U.S. Joint Fire Science Program


Date of this Version


Document Type



Fire Science Brief, Issue 26, December 2008


US government work.


Strong, abrupt climate changes on a time scale of annual to decadal length, can have widespread and long-lasting effects on forest communities. Climate affects forests by creating conditions that kill trees through severe drought, or that promote tree establishment during rainy periods that foster seedlings and saplings. Climate also affects disturbance events that produce the conditions that allow, or limit, fi re occurrence. In studying a ponderosa pine forest in southwestern Colorado, scientists found few trees older than a regional “megadrought” that lasted over two decades in the late 1500s. In the 1600s, a long rainy period allowed trees to establish in great numbers over much of the Southwest. Trees established in the early 1700s and mid-1800s grew and survived during “safe periods,” two multi decades that were fi re-quiet. These periods, of drought and tree death, and ample rain and tree establishment, correspond to shifts in global weather patterns such as El Niño. Similar aged trees in ponderosa pine forests are likely the result of climatedriven tree establishment opportunities rather than episodes caused by severe fi re. Fire exclusion since Euroamerican settlement has eliminated the historical fi re return interval that occurred approximately every eleven years, excluding the rainy periods. Ponderosa pine forests across the southwestern United States have been relatively fi re-free for 100+ years, increasing tree density, changing forest structure and contributing to increasingly severe wildfi res in recent years. A clear link between climate, fi res, and tree establishment episodes has been identifi ed, showing the importance of regional historical processes on the composition and structure of our current forests.