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The encroachment of woody plants into grasslands is a global phenomenon with implications for biodiversity and ecosystem function. Understanding and predicting the pace of expansion and the underlying processes that control it are key challenges in the study and management of woody encroachment. Theory from spatial population biology predicts that the occurrence and speed of expansion should depend sensitively on the nature of conspecific density dependence. If fitness is maximized at the low-density encroachment edge, then shrub expansion should be “pulled” forward. However, encroaching shrubs have been shown to exhibit positive feedbacks, whereby shrub establishment modifies the environment in ways that facilitate further shrub recruitment and survival. In this case there may be a fitness cost to shrubs at low density causing expansion to be “pushed” from behind the leading edge. We studied the spatial dynamics of creosotebush (Larrea tridentata), which has a history of encroachment into Chihuahuan Desert grasslands over the past century. We used demographic data from observational censuses and seedling transplant experiments to test the strength and direction of density dependence in shrub fitness along a gradient of shrub density at the grass–shrub ecotone. We also used seed-drop experiments and wind data to construct a mechanistic seed-dispersal kernel, then connected demography and dispersal data within a spatial integral projection model (SIPM) to predict the dynamics of shrub expansion. Contrary to expectations based on potential for positive feedbacks, the shrub encroachment wave is “pulled” by maximum fitness at the low-density front. However, the predicted pace of expansion was strikingly slow (ca. 8 cm/year), and this prediction was supported by independent resurveys of the ecotone showing little to no change in the spatial extent of shrub cover over 12 years. Encroachment speed was acutely sensitive to seedling recruitment, suggesting that this population may be primed for pulses of expansion under conditions that are favorable for recruitment. Our integration of observations, experiments, and modeling reveals not only that this ecotone is effectively stalled under current conditions but also why that is so and how that may change as the environment changes.
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Shrub encroachment of coastal ecosystems depends on dune elevation
Woody plant encroachment is infuenced by interactions between the physical environment and vegetation, which create heterogenous microenvironments some of which favor shrub recruitment through mitigation of the abiotic environment. Encroachment of native shrub, Morella cerifera into grasslands on Hog Island, Virginia has been attributed to warmer winter temperature; however, recruitment of seedlings in grasslands may be impacted by multiple factors at the level of the microhabitat. Our study focuses on a critical gap in understanding factors specifcally infuencing M. cerifera seedling recruitment and survival. By experimentally planting M. cerifera seedlings at varying dune elevations and grass densities, we tested hypotheses that dune elevation infuences the microclimate, soil characteristics and vegetation cover and that grass cover/density is related to shrub establishment. We tested these hypotheses through gathering data from temperature data loggers, conducting soil water content and chloride analyses, and determining percent cover of grasses relative to dune elevation. Results indicate that dune elevation was positively related to moderated temperatures with reduced temperature extremes and vegetation cover/composition that led to favorable locations for M. cerifera establishment and growth. Where dune elevation is>2 m, we document an upper limit of grass cover on natural seedling establishment, suggesting a switch from facilitative to competitive efects with grass density. Overall, our work demonstrates interactions between dune elevation and medium grass density has a facilitative infuence on M. cerifera establishment and can be used for future predictions of shrub growth with rising sea-levels.
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- Award ID(s):
- 1832221
- PAR ID:
- 10534139
- Publisher / Repository:
- Plant Ecology, Springer
- Date Published:
- Journal Name:
- Plant Ecology
- ISSN:
- 1385-0237
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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{"Abstract":["The encroachment of woody plants into grasslands is a global\n phenomenon with implications for biodiversity and ecosystem\n function. Understanding and predicting the pace of expansion and the\n underlying processes that control it are key challenges in the study\n and management of woody encroachment. Theory from spatial population\n biology predicts that the occurrence and speed of population\n expansion should depend sensitively on the nature of conspecific\n density dependence. If fitness is maximized at the low-density\n encroachment edge then shrub expansion should be "pulled"\n forward. However, encroaching shrubs have been shown to exhibit\n positive feedbacks, whereby shrub establishment modifies the\n environment in ways that facilitate further shrub recruitment and\n survival. In this case there may be a fitness cost to shrubs at low\n density causing expansion to be "pushed" from behind the\n leading edge. We studied the spatial dynamics of creosotebush\n (Larrea tridentata), which has a history of\n encroachment into Chihuahuan Desert grasslands over the past\n century. We used demographic data from observational censuses and\n seedling transplant experiments to test the strength and direction\n of density dependence in shrub fitness along a gradient of shrub\n density at the grass-shrub ecotone. We also used seed-drop\n experiments and wind data to construct a mechanistic seed dispersal\n kernel, then connected demography and dispersal data within a\n spatial integral projection model (SIPM) to predict the dynamics of\n shrub expansion. The SIPM predicted that, contrary to expectations\n based on potential for positive feedbacks, the shrub encroachment\n wave is "pulled" by maximum fitness at the low-density\n front. However, the predicted pace of expansion was strikingly slow\n (ca. 8 cm/yr), and this prediction was supported by independent\n re-surveys of the ecotone showing little to no change in spatial\n extent of shrub cover over 12 years. Encroachment speed was acutely\n sensitive to seedling recruitment, suggesting that this population\n may be primed for pulses of expansion under conditions that are\n favorable for recruitment. Our integration of observations,\n experiments, and modeling reveals not only that this ecotone is\n effectively stalled under current conditions, but also\n why that is so and how that may change as the\n environment changes."]}more » « less
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null (Ed.)Abstract Aims 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 , and Aristida purpurea ) and shrub ( Prosopis glandulosa , Atriplex canescens , and Larrea 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 N 2 -fixing deciduous shrub P. glandulosa was 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 give P. glandulosa a 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.more » « less
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Abstract Herbivores can be drivers of ecosystem change by triggering and reinforcing vegetation transitions. Such processes may be prevalent in drylands with low productivity where herbivore abundances are linked to climate‐driven resource pulses. In the Chihuahuan Desert, ecosystems are being transformed from black grama (Bouteloua eriopoda) grasslands to honey mesquite (Neltuma[formerlyProsopis]glandulosa) shrublands. Domestic livestock, exotic African oryx (Oryx gazella), and native rodents and lagomorphs have all been implicated as drivers of these transitions through multiple mechanisms affecting different plant life stages. Across shrub encroachment gradients, we paired a long‐term (21 years) herbivore exclusion experiment focused on established perennial grasses with field trials measuring herbivory risk for perennial grass seedlings. We evaluated the roles of cattle, oryx, and native herbivores in reducing grass cover, and tested whether herbivore effects on grass cover and seedling mortality varied among ecosystem states (grassland, ecotone, and shrubland). Cattle and African oryx did not contribute strongly to vegetation dynamics. However, long‐term exclusion of rodents and lagomorphs led to two‐to‐threefold increases in perennial grass cover compared to control plots (with open access to all herbivores) in shrub‐encroached states where mesquite shrubs provided these herbivores with cover from predators. Likewise, herbivory of perennial grass seedlings was highest in the shrub‐encroached states and was driven by rodents. Our results indicate that native rodents and lagomorphs exert strong control over perennial grass dynamics, creating positive feedbacks mediated by changes in habitat structure that can reinforce grassland–shrubland transitions in drylands.more » « less
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Dryland environments are experiencing shifting ecogeomorphic patterns due to climatic changes and anthropogenic activities, resulting in a shift from grasslands to shrub-dominated landscapes. This dissertation investigates the effects from increasingly variable monsoonal precipitation and ecogeomorphic connectivity on perennial grass growth, litter distribution, and soil organic matter in drylands, with a focus on grass-shrub ecotones. Field experiments were conducted in the Chihuahuan Desert at the Jornada Basin Long-Term Ecological Research (LTER) site using a precipitation manipulation system and connectivity modifiers (ConMods) to assess their effects on plant productivity, recruitment, and soil nutrient distribution. Results show that reducing connectivity, combined with increased monsoonal precipitation, can enhance perennial grass productivity and recruitment, and affect the distribution of soil organic matter and non-photosynthetic vegetation. These findings contribute to our understanding of how aeolian processes and shifting precipitation regimes will shape vegetation patterns and soil properties in dryland environments under future climate scenarios. This research provides insights into potential mitigation strategies for combating shrub encroachment and promoting the sustainability of dryland ecosystems.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1007/s11258-024-01453-2
