Grass-Shrub Competition in Arid Lands: An Overlooked Driver in Grassland–Shrubland State Transition?

Pierce, Nathan A.; Archer, Steven R.; Bestelmeyer, Brandon T.; James, Darren

doi:10.1007/s10021-018-0290-9

Cited by 39 publications

(33 citation statements)

References 51 publications

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“…This coincides with a number of recent studies (Maestre et al 2004, Poulos et al 2014Sheley and James 2014;Wright et al 2014;Reisner et al 2015;Llambi et al 2018). The relative importance (sensu Brooker et al 2005) of these biotic interactions likely depends upon abiotic conditions and the ontological development of interacting species (Callaway et al 1996;Fagundes et al 2018;Gao et al 2018;Pierce et al 2018).…”

Section: Discussionsupporting

confidence: 89%

The dark side of facilitation: native shrubs facilitate exotic annuals more strongly than native annuals

Lucero¹,

Noble²,

Haas³

et al. 2019

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Positive interactions enhance biodiversity and ecosystem function, but can also exacerbate biological invasions. Facilitation of exotic invaders by exotic foundation species (invasional meltdown) has been studied extensively, but facilitation of exotic invaders by native foundation species has attracted less attention. Specifically, very few studies have examined the extent that native foundation species facilitate native and exotic competitors. Understanding the processes that mediate interactions between native and exotic species can help explain, predict, and improve management of biological invasions. Here, we examined the effects of native foundation shrubs on the relative abundance of the annual plant community – including native and exotic taxa – from 2015–2018 in a desert ecosystem at Carrizo Plain National Monument, California, USA (elevation: 723 m). Shrub effects varied by year and by the identity of annual species, but shrubs consistently enhanced the abundance of the annual plant community and facilitated both native (n=17 species) and exotic (n=4 species) taxa. However, at the provenance level, exotic annuals were facilitated 2.75 times stronger in abundance than native annuals, and exotic annuals were always more abundant than natives both near and away from shrubs. Our study reaffirms facilitation as an important process in the organisation of plant communities and confirms that both native and exotic species can form positive associations with native foundation species. However, facilitation by native foundation species can exacerbate biological invasions by increasing the local abundance of exotic invaders. Thus, the force of facilitation can have a dark side relevant to ecosystem function and management.

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Section: Discussionsupporting

confidence: 89%

The dark side of facilitation: native shrubs facilitate exotic annuals more strongly than native annuals

Lucero¹,

Noble²,

Haas³

et al. 2019

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“…Furthermore, grasses, with their dense, fibrous root systems, likely utilize shallow soil moisture before it has a chance to percolate to depths where it would be accessible primarily to shrubs (Gherardi & Sala, ; Holdo & Brocato, ; Ward et al, ). Our results are also consistent with observations that in stressful environments, competitive interactions increase with increasing resource availability, as has been found for other Chihuahuan Desert species (Briones, Montana, & Ezcurra, ; Pierce, Archer, Bestelmeyer, & James, ) and as proposed by the stress‐gradient hypothesis (Maestre, Callaway, Valladares, & Lortie, ; Miriti, ), which argues that interactions between plants shift from net positive (facilitation) to net negative (competition) with decreasing environmental stress. The competitive influence of grasses upon small shrubs that we observed would slow the rate at which those individuals achieve the size necessary to tolerate or escape mortality factors such as fire (Wakeling, Staver, & Bond, ) and lengthen the time required for them to begin modifying the physical environment in self‐reinforcing ways, such as concentrating resources beneath their canopies (Li et al, ).…”

Section: Discussionsupporting

confidence: 92%

“…Our experimental manipulations also indicated that shrubs have a neutral effect on one another. Previous work at this research site has demonstrated that P. glandulosa shrubs have a negative (competitive) influence on B. eriopoda grasses (Pierce et al, ), and that B. eriopoda plants can have a negative effect on one another, depending on patch size (Svejcar, Bestelmeyer, Duniway, & James, ). Thus, B. eriopoda experiences both inter‐ and intraspecific competition, whereas P. glandulosa experiences only interspecific competition, and only in early life‐history stages.…”

Section: Discussionmentioning

confidence: 89%

Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition

2019

Self Cite

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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–shrubland state transition, respectively, and to assess the extent to which these interactions might influence arid land state transition dynamics. 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. 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–shrub competition was at play, but our experimental manipulations revealed no such influence. 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–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. A plain language summary is available for this article.

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“…Positive plant effects on resources may result both directly from plant traits and indirectly via soil microbes (Chapman, Langley, Hart, & Koch, 2006; Cornwell et al., 2008). Plant trait‐related mechanisms proposed to underlie soil resource increases beneath woody plants include increased shading, the attraction of grazers and their nutrient‐rich dung, plant access to deeper soil water (SoilW), mycorrhizae and enhanced decomposition of contrasting litter types (Hobbie, 2015; Kéfi, Holmgren, & Scheffer, 2016; Pierce, Archer, Bestelmeyer, & James, 2018; Wilson, 1998).…”

Section: Introductionmentioning

confidence: 99%

Per‐gram competitive effects and contrasting soil resource effects in grasses and woody plants

Wilson

Peltzer

2020

Journal of Ecology

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Plant species differ in their competitive effects by decreasing resource availability via uptake, but in some cases may increase resource availability via non‐uptake pathways. Here we explore differences between grasses and woody plants in their competitive effects, and relate these to differences in resource effects. We grew five species each of grasses and woody plants in monocultures for 8 years. In the final two growing seasons, competitive effects were measured by growing transplants in all monocultures and in plots without neighbours. Total competitive effects were significantly greater for woody plants than that for grasses. In contrast, the competitive effect per gram of grasses was about 17 times greater than that of woody plants. For grasses, soil water (SoilW) and soil available N (SoilN) decreased significantly with increasing biomass. In contrast, for woody plants, SoilW and SoilN increased significantly with increasing biomass. The results suggest that the intense per‐gram competitive effects in grasses are related to the uptake of soil resources, and that the significantly lower per‐gram competitive effects of woody plants may be related to their positive effects on soil resources. Synthesis. The results link differences in competitive effects between grasses and woody plants to differences in the direction of their effects on soil resources. These differences may contribute to the entrainment of negative feedback in grasslands, excluding trees by means of strong competition, and the entrainment of positive feedback beneath woody plants establishing in grasslands, resulting in a state change from grassland to woody vegetation.

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Grass-Shrub Competition in Arid Lands: An Overlooked Driver in Grassland–Shrubland State Transition?

Cited by 39 publications

References 51 publications

The dark side of facilitation: native shrubs facilitate exotic annuals more strongly than native annuals

The dark side of facilitation: native shrubs facilitate exotic annuals more strongly than native annuals

Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition

Per‐gram competitive effects and contrasting soil resource effects in grasses and woody plants

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