Tree rings reveal persistent Western Apache (Ndee) fire stewardship and niche construction in the American Southwest

Abstract

Identifying the influence of low-density Indigenous populations in paleofire records has been methodologically challenging. In the Southwest United States, well-replicated fire histories suggest that abundant lightning and suitable climate conditions drove frequent low-severity wildfires in dry pine forests independent of human activities even as ethnography provided hints that highly mobile indigenous populations used fire in myriad land use contexts. Here, we leverage published and unpublished tree-ring fire history records from pine forests in Western Apache (Ndee) traditional territory in central and eastern Arizona (N = 34 sites, N = 649 trees) to demonstrate that historical fire regimes were overwhelmingly influenced by Ndee cultural burning. Our tree-ring synthesis shows significantly more frequent fires in Ndee territory than elsewhere in the region for centuries before the establishment of reservations (1600-1870 CE). Despite the heightened fire activity, fires were largely small and asynchronous, occurred disproportionately in late April and May, when Ndee invested significant subsistence activities in these pine forests, and occurred independent of climate drivers. This suggests that Ndee fire stewardship created a patchwork of nearly annual small, spring fires that inhibited natural fire spread and limited the influence of drought on fire activity. Our work shows that even relatively small, highly mobile populations of forager-gardeners had significant influence on some pre-Euroamerican fire regimes despite abundant natural ignitions. Our study shows clearly that Indigenous fire management impacted fire-size distributions, fire frequencies, and fire seasonality in ways that cannot be explained by seasonal and annual lightning densities.

Keywords: Western Apache; cultural burning; fire ecology; ponderosa pine forests.

Fig. 1.

The location of tree-ring fire history sites used in this study (red diamonds; N = 34 sites) within Western Apache traditional territory (pink area). Regional fire history sites from the International Multiproxy Paleofire Database (IMPD; white dots; N = 277 sites) used in regional comparisons (21), the traditional territory of Chiricahua Apaches (blue area), and the distribution of dry conifer forests dominated by ponderosa pine (Pinus ponderosa; gray areas) are also indicated.

Fig. 2.

Photomicrograph (A) of multiple every-other-year fire scars from sample LMF3 from Limestone Flat within the Western Apache (Ndee) homelands (50), and a scatterplot (B) and boxplot (C) of mean fire return intervals (MFI_all) for Ndee and all other (regional) sites. In (B), there are significant, negative log–linear relationships between sample size and MFI_all for regional sites and Ndee sites. Ndee sites have significantly shorter fire return intervals compared to the rest of the region (Table 2).

Fig. 3.

Scatterplots of mean fire return interval (MFI_all) and annual (A), April–September/fire-season (B), and April–June seasonal drought (C) lighting densities, as well as MFI_all and fire-season (D) and April–June (E) precipitation for Ndee and regional sites along with a boxplot of annual lightning density (F).

Fig. 4.

SEA of fire–climate relationships at the local (fire history site, solid dots), cultural landscape (all of Ndee traditional territory north of the Gila River; open diamonds), and regional scales (all of Arizona and New Mexico, solid triangles). (A and B) plot fire-year PDSI (an index of summer moisture or drought) and the maximum PDSI in the 3 y prior to fire years for Ndee (A) and regional (B) sites. At the landscape scale, years that saw fire at least 10% or 25% of all Ndee sites lack a strong climate pattern (diamonds in A). At the 10% filter (at least 10% of sites recording a fire), there is no apparent fire–climate association (C), especially compared to regional 10% fire years (A and B). The largest fire years (scarring at least 25% of all sites) presented a statistically significant “canonical” pattern of wet prior conditions and dry fire conditions that were only trivially wet and dry (A and D), particularly when compared to the region (B and D).

Fig. 5.

Scatterplot (A) and boxplot (B) of mean fire intervals (MFI_all) and MFI_25%:MFI_all ratios for Ndee and regional fire history sites. This ratio is a measure of the importance of relatively small fires (low levels of fire-year synchrony) when MFI_25%:MFI_all ratios are higher. MFI_25%:MFI_all ratios are significantly higher at Ndee sites (Table 2) indicating a greater prevalence for fires that scarred few trees, which we interpret as qualitatively smaller fires.

Fig. 6.

Seasonality for Western Apache sites (red) compared to non-Apache Arizona sites (gray). Density plots show the frequencies with which particular percentages of scar placements occur across sites in each category (A–E). Across all trees aggregated for each category, early earlywood dates are unusually well represented at Western Apache sites (F). Individual sites in the Western Apache homeland have unusually high occurrences of early earlywood scars (late April or May) (B) but lower representation of middle earlywood (June) (C and F) scars compared to non-Apache Arizona sites.