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Abstract
Interannual climate variations have been important drivers of wildfire occurrence in ponderosa pine forests across western North America for at least 400 years, but at finer scales of mountain ranges and landscapes human land uses sometimes over-rode climate influences. We reconstruct and analyse effects of high human population densities in forests of the Jemez Mountains, New Mexico from ca 1300 CE to Present. Prior to the 1680 Pueblo Revolt, human land uses reduced the occurrence of widespread fires while simultaneously adding more ignitions resulting in many small-extent fires. During the 18th and 19th centuries, wet/dry oscillations and their effects on fuels dynamics controlled widespread fire occurrence. In the late 19th century, intensive livestock grazing disrupted fuels continuity and fire spread and then active fire suppression maintained the absence of widespread surface fires during most of the 20th century. The abundance and continuity of fuels is the most important controlling variable in fire regimes of these semi-arid forests. Reduction of widespread fires owing to reduction of fuel continuity emerges as a hallmark of extensive human impacts on past forests and fire regimes.This article is part of the themed issue ‘The interaction of fire and mankind’.
Highlights
People living within fire-prone forested environments over long periods of time have profound impacts on forest structures and fire regimes, and vice versa
At the same time that fuel connectivity and widespread fires were reduced by human land uses, purposeful and accidental ignitions were added to those occurring from lightning strikes [6]
In general, the patterns identified in these areas are consistent, even when accounting for sample size changes between periods
Summary
People living within fire-prone forested environments over long periods of time have profound impacts on forest structures and fire regimes, and vice versa. We evaluate climate and human controls over past fire regimes using recently compiled networks of fire scar-based chronologies from three spatial scales: (i) a subcontinental region of western North America, (ii) landscapes within the Jemez Mountains, a large mountain range in north central New Mexico, USA, and (iii) forest stands within areas of different human land-use intensities and timing within the Jemez Mountains. For broad-scale context, we use the largest network of treering-based fire scar chronologies in the world to assess interannual fire –climate relations at the subcontinental scale This data network has recently been compiled from western North America and it provides dates and estimates of the relative extent of fires within forests and woodlands (figure 1 and electronic supplementary material, figure S1 [54]). We used the software program called FHAES to carry out the SEA, the sample size/fire frequency assessment, and to produce initial versions of the fire scar chronology graphics [66,67]
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