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Abstract Hysteresis is a fundamental characteristic of alternative stable state theory, yet evidence of hysteresis is rare. In mesic grasslands, fire frequency regulates transition from grass‐ to shrub‐dominated system states. It is uncertain, however, if increasing fire frequency can reverse shrub expansion, or if grass‐shrub dynamics exhibit hysteresis. We implemented annual burning in two infrequently burned grasslands and ceased burning in two grasslands burned annually. With annual fires, grassland composition converged on that of long‐term annually burned vegetation due to rapid recovery of grass cover, although shrubs persisted. When annual burning ceased, shrub cover increased, but community composition did not converge with a long‐term infrequently burned reference site because of stochastic and lagged dispersal by shrubs, reflecting hysteresis. Our results demonstrated that annual burning can slow, but not reverse, shrub encroachment. In addition, reversing fire frequencies resulted in hysteresis because vegetation trajectories from grassland to shrubland differed from those of shrubland to grassland.
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Minimal mortality and rapid recovery of the dominant shrub Larrea tridentata following an extreme cold event in the northern Chihuahuan Desert
Abstract QuestionsWoody encroachment into grasslands is a worldwide phenomenon partially influenced by climate change, including extreme weather events.Larrea tridentatais a common shrub throughout the warm deserts of North America that has encroached into grasslands over the past 150 years. Physiological measurements suggest that the northern distribution ofL. tridentatais limited by cold temperatures; thus extreme winter events may slow or reverse shrub expansion. We tested this limitation by measuring the response of individualL. tridentatashrubs to an extreme winter cold (−31°C) event to assess shrub mortality and rate of recovery of surviving shrubs. LocationSevilleta National Wildlife Refuge, Socorro County, New Mexico, USA. MethodsCanopy dieback and recovery following an extreme cold event were measured for 869 permanently marked individualL. tridentatashrubs in grass–shrub ecotone and shrubland sites. Individual shrubs were monitored for amount of canopy dieback, rate of recovery, and seed set for three growing seasons after the freeze event. ResultsShrubs rapidly suffered a nearly complete loss of canopy leaf area across all sites. Although canopy loss was high, mortality was low and 99% of shrubs resprouted during the first growing season after the freeze event. Regrowth rates were similar within ecotone and shrubland sites, even when damage by frost was larger in the latter. After three years of recovery,L. tridentatacanopies had regrown on average 23–83% of the original pre‐freeze canopy sizes across the sites. ConclusionsWe conclude that isolated extreme cold events may temporarily decrease shrubland biomass but they do not slow or reverse shrub expansion. These events are less likely to occur in the future as regional temperatures increase under climate change.
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- Award ID(s):
- 1655499
- PAR ID:
- 10459622
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Vegetation Science
- Volume:
- 30
- Issue:
- 5
- ISSN:
- 1100-9233
- Page Range / eLocation ID:
- p. 963-972
- 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
