Section 1 Executive Summary

“What are the vulnerabilities of our forests to climate change?”

— Navajo Nation Forestry Department, March, 2013.

It is crucial for the future of forest management on the Navajo Nation that this question be addressed. This report derives from a collaborative, multi-faceted effort to identify and describe the vulnerabilities of Navajo forests to climate change. By providing place-based ecological data, this report can aid in developing priorities for forest management. Through the implementation of adaptive, science-based management, the Navajo forests can become more resilient to the effects of climate change.

The Navajo Nation includes over five million acres of forest and woodland. This report focuses on two large landscapes - the Chuska Mountains and Defiance Plateau - that together comprise over 600,000 acres of ponderosa pine and associated dry conifer species. Using analyses from tree-ring collections, long-term forest plot data, aerial surveys, and remote sensing, this report provides long-term perspectives on four forest processes that influence forest vulnerability: (1) changing fire regimes, (2) patterns of tree recruitment, (3) sensitivity of forest growth to climate, and (4) patterns of tree mortality. Each is presented in a different section of the report.

Climate change projections for the southwestern United States show unequivocally that temperatures are increasing and will continue to rise through at least the end of this century. Future changes in precipitation levels are less certain, but it is likely that precipitation will be more variable. Rising temperatures are enough to cause concern, because they amplify the severity of drought. Large precipitation increases would be necessary to overcome the decreased available moisture due to increased temperatures. Evidence is mounting that the Southwest and the Navajo Nation are already progressing toward a warmer, drier climate.

Drought events, especially when combined with high temperatures and heat stress, can be enough to kill trees, but they also weaken tree resistances to disturbances like fire and insects. The vulnerabilities of the Navajo forest to climate change stem from reduced forest resilience due to (1) the disruption of historically frequent and extensive surface fire activity, (2) overly dense ponderosa pine-dominated forests in the absence of fires, (3) enhanced sensitivity of tree growth to climate variability, especially at higher elevations, and (4) increasing rates and spatial extent of tree mortality over the last two decades. These problems will grow worse with time, and areas of the Navajo forest could be at or approaching ecological tipping points. Climate change could contribute to a future with altered forest compositions and structures that would lead to losses of the ecosystem services provided by healthy forests.

Tree-ring fire-scar data indicate that for hundreds of years, low-intensity fires burned roughly once a decade through the understory of ponderosa pine forests on the Navajo Nation. These surface fires covered extensive areas, especially in the spring or early summer following a dry winter. In 1870, for example, fire-scarred trees document burning across 35 miles of the Chuska Mountain crest. But the historical fire regimes were sensitive to human impacts. Fire regimes in the Chuska Mountains began to decline early, in the 1830s as grazing intensified. Interestingly, fire activity remained higher for longer on the Defiance Plateau, likely due to less grazing. By 1886, though, fire regimes had collapsed throughout the Navajo forest due to ever increasing numbers of livestock. Now, many areas have not burned for 140 years, and some areas are experiencing the consequences when fire finally returns. Probably the most dramatic wildfire event of recent decades was the 2014 Asaayi Lake Fire in the Chuska Mountains. It burned 15,000 acres, roughly half of which included moderate to high severity fire that killed most or all of the trees. Large high-severity fire events like this are likely unprecedented in Navajo ponderosa pine forests over the last four centuries and was clearly influenced by the increased fuels from over a century without fire. This pattern of increasing fire severity due to increased fuels and forest density from over a century of fire exclusion is common across the Southwest. Extreme fires are the most rapid means to tip an ecosystem into a new, potentially undesirable state, such as grassland or shrubfield. In a hotter, drier climate, and after a century or longer without fire to reduce fuel loads, large high-severity fires will continue to be a major source of vulnerability to Navajo forests.

Patterns of tree recruitment over the last 400 years in the Navajo forest underscore two important processes. First, historical recruitment was largely episodic, including only small tree cohorts establishing at a time. Fires acted to thin the new, young stands, creating open, park-like conditions in many areas. After fire regimes declined in the late 1800s, the density of stands increased at an accelerated, unprecedented rate. In some areas, stand densities are now more than double what they were in 1870. These additional trees contribute to the increased probability of high-severity fires. Second, ponderosa pine recruitment persisted through recurrent surface fire and drought. This suggests that future prescribed or managed lightning fires will not put the tree regeneration or the future of the forests at excessive risk. Furthermore, restoration of historical stand structures that were shaped by fire will likely increase forest resilience to future hotter droughts. Diverse multi-aged stands act to buffer climate extremes for young understory trees, raising their capacity to survive in a drier climate.

Climate exerts a strong influence on tree diameter growth patterns across the Navajo forest. Growth is mostly limited by the availability of moisture in this semi-arid landscape, meaning that precipitation deficits and warmer conditions reduce productivity. The most climatically sensitive trees are on the Defiance Plateau, where there there is relatively reduced precipitation and higher temperatures than in the Chuska Mountains. Lower elevation trees are highly adapted to drought, slowing or stopping their growth during the driest years and rapidly recovering in wetter years. Trees on top of the Chuska Mountains experience greater precipitation levels and lower temperatures than their lower-elevation counterparts. The wetter/cooler conditions of the Chuskas buffered drought effects over the 20th century, and trees maintained high growth rates through drought. Since the mid-1980s, however, increased temperatures and more variable precipitation appear to be changing the growth patterns of upper-elevation trees. Both ponderosa pine and Douglas-fir in the Chuska Mountains now have greater sensitivity to climate. Their response to dry years is now more like the mid-elevation trees, but they may be less adapted to drought. These changing growth patterns signal the arrival of climate change to the mountain top. With more limited moisture availability on average, future droughts could affect a broader area of the forest than it did historically. This increases the vulnerability of more of the forested landscape to drought and associated disturbances.

Rates of tree mortality across the Navajo forests have increased over the last 20 years, but remain below regional levels. Continuous Forest Inventory (CFI) plots throughout the Navajo forest show an average mortality rate of 0.36% per year from 1974 to 2015 (1 out of every 400 trees died). During the 2004-2015, it was 0.59% per year. What’s more, over the last 35 years the percent of plots with dead trees increased from less than 35% to over 50%. Aerial surveys of forest health and disease conducted by the USDA Forest Service showed that some of this increase is attributable to the extreme drought years of 2002-2003, when 140,000 acres (23%) of the forested area had high mortality. Most mortality during the 2002 drought was caused by bark beetles taking advantage of the drought-stressed trees. Tree mortality recorded in CFI plots is largely centered in the Chuska Mountains, where it is probable that decreasing available moisture, due to increased competition and drought, is contributing to tree mortality. As the upper-elevation forests respond to increasing drought stress, mortality levels may stay relatively high, or continue to increase. These trends underscore the need for future monitoring for climate change impacts through CFI plot inventories, aerial surveys, and by other means.

Management to return historically frequent, low-severity fire and associated less-dense forest structure would be ecologically beneficial and likely reduce the vulnerability of ponderosa pine forests of the Navajo Nation to climate change. It would provide resilience to the forests to withstand worsening future disturbances such as high-severity fire, drought stress, and insect outbreaks. Various strategies of adaptive forest restoration are well documented and tested, and are being applied across the Southwest. This report details some of the primary sources of climate change vulnerabilities and can thus be used to set priorities on where and what management actions can be taken.