Native American geography shaped historical fire frequency in forests of eighteenth‑century Pennsylvania, USA

Summary:

Fire scars in tree rings across a network of study sites suggest that wildland fires were more common near Native American trails and towns in 18th-century Pennsylvania; fires were also more common in warmer climates with gentle terrain and windy conditions.

Abstract:

Researchers have debated the relative importance of environmental versus Indigenous effects on past fire regimes in eastern North America. Tree-ring fire-scar records (FSRs) provide local-resolution physical evidence of past fire, but few studies have spatially correlated fire frequency from FSRs with environmental and anthropogenic variables. No study has compared FSR locations to Native American settlement features in the eastern United States. We assess whether FSRs in the eastern US are located near regions of past Native American settlement. We also assess relationships between distance to Native American settlement, environmental conditions, and fire frequency in central Pennsylvania (PA), US, using an “ensemble of small models” approach for low sample sizes. Regression models of fire frequency at 21 locations in central PA often selected distance-based proxies of Indigenous land use. Models with mean annual temperature and Native American variables as predictors explained > 70% of the variation in fire frequency. Alongside temperature and wind speed, “distance to nearest trail” and “mean distance to nearest town” were significant and important predictors. In 18th-century central PA, fires were more frequent near Indigenous trails and towns, and further south due to increasing temperature and pyrophilic vegetation. However, for the entire eastern US, FSRs are located far from past settlement, limiting their effectiveness in detecting fire patterns near population centers. Improving understanding of historical fire will require developing FSRs closer to past Native American settlement.

Research questions:

1. Was wildland fire more frequent near Native American towns and trails in 18th-century Pennsylvania? 
2. How frequent was wildland fire in 18th-century Pennsylvania? 
3. What other environmental variables (e.g., climate, terrain, soil) were correlated with wildland fire frequency?

What previous research has shown:

Native Americans used fire to burn wildlands for many purposes, such as clearing land, making hunting and travel easier, promoting advantageous fire-tolerant tree species, and attracting game to areas with vigorous regrowth.

What this research adds to the conversation:

It is one of the few studies correlating fire frequency recorded in tree-ring records with variables representing Native American geography. It shows that past Native American geography (i.e., trail and town locations) is predictive of fire frequency. There were more wildland fires near trails and towns. Forests relatively close to trails and towns burned as often as every three years (but due to the nature of fires, the frequency could have even been greater). However, environmental variables were also important: wildland fire occurred in warmer climates (warmer = easier to burn), windier locations (windier = easier to spread fire), and gentler terrain (gentle terrain = easier for fire to spread in the absence of gullies).

Novel methodology:

Ensemble of small models to deal with small sample sizes—there were only about 20 sites with tree-ring fire-scar records in the study area, which requires the training of multiple "small models" that each relate fire frequency to at most two independent variables. The model predictions from each small model are then averaged together in order to predict fire frequency historically at a location.

Implications for society:

To restore fire-dependent communities with rare plant species via controlled wildland burns, we should focus on forests near former features of Native American settlement.

Policy implications:

Controlled burns of land do occur on managed lands. This research may re-focus controlled/prescribed burning to areas closer to past Native American trails and towns. It also suggests that burning was more widespread because correlations between fire frequency and Native American geography extend tens of kilometers from these geographic features.

Funding:

The USDA Forest Service, Rocky Mountain Research Station supported this research.