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

Abstract

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.

Shrub‐encroached grassland as an alternative stable state in semiarid steppe regions: Evidence from community stability and assembly

“…The increase of species diversity would intensify resource competition among herbaceous species (Chapin, Walker, Fastie, & Sharman, 1994; Purschke et al, 2013). In addition, there might be direct competition between shrubs and herbaceous species because higher shrubs have a greater competitive advantage in light and water resources than herbaceous species (Peng et al, 2013; Pierce, Archer, & Bestelmeyer, 2019). In contrast to the Simpson's diversity, the vegetation cover had a significant positive effect on the SES MPD , suggesting that the higher vegetation cover slows down competitive exclusion (Figures 3 and 4b).…”

Is shrub-encroached grassland a stable state in the semi-arid steppe region?

“…The decrease in abiotic stress and the increase in species richness may intensify resource competition among herbaceous species (Chapin et al,1994;Purschke et al, 2013). On the other hand, there may be direct competition between shrubs and herbaceous species, and higher shrubs have a higher competitive advantage in light and water resources than herbaceous species (Peng et al, 2013;Pierce et al, 2019), accelerating competitive ability for resources. Thus, niche overlap in open space should become significant and competition should consequently intensify.…”

“…microphylla . It may require some human interference practices that inhibit C mircophylla growth (Xiong et al, 2004;Alados et al, 2019) and incorporates biotic perspectives into the framework of restoration (Koyama et al, 2015;Pierce et al, 2019).…”

“…However, there is still a lack of research on the community assmbly mechanisms in secondary succession of shrub-encroached grassland considering the significant differences between nonshrub-encroached degraded grassland and shrub-encroached grassland. Although some reseachers pointed out that shrubs can facilitate the establishment of surrounding herbaceous communities (Koyama et al, 2015) by reducing abiotic stress (Maestre et al, 2009;Soliveres and Eldridge, 2014), many studies comfirmed that shrub encroachment influenced the cycling of nutrient such as nitrogen, carbon and surface water and energy flow (Peng et al, 2013), causing grassland patches or degradation (Xiong et al, 2004;Zhou et al, 2018;Pierce et al, 2019).…”

ExaminingFouquieria splendensin an environmental and ecological context: Effect of topography and interspecific neighbors on ocotillo morphology and distribution

“…Arroyos are characterized as washes that fill up seasonally with sufficient rain while bajadas are slopes that exhibit a gradient of water availability (McAuliffe, 1994; Stromberg, 2007). These geological features have allowed researchers to study both diverse competition dynamics and topographic effects on the growth of various desert plant species in the field (Bowers and Lowe, 1986; Nobel and Zutta, 2007; Pierce, 2019); however these features have been overlooked in ocotillo studies. Wider studies on desert plant communities have investigated the interaction between morphology and competition.…”