Title: 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.
TargetProsopis glandulosashrubs 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.
Aplain language summaryis available for this article.
Niu, Furong; Pierce, Nathan A.; Archer, Steven R.; Okin, Gregory S.(
, Plant and Soil)
AbstractAims
Grassland-to-shrubland transition is a common form of land degradation in drylands worldwide. It is often attributed to changes in disturbance regimes, particularly overgrazing. A myriad of direct and indirect effects (e.g., accelerated soil erosion) of grazing may favor shrubs over grasses, but their relative importance is unclear. We tested the hypothesis that topsoil “winnowing” by wind erosion would differentially affect grass and shrub seedling establishment to promote shrub recruitment over that of grass.
Methods
We monitored germination and seedling growth of contrasting perennial grass (Bouteloua eriopoda,Sporobolus airoides, andAristida purpurea) and shrub (Prosopis glandulosa,Atriplex canescens, andLarrea tridentata) functional groups on field-collected non-winnowed and winnowed soils under well-watered greenhouse conditions.
Results
Non-winnowed soils were finer-textured and had higher nutrient contents than winnowed soils, but based on desorption curves, winnowed soils had more plant-available moisture. Contrary to expectations, seed germination and seedling growth on winnowed and non-winnowed soils were comparable within a given species. The N2-fixing deciduous shrubP. glandulosawas first to emerge and complete germination, and had the greatest biomass accumulation of all species.
Conclusions
Germination and early seedling growth of grasses and shrubs on winnowed soils were not adversely nor differentially affected comparing with that observed on non-winnowed soils under well-watered greenhouse conditions. Early germination and rapid growth may giveP. glandulosaa competitive advantage over grasses and other shrub species at the establishment stage in grazed grasslands. Field establishment experiments are needed to confirm our findings in these controlled environment trials.
Archer, Steven R.; Peters, Debra P. C.; Burruss, N. Dylan; Yao, Jin(
, Ecosphere)
Abstract
Grassland‐to‐shrubland state change has been widespread in arid lands globally. Long‐term records at the Jornada Basin USDA‐LTER site in the North American Chihuahuan Desert document the time series of transition from grassland dominance in the 1850s to shrubland dominance in the 1990s. This broadscale change ostensibly resulted from livestock overgrazing in conjunction with periodic drought and represents the classic “grassland‐to‐shrubland” regime shift. However, finer‐scale observations reveal a more nuanced view of this state change that includes transitions from dominance by one shrub functional type to another (e.g., based on leaf habit [evergreen vs. deciduous], N2fixation potential, and drought tolerance). We analyzed the Jornada Basin historic vegetation data using a fine‐scale grid and classified the dominant vegetation in the resulting 890 cells on each of four dates (1858, 1915, 1928, and 1998). This analysis allowed us to quantify on contrasting soil geomorphic units the rate and spatial distribution of: (1) state change from grasslands to shrublands across the Jornada Basin, (2) transitions between shrub functional groups, and (3) transitions from shrub‐to‐grass dominance. Results from our spatially explicit, decadal timescale perspective show that: (1) shrubland ecosystems developing on former grasslands were spatially and temporally more dynamic than has been generally presumed, (2) in some locations, shrublands initially developing on grasslands subsequently transitioned to ecosystems dominated by a different shrub functional type, with these changes in shrub composition likely involving changes in soil properties, and (3) some shrub‐dominated locations have reverted to grass dominance. Accordingly, traditional, broad characterizations of “grassland‐to‐shrubland” state change may be too simplistic. An accounting of these complexities and transitions from one shrub functional group to another is important for projecting state change consequences for ecosystem processes. Understanding the mechanisms, drivers, and influence of interactions between patterns and processes on transitions between shrub states defined by woody plant functional types will be germane to predicting future landscape change.
Muldavin, Esteban; Collins, Scott(
, Environmental Data Initiative)
Disturbance from fire can affect the abundance and distribution of shrubs and grasses in arid ecosystems. In particular, fire may increase grass and forb production while hindering shrub encroachment. Therefore, prescribed fires are a common management tool for maintaining grassland habitats in the southwest. However, Bouteloua eriopoda (black grama), a dominant species in Chihuahuan Desert grassland, is highly susceptible to fire resulting in death followed by slow recovery rates. A prescribed fire on the Sevilleta National Wildlife refuge in central New Mexico in 2003 provided the opportunity to study the effects of infrequent fires on vegetation in this region. This study was conducted along a transition zone where creosote bushes (Larrea tridentata) are encroaching on a black grama grassland. Before and after the fire, above ground plant productivity and composition were monitored from 2003 to present. Following the prescribed fire, there were fewer individual grass clumps and less above ground grass cover in burned areas compared to unburned areas. This decrease in productivity was primarily from a loss of B. eriopoda. Specifically, B. eriopoda density and cover were significantly lower following the fire with a slow recovery rate in the five years following the fire. Other grasses showed no such adverse response to burning. Data were collected from 2004-2013 and 2018. Data were not collected for 2014-2017.
Mount, Hailey E.; Smith, Melinda D.; Knapp, Alan K.; Griffin‐Nolan, Robert J.; Collins, Scott L.; Atkins, David H.; Stears, Alice E.; Laughlin, Daniel C.(
, Journal of Ecology)
Abstract
Plant populations are limited by resource availability and exhibit physiological trade‐offs in resource acquisition strategies. These trade‐offs may constrain the ability of populations to exhibit fast growth rates under water limitation and high cover of neighbours. However, traits that confer drought tolerance may also confer resistance to competition. It remains unclear how fitness responses to these abiotic conditions and biotic interactions combine to structure grassland communities and how this relationship may change along a gradient of water availability.
To address these knowledge gaps, we estimated the low‐density growth rates of populations in drought conditions with low neighbour cover and in ambient conditions with average neighbour cover for 82 species in six grassland communities across the Central Plains and Southwestern United States. We assessed the relationship between population tolerance to drought and resistance to competition and determined if this relationship was consistent across a precipitation gradient. We also tested whether population growth rates could be predicted using plant functional traits.
Across six sites, we observed a positive correlation between low‐density population growth rates in drought and in the presence of interspecific neighbours. This positive relationship was particularly strong in the grasslands of the northern Great Plains but weak in the most xeric grasslands. High leaf dry matter content and a low (more negative) leaf turgor loss point were associated with high population growth rates in drought and with neighbours in most grassland communities.
Synthesis: A better understanding of how both biotic and abiotic factors impact population fitness provides valuable insights into how grasslands will respond to extreme drought. Our results advance plant strategy theory by suggesting that drought tolerance increases population resistance to interspecific competition in grassland communities. However, this relationship is not evident in the driest grasslands, where above‐ground competition is likely less important. Leaf dry matter content and turgor loss point may help predict which populations will establish and persist based on local water availability and neighbour cover, and these predictions can be used to guide the conservation and restoration of biodiversity in grasslands.
Wojcikiewicz, Robert; Ji, Wenjie; Hanan, Niall P.(
, New Phytologist)
Summary
The Jornada Basin Long‐Term Ecological Research Site (JRN‐LTER, or JRN) is a semiarid grassland–shrubland in southern New Mexico, USA. The role of intraspecific competition in constraining shrub growth and establishment at the JRN and in arid systems, in general, is an important question in dryland studies.
Using information on shrub distributions and growth habits at the JRN, we present a novel landscape‐scale (c. 1 ha) metric (the ‘competition index’, CI), which quantifies the potential intensity of competitive interactions. We map and compare the intensity of honey mesquite (Prosopis glandulosa, Torr.) competition spatially and temporally across the JRN‐LTER, investigating associations of CI with shrub distribution, density, and soil types.
The CI metric shows strong correlation with values of percent cover. Mapping CI across the Jornada Basin shows that high‐intensity intraspecific competition is not prevalent, with few locations where intense competition is likely to be limiting further honey mesquite expansion.
Comparison of CI among physiographic provinces shows differences in average CI values associated with geomorphology, topography, and soil type, suggesting that edaphic conditions may impose important constraints on honey mesquite and growth. However, declining and negative growth rates with increasing CI suggest that intraspecific competition constrains growth rates when CI increases abovec. 0.5.
Pierce, Nathan A., Archer, Steven R., Bestelmeyer, Brandon T., and Yahdjian, ed., Laura. Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition. Functional Ecology 33.8 Web. doi:10.1111/1365-2435.13336.
Pierce, Nathan A., Archer, Steven R., Bestelmeyer, Brandon T., & Yahdjian, ed., Laura. Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition. Functional Ecology, 33 (8). https://doi.org/10.1111/1365-2435.13336
Pierce, Nathan A., Archer, Steven R., Bestelmeyer, Brandon T., and Yahdjian, ed., Laura.
"Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition". Functional Ecology 33 (8). Country unknown/Code not available: Wiley-Blackwell. https://doi.org/10.1111/1365-2435.13336.https://par.nsf.gov/biblio/10460551.
@article{osti_10460551,
place = {Country unknown/Code not available},
title = {Competition suppresses shrubs during early, but not late, stages of arid grassland–shrubland state transition},
url = {https://par.nsf.gov/biblio/10460551},
DOI = {10.1111/1365-2435.13336},
abstractNote = {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.TargetProsopis glandulosashrubs 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. Aplain language summaryis available for this article.},
journal = {Functional Ecology},
volume = {33},
number = {8},
publisher = {Wiley-Blackwell},
author = {Pierce, Nathan A. and Archer, Steven R. and Bestelmeyer, Brandon T. and Yahdjian, ed., Laura},
}
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