Drought stress detected in tree rings suggests an impending tipping point for forests on the Navajo Nation

Abstract

Forests across the southwestern U. S. face escalating challenges from changing climate. The objective of this study, conducted in partnership with the Navajo Forestry Department (NFD), was to assess the impact of climate variability and change on forest health on the Navajo Nation by adding tree-ring sampling to a stratified random subsample of NFD’s Continuous Forest Inventory plot network. Increment cores from 1075 trees at 56 plots were used to calculate basal area increment (BAI) and assess growth patterns at three scales - trees, forest stands, and the landscape. Tree-level regressions showed that warmer-than-average temperatures in the current year’s spring and previous year’s fall reduced growth of all three dominant tree species. At the forest stand scale, we found a multi-decade decline in productivity from 1990-present, corresponding to the period of rising temperatures. Landscape-scale analysis demonstrated reduced spatial heterogeneity of growth, hence increased synchrony. Higher-elevation forests that historically acted as climate change refugia are now being negatively affected by unusually warm spring temperatures. Declining growth and increased synchrony suggest the forest is approaching a tipping point, such that further warming and drought may exceed the limits of its resilience. We identify pathways to improve forest resilience via strategic fuel reduction treatments that simultaneously reduce the risk of high-severity fire, increase soil moisture availability for residual trees, and generate stand and landscape diversity. We emphasize the value of collaborative research approaches that have broad relevance for sustainable forest management practices, fostering resilience and protection of culturally significant services for Tribal communities.