Competition suppresses shrubs during early, but not late, stages of arid grassland-to-shrubland transition

Nathan A. Pierce, Steven R. Archer, Brandon T. Bestelmeyer

 

Jornada Grassland

Desert grasslands worldwide are experiencing a form of degradation wherein the landscape becomes dominated by shrubs and bare ground. This is a potentially irreversible change that has numerous negative consequences for ecosystems and the goods and services they provide (e.g. soil erosion, loss of suitable grazing land for ranchers and herders, etc.). Traditionally, these arid grassland-to-shrubland transitions have been attributed to drought and overgrazing, which set into motion changes in the physical environment that impede grass survival while promoting shrub proliferation. The role that interactions between plants, such as competition for soil water or nutrients, play in this degradation has been largely overlooked, despite considerable evidence suggesting that grass-shrub interactions influence rates, dynamics and patterns of vegetation change in ecosystems with higher rainfall. To assess how plant-plant interactions may influence grassland-to-shrubland transitions we conducted an experiment to determine if a common shrub invader of Chihuahuan Dessert grasslands in New Mexico, USA, experiences (1) competition from neighboring grasses in the early stages of grassland-to-shrubland transition, and (2) competition from neighboring shrubs in latter stages of the transition. We did this by monitoring, over several years, individual shrubs that had either their grass or shrub neighbors left intact or removed. Small shrubs with grass neighbors removed grew more than those with grass neighbors intact, but large shrubs grew the same regardless of grass neighbor presence/absence. Grasses may therefore slow the rate at which shrubs proliferate, but only to a certain point. Growth of shrubs whose shrub neighbors were killed was comparable to that of shrubs whose shrub neighbors were intact. This unexpected lack of interaction means that shrub-shrub competition will not slow the rate of shrub proliferation. We conclude (i) that grass effects on shrubs should be included in predictions of desert grassland-to-shrubland transition probabilities and rates, and (ii) that desertification models in arid ecosystems that have traditionally focused on disturbance and modification of the physical environment should be broadened to incorporate competitive effects.

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