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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."]}
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Mega-Monsoon Experiment (MegaME) Vegetation Sampling Data from the Sevilleta National Wildlife Refuge, New Mexico
Shrub encroachment is a global phenomenon. Both the causes and consequences of shrub encroachment vary regionally and globally. In the southwestern US a common native C3 shrub species, creosotebush, has invaded millions of hectares of arid and semi-arid C4-dominated grassland. At the Sevilleta LTER site, it appears that the grassland-shrubland ecotone is relatively stable, but infill by creosotebush continues to occur. The consequences of shrub encroachment have been and continue to be carefully documented, but the ecological drivers of shrub encroachment in the southwestern US are not well known. One key factor that may promote shrub encroachment is grazing by domestic livestock. However, multiple environmental drivers have changed over the 150 years during which shrub expansion has occurred through the southwestern US. Temperatures are warmer, atmospheric CO2 has increased, drought and rainy cycles have occurred, and grazing pressure has decreased. From our prior research we know that prolonged drought greatly reduces the abundance of native grasses while having limited impact on the abundance of creosotebush in the grass-shrub ecotone. So once established, creosotebush populations are persistent and resistant to climate cycles. We also know that creosotebush seedlings tend to appear primarily when rainfall during the summer monsoon is well above average. However, high rainfall years also stimulate the growth of the dominant grasses creating a competitive environment that may not favor seedling establishment and survival. The purpose of the Mega-Monsoon Experiment (MegaME) is twofold. First, this experiment will determine if high rainfall years coupled with (simulated) grazing promote the establishment and growth of creosotebush seedlings in the grassland-shrubland ecotone at Sevilleta, thus promoting infill and expansion of creosotebush into native grassland. Second, MegaME will determine if a sequence of wet summer monsoons will promote the establishment and growth of native C4 grasses in areas where creosotebush is now dominant, thus demonstrating that high rainfall and dispersal limitation prevent grassland expansion into creosotebush shrubland.
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- Award ID(s):
- 1655499
- PAR ID:
- 10424110
- Publisher / Repository:
- Environmental Data Initiative
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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