Can long-term population growth explain woody encroachment?
Pinyon-juniper woodlands are one of the most widespread forest types in North America, and they support diverse wildlife communities, Indigenous cultural values, and many other ecosystem services. Contemporary woodland management has focused primarily on responding to observations of increasing woodland density and expansion, also known as “encroachment,” over the last century. Woody encroachment in dryland ecosystems 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.
Photo of a dense pinyon-juniper forest. Location: Sweetwater Valley, Nevada. 2024.
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. This is important because understanding the drivers of increasing woodland density, and whether those drivers are "natural" or not, frames management perspectives of desired landscape conditions.
The study also found that tree establishment rates in the last century have mostly declined, and by the late 20th century, they reached their lowest level in 400 years. Thus, there exists a possibility of future woodland decline and contraction, given the current low establishment rates and widespread drought-related tree mortality. While the study focuses on establishment rates within existing populations (infilling), it acknowledges that fire exclusion and settlement activities might have played a role in woodland range expansion, a process our approach doesn't directly address.