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  1. Abstract

    Janus is the Roman god of transitions. In many environments, state transitions are an important part of our understanding of ecological change. These transitions are controlled by the interactions between exogenous forcing factors and stabilizing endogenous feedbacks. Forcing factors and feedbacks are typically considered to consist of different processes. We argue that during extreme events, a process that usually forms part of a stabilizing feedback can behave as a forcing factor. And thus, like Janus, a single process can have two faces. The case explored here pertains to state change in drylands where interactions between wind erosion and vegetation form an important feedback that encourages grass‐to‐shrub state transitions. Wind concentrates soil resources in shrub‐centered fertile islands, removes resources through loss of fines to favor deep‐rooted shrubs, and abrades grasses' photosynthetic tissue, thus further favoring the shrub state that, in turn, experiences greater aeolian transport. This feedback is well documented but the potential of wind to act also as a forcing has yet to be examined. Extreme wind events have the potential to act like other drivers of state change, such as drought and grazing, to directly reduce grass cover. This study examines the responses of a grass‐shrub community after two extreme wind events in 2019 caused severe deflation. We measured grass cover and root exposure due to deflation, in addition to shrub height, grass patch size, and grass greenness along 50‐m transects across a wide range of grass cover. Root exposure was concentrated in the direction of erosive winds during the storms and sites with low grass cover were associated with increased root exposure and reduced greenness. We argue that differences between extreme, rare wind events and frequent, small wind events are significant enough to be differences in kind rather than differences in degree allowing extreme winds to behave as endogenous forcings and common winds to participate in an endogenous stabilizing feedback. Several types of state change in other ecological systems in are contextualized within this framework.

     
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  2. Abstract

    Several studies have shown that dust emission (vertical flux,F) can be considered a constant fraction (k) of the saltating flux (horizontal flux,Q), that is, . This coefficient of proportionality, or dust production efficiency factor, is often called the ‘kfactor’ and is fundamentally related to soil properties especially soil texture. Beyond regional and global modeling applications, a practical utility ofkis for air quality regulatory agencies wherekcan be used to estimateFbased on only measurements ofQ, which is more easily measured in the field. Only a few studies have directly estimated thekfactor from soils within potential dust sources even though dust models that represent the sandblasting process typically utilizek. The goal of this study was to compare two methods to calculatekfrom sandy sediments and compare those estimates with an empirical method of calculating thekfactor. The first method (method 1) used the difference betweenFcalculated from two sets of sediment samplers whereas the second method (method 2) used a set of aerosol monitors to measureF. We found that the range ofkvalues from our study are consistent with soil texture‐based estimates ofkand also have the correct order of magnitude. Thus, any of the methods described in our study are appropriate for estimation ofkfor sandy soils.

     
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  3. Abstract

    Plants with crassulacean acid metabolism (CAM) are increasing in distribution and abundance in drylands worldwide, but the underlying drivers remain unknown. We investigate the impacts of extreme drought and CO2enrichment on the competitive relationships between seedlings ofCylindropuntia imbricata(CAM species) andBouteloua eriopoda(C4grass), which coexist in semiarid ecosystems across the Southwestern United States. Our experiments under altered water and CO2water conditions show thatC. imbricatapositively responded to CO2enrichment under extreme drought conditions, whileB. eriopodadeclined from drought stress and did not recover after the drought ended. Conversely, in well‐watered conditionsB. eriopodahad a strong competitive advantage onC. imbricatasuch that the photosynthetic rate and biomass (per individual) ofC. imbricatagrown withB. eriopodawere lower relative to when growing alone. A meta‐analysis examining multiple plant families across global drylands shows a positive response of CAM photosynthesis and productivity to CO2enrichment. Collectively, our results suggest that under drought and elevated CO2concentrations, projected with climate change, the competitive advantage of plant functional groups may shift and the dominance of CAM plants may increase in semiarid ecosystems.

     
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