Section 2 Introduction

Climate change is bringing more frequent, longer-lasting, and more severe droughts to the southwestern United States^1–3. Drought is an important component of the Southwest climate, and under natural conditions they can last years to decades⁴. Already, however, climate change is making the region progressively drier through warming⁵. Temperatures in the region have risen over the last century due to increased human emission of greenhouse gases⁶. Temperatures will continue to rise even under the most optimistic climate projections⁷. Increasing temperatures reduce moisture availability to plants by drying soils and increasing plant water uptake and transpiration to the air^8,9. Changing patterns of precipitation could offset these temperature-induced losses, but climate projections are less certain of trends in precipitation. Although increasing temperatures have already led to declines in winter snowpack and shifting storm tracks as large-scale ocean-atmosphere connections are potentially disrupted^10–13.

Rising temperatures in the Southwest are driving trends in aridity, and plants are recording these changes. Some recent drought years have been among the worst droughts in over 500 years^14–17. The risk of future severe, long-lasting droughts is also on the rise with warming^18,19. The era of hotter droughts has arrived.

A multitude of factors can and will impact southwestern forests with hotter droughts²⁰. For example, forest disturbances processes will be amplified and accelerated, extending further and affecting more areas across landscapes and regions. Tree-die off events associated with increased temperatures and heat-stress have been reported globally²¹ as background rates of tree mortality rise in the western US²². During the early 2000s, bark beetle outbreaks spanned from at least southern Arizona to Alaska and northern British Columbia²³. Wildfire events over the last couple of decades have grown in size and severity as forests dry out and the fire season lengthens with warming^24–26. Roughly 20% of southwestern forests have already been severely affected by beetle kill and high-severity fire²⁷. In some areas, tree-killing disturbances could lead to permanent conversions of forests to other vegetation types^28,29. Increasing temperatures are also attributed to greater levels of forest drought stress leading to declines in forest productivity and reducing resilience to disturbance events^15,30. Under current climate predictions, an average year for tree growth in the 2050s will be more like the worst drought years over the last millennium^15,31.

For a forested landscape like that present on the Navajo Nation, these climate change and drought trends pose significant threats to biodiversity, infrastructure, forest products, traditional forest uses, and vital ecosystem services^32–35. Many communities on reservation lands are disproportionately dependent on the ecosystem services provided by local forests and watersheds.

However, regional trends are not always manifested in every forest. To prepare for future hotter droughts with adaptive forest management, we need to reduce uncertainties in how, when, and where climate change threats will play out on individual landscapes. Landscape-scale studies provide the kind of place-based perspective that is critical to understanding ongoing and potential future impacts of climate change. The objective of this report is document and describe recent and historical changes, trends, and patterns in forest processes on the Navajo Nation. These data and the insights gleaned from the various studies presented below should aid in developing, prioritizing, and implementing forest management actions in light of identified vulnerabilities to Navajo forests in a warmer, drier future.

References

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3. Williams, A. P. et al. Causes and implications of extreme atmospheric moisture demand during the record-breaking 2011 wildfire season in the southwestern United States. Journal of Applied Meteorology and Climatology 53, 2671–2684 (2014).

4. Woodhouse, C. A. & Overpeck, J. T. 2000 Years of Drought Variability in the Central United States. Bulletin of the American Meteorological Society 79, 2693–2714 (1998).

5. Prein, A. F., Holland, G. J., Rasmussen, R. M., Clark, M. P. & Tye, M. R. Running dry: The U.S. Southwest’s drift into a drier climate state. Geophysical Research Letters 43, 1272–1279 (2016).

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9. Breshears, D. D. et al. The critical amplifying role of increasing atmospheric moisture demand on tree mortality and associated regional die-off. Frontiers in plant science 4, 266 (2013).

10. Pederson, G. T. et al. The Unusual Nature of Recent Snowpack Declines in the North American Cordillera. Science 333, 332–336 (2011).

13. Alfaro-Sánchez, R. et al. Climatic and volcanic forcing of tropical belt northern boundary over the past 800 years. Nature Geoscience 11, (2018).

14. Touchan, R., Woodhouse, C. A., Meko, D. M. & Allen, C. D. Millennial precipitation reconstruction for the Jemez Mountains, New Mexico, reveals changing drought signal. International Journal of Climatology 31, 896–906 (2011).

17. Belmecheri, S., Babst, F., Wahl, E. R., Stahle, D. W. & Trouet, V. Multi-century evaluation of Sierra Nevada snowpack. Nature Climate Change 6, 2–3 (2016).

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19. Cook, B. I., Ault, T. R. & Smerdon, J. E. Unprecedented 21st century drought risk in the American Southwest and Central Plains. Science Advances 1, 1–7 (2015).

20. Allen, C. D., Breshears, D. D. & McDowell, N. G. On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. Ecosphere 6, art129 (2015).

21. Allen, C. D. et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259, 660–684 (2010).

22. van Mantgem, P. J. et al. Widespread increase of tree mortality rates in the western United States. Science (New York, N.Y.) 323, 521–524 (2009).

23. Raffa, K. F. et al. Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions. BioScience 58, 501 (2008).

24. Westerling, A. L., Hidalgo, H. G., Cayan, D. R. & Swetnam, T. W. Warming and earlier spring increase western U.S. forest wildfire activity. Science 313, 940–943 (2006).

26. Singleton, M., Thode, A., Sanchez Meador, A. & Iniguez, P. Increasing trends in high-severity fire in the southwestern USA from 1984-2015. Forest Ecology and Management 433, 709–719 (2019).

27. Williams, A. P. et al. Forest responses to increasing aridity and warmth in the southwestern United States. Proceedings of the National Academy of Sciences of the United States of America 107, 21289–94 (2010).

28. Savage, M. & Mast, J. N. How resilient are southwestern ponderosa pine forests after crown fires? Canadian Journal of Forest Research 977, 967–977 (2005).

29. Guiterman, C. H., Margolis, E. Q., Allen, C. D., Falk, D. A. & Swetnam, T. W. Long-Term Persistence and Fire Resilience of Oak Shrubfields in Dry Conifer Forests of Northern New Mexico. Ecosystems 21, 943–959 (2018).

15. Williams, A. P. et al. Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change 3, 292–297 (2013).

30. van Mantgem, P. J., Falk, D. A., Williams, E. C., Das, A. J. & Stephenson, N. L. Pre-fire drought and competition mediate post-fire conifer mortality in western U.S. National Parks. Ecological Applications 28, 1730–1739 (2018).

31. Klesse, S. et al. Sampling bias overestimates climate change impacts on forest growth in the southwestern United States. Nature Communications 9, 5336 (2018).

32. Ferguson, D. et al. Drought preparedness for tribes in the four corners region. Report from April 2010 workshop. (Climate Assessment for the Southwest, 2011).

35. Voggesser, G., Lynn, K., Daigle, J., Lake, F. K. & Ranco, D. Cultural impacts to tribes from climate change influences on forests. Climatic Change 615–626 (2013). doi:10.1007/s10584-013-0733-4