Long-term tree population growth can predict woody encroachment patterns

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Increasing presence of woody plants in dryland ecosystems, also known as “woody encroachment,” is commonly attributed to anthropogenic land-use changes such as livestock grazing and wildfire suppression. However, empirical evidence to support these external drivers has not uncovered a unifying mechanism. We test whether plant demographic processes could be responsible for woody encroachment using tree-ring data from pinyon and juniper woodland populations in the western United States. Our results indicate that woody encroachment patterns can largely be predicted by a null model based only on steady tree population growth. Modern increases in woodland density, which are typically viewed as a natural resource management problem, may therefore be a result of long-term population expansion and recovery.