@article {JRN55274, title = {Competition suppresses shrubs during early, but not late, stages of arid grassland{\textendash}shrubland state transition}, journal = {Functional Ecology}, volume = {33}, year = {2019}, month = {3/7/2019}, pages = {1480{\textendash}1490}, abstract = {1. Transitions from grass to woody plant dominance, widely reported in arid systems, are typically attributed to changes in disturbance regimes in combination with abiotic feedbacks, whereas biotic mechanisms such as competition and facilitation are often overlooked. Yet, research in semi-arid and subhumid savannas indicates that biotic interactions are important drivers in systems at risk for state transition. We sought to bridge this divide by experimentally manipulating grass-on-shrub and shrub-on-shrub interactions in early and late stages of grassland{\textendash}shrubland state transition, respectively, and to assess the extent to which these interactions might influence arid land state transition dynamics. 2. Target Prosopis glandulosa shrubs had surrounding grasses or conspecific neighbours left intact or killed with foliar herbicide, and metrics of plant performance were monitored over multiple years for shrubs with and without grass or shrub neighbours. 3. Productivity of small shrubs was enhanced by grass removal in years with above average precipitation, a result not evident in larger shrubs or during dry years. Proxy evidence based on nearest neighbour metrics suggested shrub{\textendash}shrub competition was at play, but our experimental manipulations revealed no such influence. 4. Competition from grasses appears to attenuate the rate at which shrubs achieve the size necessary to modify the physical environment in self-reinforcing ways, but only during the early stages of shrub encroachment. Our results further suggest that at late stages of grassland-to-shrubland state transitions, shrub{\textendash}shrub competition will not slow the rate of shrub expansion, and suggest that maximum shrub cover is regulated by something other than density-dependent mechanisms. We conclude that grass effects on shrubs should be included in assessments of desert grassland state transition probabilities and rates, and that desertification models in arid ecosystems that traditionally focus on disturbance and abiotic feedbacks should be broadened to incorporate spatial and temporal variations in competitive effects.}, keywords = {Chihuahuan Desert, competition, grassland, shrubland, state transition, woody plant encroachment}, doi = {10.1111/1365-2435.13336}, url = {files/bibliography/19-029.pdf}, author = {Nathan Pierce and Steven R. Archer and Brandon T Bestelmeyer} } @article {JRN55134, title = {Grass-Shrub competition in arid lands: An overlooked driver in grassland-shrubland state transition?}, journal = {Ecosystems}, volume = {22}, year = {2019}, month = {04/2019}, pages = {619-628}, chapter = {619}, abstract = {

Traditional models of state transition in arid lands emphasize changes in disturbance regimes and abiotic feedbacks that promote the degradation of grassland into shrubland, while biotic interactions like competition and facilitation are often overlooked. Here, we conducted an experiment to determine whether shrubs have a positive, neutral, or negative effect on grasses, and if these interactions may play a role in grassland-shrubland state transition. Prosopis glandulosa shrub neighbors within 5 m of Bouteloua eriopoda grass patches were left intact (controls) or killed with foliar herbicide, and metrics of grass performance were evaluated over 5 years. We saw no evidence of shrub facilitation of grasses. Instead, grass ANPP responded positively to shrub removal in all years, but more so in years with above-average rainfall. Grass allocation to vegetative reproduction and grass patch size also increased when shrub neighbors were removed. These results demonstrate that biotic interference by shrubs upon grasses reinforce and magnify grazing- and drought-induced abiotic feedbacks during grassland{\textendash}shrubland transitions. Shrub effects on grass should therefore be considered a key process in desert grassland state transitions.

}, keywords = {Bouteloua eriopoda, Chihuahuan Desert, competition, grassland, prosopis glandulosa, Scrublands, state transition, woody plant encroachment}, doi = {doi.org/10.1007/s10021-018-0290-9}, url = {files/bibliography/19-016.pdf}, author = {Nathan Pierce and Steven R. Archer and Brandon T Bestelmeyer and D. K. James} } @article {JRN54925, title = {The Grassland{\textendash}Shrubland Regime Shift in the Southwestern United States: Misconceptions and Their Implications for Management}, journal = {Bioscience}, volume = {68}, year = {2018}, month = {09/2018}, pages = {678-690}, chapter = {678}, abstract = {

Transitions from semiarid grassland to shrubland states are among the most widely recognized examples of regime shifts in terrestrial ecosystems. Nonetheless, the processes causing grassland{\textendash}shrubland transitions and their consequences are incompletely understood. We challenge several misconceptions about these transitions in desert grasslands, including that (a) they are currently controlled by local livestock grazing and drought events, (b) they represent severe land degradation, and (c) restoration of grassland states is impossible. Grassland{\textendash}shrubland transitions are the products of multiple drivers and feedback systems, both ecological and social, interacting at multiple scales of space and time. Grass recovery within shrubland states{\textemdash}with and without shrub removal{\textemdash}produces novel ecosystems that are dissimilar from historical grasslands but that provide important ecosystem services. Projected increases in climate variability are likely to promote the further displacement of perennial grasses by xerophytic shrubs. This article offers guidelines for managing grassland{\textendash}shrubland transitions in the face of changing biophysical and socioeconomic circumstances.

}, keywords = {long term data, novel ecosystems, restoration, social-ecological, state transition, threshold}, doi = {doi.org/10.1093/biosci/biy065}, url = {files/bibliography/18-016.pdf}, author = {Brandon T Bestelmeyer and Debra C. Peters and Steven R. Archer and Dawn M. Browning and Gregory S. Okin and Robert L. Schooley and Nicholas Webb} } @article {JRN54151, title = {Scale-dependent feedbacks between patch size and plant reproduction in desert grassland}, journal = {Ecosystems}, volume = {18}, year = {2015}, month = {01/2015}, pages = {146-153}, chapter = {146}, abstract = {

Theoretical models suggest that scale-dependent feedbacks between plant reproductive success and plant patch size govern transitions from highly to sparsely vegetated states in drylands, yet there is scant empirical evidence for these mechanisms. Scale-dependent feedback models suggest that an optimal patch size exists for growth and reproduction of plants and that a threshold patch organization exists below which positive feedbacks between vegetation and resources can break down, leading to critical transitions. We examined the relationship between patch size and plant reproduction using an experiment in a Chihuahuan Desert grassland. We tested the hypothesis that reproductive effort and success of a dominant grass (Bouteloua eriopoda) would vary predictably with patch size. We found that focal plants in medium-sized patches featured higher rates of grass reproductive success than when plants occupied either large patch interiors or small patches. These patterns support the existence of scale-dependent feedbacks in Chihuahuan Desert grasslands and indicate an optimal patch size for reproductive effort and success in B. eriopoda. We discuss the implications of these results for detecting ecological thresholds in desert grasslands.

}, keywords = {Bouteloua eriopoda, Chihuahuan Desert, critical threshold, desertification, dryland, feedbacks, patch size, resilience, state transition}, doi = {10.1007/s10021-014-9818-9}, url = {/files/bibliography/15-008.pdf}, author = {L.N. Svejcar and Brandon T Bestelmeyer and Danielle Duni and D. K. James} }