystems. This integration is clearest in ecosystems dominated by a single species or life form, such as seasonally dynamic grasslands or more temporally constant evergreen forests. Yet many ecosystems have substantial contribution of cover from both herbaceous and woody evergreen plants. Responses of mixed woody—herbaceous ecosystems to climate are of increasing concern due to their extensive nature, the potential for such systems to yield more complex responses than those dominated by a single life form, and projections that extreme climate and weather events will increase in frequency and intensity with global warming. We present responses of a mixed woody—herbaceous ecosystem type to an extreme event: regional-scale piñon pine mortality following an extended drought and the subsequent herbaceous green-up following the first wet period after the drought. This example highlights how reductions in greenness of the slower, more stable evergreen woody component can rapidly be offset by increases associated with resources made available to the relatively more responsive herbaceous component. We hypothesize that such two-phase phenological responses to extreme events are characteristic of many mixed woody—herbaceous ecosystems.