Demography and dispersal at a grass‐shrub ecotone: A spatial integral projection model for woody plant encroachment

Abstract

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.

Correlative ecological niche model applications to predicting landscape-scale woody plant encroachment in Kansas tallgrass prairie systems

“…Considerable effort has been invested in developing remote-sensing approaches to characterizing WPE processes retrospectively [ 8 , 25 , 32 ]. However, less attention has been paid to the more exploratory challenge of anticipating areas with greater or lesser potential for further WPE [ 33 , 34 ]. A notable exception is the work of Donovan, Burnett [ 35 ], who found that WPE from windbreaks was most likely along roadways, and less along other features, suggesting a predictive local nature to WPE, albeit in a different landscape context than that of this study.…”

Sensitivity of root production to long‐term aridity under environmental perturbations in Chihuahuan Desert ecosystems

“…The focal dryland ecosystems, while geographically proximal, are undergoing state transitions over time (Zinnert et al, 2021). Under future climates, we expect the Chihuahuan Desert shrubland to expand to replace Chihuahuan Desert grassland (Caracciolo et al, 2016;D'Odorico et al, 2010;Drees et al, 2023), and Desert grassland to overtake Plains grassland (Chung et al, 2019;Collins et al, 2020).…”

Shrub expansion raises both aboveground and underground multifunctionality on a subtropical plateau grassland: coupling multitrophic community assembly to multifunctionality and functional trade-off

“…Shrub expansion triggers major ecosystem changes ( Cannone et al, 2022 ) with positive and negative effects on aboveground vegetation ( Drees et al, 2023 ) and belowground soil ecosystems ( Yannarell et al, 2014 ; Ding et al, 2020 ; Ma et al, 2023 ). However, it remains debatable whether shrub expansion improves or degrades land.…”

“…However, it remains debatable whether shrub expansion improves or degrades land. Some studies have shown that shrub expansion should be regarded as a degradation process ( Khazieva et al, 2022 ; Root-Bernstein and Hoag, 2022 ; Qu et al, 2023 ) due to its generally negative effects on biodiversity ( Ding et al, 2020 ; Guo et al, 2022 ; Song and Wang, 2022 ; Gómez-García et al, 2023 ), livestock production ( Eldridge and Ding, 2020 ; Root-Bernstein and Hoag, 2022 ), economy ( Drees et al, 2023 ), and ecosystem functions ( Li et al, 2022b ). Other studies have revealed that shrubs do not have a negative effect on soil quality ( Ambrosino et al, 2023 ) but enhance nutrient cycling, primary production, soil water and nutrients ( Chandregowda et al, 2018 ; Li D. et al, 2023 ), and bacterial diversity ( Guo et al, 2022 ).…”