Density-dependent demography of creosote bush (Larrea tridentata) along grass-shrub ecotones.

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 population
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. The SIPM predicted that, 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/yr), and this prediction was supported by independent
re-surveys of the ecotone showing little to no change in 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.