Joint Fire Science Program, January 8, 2009
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, fire 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, and grew and survived during “safe periods,” two multi decades that were fire-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 climate-driven tree establishment opportunities rather than episodes caused by severe fire. Fire exclusion since Euroamerican settlement has eliminated the historical fire return interval that occurred approximately every eleven years, excluding the rainy periods. Ponderosa pine forests across the southwestern United States have been relatively fire-free for 100+ years, increasing tree density, changing forest structure and contributing to increasingly severe wildfires in recent years. A clear link between climate, fires, and tree establishment episodes has been identified, showing the importance of regional historical processes on the composition and structure of our current forests.
• Strong but short climate changes, as short as several years or a decade, can have widespread and long-lasting effects on forest communities.
• Climate affects forests by causing trees to die, by producing conditions favorable to tree establishment and growth, or by affecting disturbances such as fire that control tree establishment, growth and mortality.
• In the ponderosa pine forest studied in southwestern Colorado, few trees were older than the region-wide megadrought that lasted through multiple years in the 1580s.
• A lengthy rainy period in the 1600s created an episode in which trees established in the study area, as they did throughout much of the Southwest.
• Wet cycles, followed by dry cycles, contributed to fuel build up and drying. Fire, affected by the climate variations, killed seedlings and saplings during periods of more frequent burning.
• Fire exclusion since Euroamerican settlement began has changed forest composition and increased tree density—factors that have contributed to increasingly severe wildfires in Southwestern ponderosa pine forests.
• In considering the forest studied, the scientists found a clear link between climate, fire, and episodes of tree establishment and growth, revealing the importance of regional processes on the composition and structure of our forests today.
When the Soviet Union launched the first artficial satellite off the surface of the earth and into space, many Americans viewed sputnik as a direct threat to the United States’ technological supremacy, and were shocked the achievement had been made elsewhere. The Cold War climate forced the creation and rapid expansion of hundreds of thousands of technicians, engineers, scientists, service people, and average citizens into producers and credulous supporters of America’s space program, a program of technological recovery that twelve years from its beginning delivered a man to walk on the moon. Throughout human history, such climates have produced major shifts, both in the rapid expansion of our endeavors, and in our quick recovery from catastrophe. What metaphoric climate does for human histories, real climate does for forest histories. Peter M. Brown, Director of Rocky Mountain Tree-Ring Research, and Rosalind Wu, Fire Ecologist with the San Juan National Forest, have looked at the evidence in a familiar southwestern ponderosa pine forest, and made intriguing discoveries about the pace in which catastrophe and recovery can occur across these types of landscapes.
The lore of the rings and the stories of the stand
The dramatic changes in some of our western forests is a familiar story: Euroamerican settlers introduced grazing animals which altered landscapes beginning in the 19th century, and later, efforts to exclude fire had a dramatic effect on fuel build up and forest density. Assuming that fire exclusion would have the biggest impact on ecosystems where plants’ life histories are adapted to more frequent fire return intervals, such as ponderosa pine forests, we could also assume the impact would not be as great in less fire-prone forest types such as higher elevation mixed conifer or subalpine forests. Brown and Wu, researchers familiar with southwestern forests, examined this assumption in the San Juan National Forest. With their team, they found that while this might be the case at the individual stand level, big changes had occurred across larger landscape-scales in even upper elevation mixed-conifer and subalpine forests. What were the drivers of change, the scientists wondered? In order to answer this question, the scientists had to look at their subjects, from the inside and out.
Having selected three study areas in the middle Piedra River watershed on the San Juan National Forest in southwestern Colorado that were both representative of forest types in the area and were largely unharvested, the team collected samples from over 3,700 trees. The scientists gathered tree-ring evidence that could tell them what the trees had experienced: stand-level fires that might not have killed the tree; the date the tree established, possibly following a stand-opening fire or some other disturbance; or the beginning of the tree’s life as part of a wide-scale, climate-driven opportunity.
The scientists cross dated the cores and cross sections against master chronologies they had developed for the study areas. This dendrochronological dating of tree rings gave the scientists absolute dates for fire events and tree establishment, or recruitment—dates that they could compare across landscape scales, and with known land use and climate histories. The scientists evaluated tree establishment timelines and fire years with climate timelines—of wet years, dry years, and larger-scale climate patterns such as El Niño, Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation that affect droughts and therefore fire occurrence in the western United States. In reading the evidence, the scientists saw a story emerge.
A ponderosa pine forest tells its story…
Mature ponderosa pines, with high crowns and thick bark, are well adapted to burning. Recurrent surface fire, the dominant historical disturbance in all ponderosa pine forests, would kill most seedlings and saplings, maintaining a generally open forest structure. On Archuleta Mesa, an isolated, unlogged, old-growth forest displays this classic structure. In examining the ponderosa pine landscape of Archuleta Mesa, one of the study sites, the scientists found very few trees were older than a multi-year megadrought that occurred in the region around the 1580s. This prolonged drought, identified in tree-ring chronologies throughout the western United States and Mexico, was probably the most severe in the Southwest in at least the last thousand years, the scientists discovered. The team identified some seedlings that established a little before or during the megadrought, but the evidence shows they grew slowly until a lengthy rainy period began in the early 1600s. The numerous ponderosa pine trees that established during the early 1600s probably resulted from a combination of factors, the team believes—openings in the forest owing to the dry conditions that killed trees, as well as rainfall that produced the right conditions for seedlings to germinate and saplings to grow in the dry ponderosa pine ecosystem.
But fire was probably not a major factor in this episode of widespread tree establishment, or recruitment. “It is doubtful that severe fires were responsible for stand opening during the megadrought,” Brown explains, “since we found no scars on any trees surviving from this period. It is, however, possible that other disturbance factors, such as bark beetles, contributed to widespread and synchronized mortality during the megadrought.” The vast swaths of brown, standing dead trees in the four corners area of the Southwest—stark images of massive forest death from severe drought and beetle outbreaks during 2002–2004—were familiar to any average person reading the paper or watching the news. If the key to the past is the present, as the adage in geology goes, the contemporary event would seem to offer a good indication of the scenario in the centuries-old past. But Brown, Wu and the team could not find evidence of bark beetles, such as blue stain in sapwood, that has survived from the mega drought period. Decomposition, long since accomplished, has eliminated any dead trees that could have offered the story.
In addition to being a long wet period, the early seventeenth century also had few fires. Seedlings and saplings that would normally be killed by surface fires were able to grow to a height that allowed them to survive later fires. After the era of tree establishment in the early 1600s, the scientists identified two long episodes that had few fires, allowing trees to establish—from 1684 to 1724, a period of 40 years, and from 1818 to 1851, a period of 33 years. With the exception of these episodes, the scientists found fires occurred, on average, every eleven years from 1632 to 1871. They identified a climate pattern in which rainy years allowed grasses and forbs to build up, increasing surface fuels, followed by dry years in which the added fuels then burned more easily, and over greater areas. Brown and the team found this pattern in both tree-ring and modern fire-atlas datasets. They believe that climate variations were a crucial factor that changed fire occurrence, and produced the wet conditions that allowed trees to establish and grow.
They also found this pattern occurred in other ponderosa pine forests in the Southwest, which they feel supports the idea that region-wide climate affected fire occurrence, and caused related episodes in which trees were able to establish and thrive. The more important effects on broader-scale forest structure, the scientists explain, are what they call “safe periods”—the extended periods where fire did not occur that produced the episodes in which trees established.
…And the story offers a regional history
Scale, Brown offers, is necessary to read an accurate story. “What is clear from our results is that by scaling up from individual plots to the entire landscape, an emergent pattern appears in which cohort structure is uncoupled from any single mortality event and instead appears to be the result of broader scale climate forcing of fire timing that resulted in successful recruitment episodes.” Brown and the team urge there are minimum scales of time that area managers must use to see patterns in tree establishment and fire histories that will allow them to infer events. The scientists suggest that managers use scales that cover more than only a few stands, and at least one to two hundred years of tree establishment and fire history data. Because Euroamerican settlement has meant land use changes, and accompanying fire exclusion efforts that eliminated surface fires from nearly all ponderosa pine forests across the western United States, Brown explains, “the 135-year-long fire-free period from 1871 to 2005 is more than three times as long as the longest historical fire-quiescent period of 40 years from 1684 to 1724.”
Comprised of fewer fire-tolerant species, and more densely packed, the canopy of ponderosa pine forests has changed. Ladder fuels, plants that allow fire to reach from surface to canopy, have contributed to more severe fires. Crown fire, the scientists offer, has largely replaced historic surface fire in many ponderosa pine forests. To this negative news, massive forest death due to climate forces has been seen as a rapid event, while tree establishment that follows has been seen as a much slower process. Brown and the team discovered, however, that after the 1580s megadrought, Southwestern ponderosa pine forests recovered fairly rapidly. Climate conditions—bringing ample rain, and few fires—drove this pace. The scientists feel this example has important implications for predicting plant responses after the recent massive die off caused by drought and insect damage over millions of acres of piñon and ponderosa pine forests in the southwestern United States. Brown offers this optimistic note, “over multicentury time scales, broad-scale dynamics in the Southwest have included abrupt and synchronized mortality that, at least once before in the relatively recent past, was followed by fairly rapid community recovery.” He stresses the data reveal the importance of events, contingent upon one another, that give rise to forest structure and composition—climate, disturbances such as fire, and the dramatic, dynamic cycles of dying and flourishing. In looking at their histories, we can see plants offer clues of their responses to climate change, and the effects that accompanying disturbances will have on their survival. While ponderosa pine forests respond to climate forces, rising and falling and rising on timescales that are long in human terms, in the millennial view it is but a season.
• Landscape-scales and timescales need to be considered to assess tree establishment and fire histories. Patterns that result from climate factors only emerge at larger landscape and longer time scales. Managers should consider the size of the scales they use, and the minimum sizes necessary when attempting to analyze patterns.
• In the past, forest recovery from climate-caused, widespread rapid tree death was viewed as a slow process, and millions of acres of trees dead from drought and insect damage have recently captured public emotion. With the example in the relatively recent past of rapid forest recovery in wet years following the 1580s megadrought, managers should consider the interaction of climate, disturbances such as fire, and forest structure when predicting the response of plants over the short, and long-term.
Further Information: Publications and Web Resources
P.M. Brown and R. Wu. 2005. Climate and disturbance forcing of episodic tree recruitment in a southwestern ponderosa pine landscape. Ecology. 86:3030-3038. http://www.rmtrr.org/data/Brown&Wu_2005.pdf (November 27, 2007).
P.M. Brown and R. Wu. 2006. Fire and Forest Structure Across Vegetation Gradients in San Juan National Forest, Colorado: A Multi-scaled Historical Analysis Final Report: JFSP Project 01-3-3-13. http://www.rmtrr.org/SJNF/SanJuanFinalReport.pdf (November 27, 2007).
Peter M. Brown is Director of Rocky Mountain Tree-Ring Research, a nonprofit research institute dedicated to using tree-ring studies in understanding climate, fire, forest, and cultural histories of the US, northern Mexico, and Mongolia. He received a PhD degree from Colorado State University, where he is also an affiliate faculty member and teaches courses in forest and fire ecology and dendrochronology.
Peter Brown can be reached at: Director, Rocky Mountain Tree-Ring Research, 2901 Moore Lane, Fort Collins, CO 80526 Phone: 970-229-9557 Email: firstname.lastname@example.org
Rosalind Wu was the Fire Ecologist and currently serves as Wilderness Specialist for the San Juan National Forest. She received her MS degree in Forest and Fire Ecology from Colorado State University, and plans to apply the knowledge gained from this study to better manage fire in the extensive San Juan National Forest wilderness areas.
Rosalind Wu can be reached at: Pagosa Ranger District, 180 Pagosa Springs, CO 81147 Phone: 970-264-1529 Email: email@example.com