Fire regime on a cultural landscape: Navajo Nation

Abstract

Fire has played an important role in the evolutionary environment of global ecosystems, and Indigenous peoples have long managed natural resources in these fire-prone environments. We worked with the Navajo Nation Forestry Department to evaluate the historical role of fire on a 50 km² landscape bisected by a natural mountain pass. We used fifty 5-ha circular plots to collect proxy fire history data on fire-scarred trees, stumps, logs, and snags in a coniferous forest centered on a key mountain pass. The fire history data were categorized into three groups: All (all 50 plots), Corridor (25 plots closest to Buffalo Pass drainage), and Outer (remaining 25 plots, farther from pass). We assessed spatial and temporal patterns of fire recurrence and fire-climate relationships. The landscape experienced frequent fires from 1644, the earliest fire date with sufficient sample depth, to 1920, after which fire occurrence was interrupted. The mean fire interval (MFI) for fire dates scarring 10% or more of the samples was 6.25 years; there were 13 large-scale fires identified with the 25% filter with an MFI of 22.6 years. Fire regimes varied over the landscape, with an early reduction in fire occurrence after 1829, likely associated with pastoralism, in the outer uplands away from the pass. In contrast, the pass corridor had continuing fire occurrence until the early 20th century.Synthesis. Fires were synchronized with large-scale top-down climatic oscillations (drought and La Niña), but the spatially explicit landscape sampling design allowed us to detect bottom-up factors of topography, livestock grazing, and human movement patterns that interacted in complex ways to influence the fire regime at fine scales. Since the early 20th century, however, fires have been completely excluded. Fuel accumulation in the absence of fire and warming climate present challenges for future management.

Keywords: Diné; Indigenous; climate; drought; fire history; tribal lands; wildfires.

Figure 1

Study area in the Lukachukai Mountains, northern region of the Chuska Mountains. Fifty 5‐ha plots were sampled for fire‐scarred trees on a 1 × 1‐km grid. Colored dots show the position of individual fire‐scarred samples with blue shading representing Corridor and yellow shading representing Outer sites. Navajo Tribal Lands are shown within the “Four Corners” region, and the gridpoints 103, 104, 118, and 119 from the North American Drought Atlas (Cook & Krusic, 2004) are identified. The three subpanels (left) depict maps of fire occurrence for selected years (1708, 1748, and 1879) with red dots representing sites where at least one tree recorded a fire that year, grey dots represent sites where that year was present in the sample but no trees recorded a fire, and empty dots represent sites where that year was not present in the sample (i.e., the sample trees were all too young)

Figure 2

Fire events at Lukachukai study site in the categories All, Corridor, and Outer. Fires with dates shown on the x‐axis met the 10% fire‐scarred filter. The orange shading depicts the post‐1829 reduced fire interval in the outer region, and grey identifies the absence of fire events meeting the 10%‐scarred threshold

Figure 3

Seasonality of fire occurrence (10% fire‐scarred filter): 36% dormant, 20% early earlywood, 24% middle earlywood, 16% late earlywood, and 4% latewood

Figure 4

Superposed epoch analysis illustrating departures from the mean value of Palmer Drought Severity Index for years 1644–1916. (a) 25% or more fire‐scarred filter (minimum two fire‐scarred trees) applied with 0 representing fire year. Dotted lines represent 95% confidence interval derived from 1,000 Bootstrap simulations, and solid lines indicate those that exceed 99% confidence interval. (b) Same analysis with 10% fire‐scarred filter

Figure 5

Superposed epoch analysis illustrating departures from the mean value of the Niño3 index for years (1644–1916). (a) 25% or more fire‐scarred filter (minimum two fire‐scarred trees) applied with 0 representing fire year. Dotted lines represent 95% confidence interval derived from 1,000 Bootstrap simulations, and solid lines indicate those that exceed 99% confidence interval. (b) Same analysis with 10% fire‐scarred filter

Figure 6

Palmer Drought Severity Index (PDSI) and fire event relationship for 25% fire‐scarred filter (triangles) in the Corridor site. The PDSI shown is the average of gridpoints 103, 104, 118, and 119 from the North American Drought Atlas (Cook & Krusic, 2004)