Long-term tree population growth can predict woody encroachment patterns

Recent increases of woody plant density in dryland ecosystems around the world are often attributed to land use changes such as increased livestock grazing and fire suppression, or to climatic trends driven by increasing atmospheric carbon dioxide1,2. While such changes have undoubtedly impacted ecosystem structure and function, the evidence linking them to woody encroachment is mixed and demographic processes underlying changes in woody plant abundance require further consideration3. After examining tree age structures from woodlands across the interior western USA using demographic models, we find little evidence of widespread increases in per-capita tree establishment rates following 19th century Euro-American settlement. Woodlands dominated by young trees have often been cited as evidence of woody encroachment driven by a number of anthropogenic processes, but we demonstrate they can also be accurately predicted by a null model including only steady long-term tree population growth. Contrary to common interpretations, we show that tree establishment rates in the last century have mostly declined, rather than increased, and in fact they are currently at their lowest rates since at least 1600 CE.