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Table 3 Spatial data used to describe fire environments, including topographic, climatic, ignition sources (including human influence), and vegetation in the Madrean Ecoregion Sky Islands, US and Mexico. The spatial density of roads, lightning, and population was calculated using a uniformly weighted kernel (~55 km radius moving window) for use in analysis. The bioclim variables graphed in figures are listed in bold. For biotic communities and anthropogenic biomes, only categories present in the study region are given in the table

From: Distant neighbors: recent wildfire patterns of the Madrean Sky Islands of southwestern United States and northwestern Mexico

Variable Description
 Terrain Terrain ruggedness index derived using a digital elevation model (1 km resolution). Defined as the mean of the absolute differences between the value of a cell and the value of its eight surrounding cells.
 Spatial climate ClimateWNA bioclim layers based on climate normals 1981 to 2010 at 1 km resolution (Wang et al. 2016;
Bioclimatic variables:
MAT: mean annual temperature (°C)
MWMT: mean temperature of the warmest month (°C)
MCMT: mean temperature of the coldest month (°C)
TD: difference between MCMT and MWMT, as a measure of continentality (°C)
MAP: mean annual precipitation (mm)
MSP: mean summer (May to Sep) precipitation (mm)
AHM: annual heat moisture index, calculated as (MAT+10)/(MAP/1000)
SHM: summer heat moisture index, calculated as MWMT/(MSP/1000)
DD_0: degree-days below 0 °C (chilling degree days)
DD5: degree-days above 5 °C (growing degree days)
DD_18: degree-days below 18 °C
DD18: degree-days above 18 °C
NFFD: the number of frost-free days
bFFP: the Julian date on which the frost-free period begins
eFFP: the Julian date on which the frost-free period ends
FFP: frost-free period
PAS: precipitation as snow (mm)
EMT: extreme minimum temperature over 30 years
EXT: extreme maximum temperature over 30 years
Eref: Hargreave’s reference evaporation
CMD: Hargreave’s climatic moisture index
RH: mean annual relative humidity (%)
Tave_wt: winter (Dec to Feb) mean temperature (°C)
Tave_sm: summer (Jun to Aug) mean temperature (°C)
PPT_wt: winter (Dec to Feb) precipitation (mm)
PPT_sm: summer (Jun to Aug) precipitation (mm)
Ignition sources
 Lightning Average annual density of lightning strikes per unit area (~10.5 km resolution), for 1995 to 2005 (strikes per km2 per year) from NASA Global Hydrology and Climate Center: annual high-resolution data on lightning (Albrecht et al. 2016;
 Roads Major roads mapped at 1:10 000 000 (CEC 2009, and resampled to 1 km resolution.
 Human population United Nations-adjusted population density Gridded Population of the World-v4 output at 30 arc seconds. (Socioeconomic Data and Applications Center:, resampled to 1 km resolution.
 Human influence Anthropogenic biomes mapped at 4 km resolution: (CEC 2008,; Ellis and Ramankutty 2008).
Dense agricultural settlements
 • Cropped and pastoral: villages with a mix of crops and pasture villages
 • Pastoral villages: villages dominated by rangeland
Annual crops mixed with other land uses and land covers
 • Residential irrigated: irrigated cropland with substantial human populations cropland
 • Residential rainfed mosaic: mix of trees and rainfed cropland with substantial human populations
 • Populated irrigated cropland: irrigated cropland with minor human populations
 • Populated rainfed cropland: rainfed cropland with minor human populations
Livestock grazing; minimal crops and forests
 • Residential rangelands: rangelands with substantial human populations
 • Populated rangelands: rangelands with minor human populations
 • Remote rangelands: rangelands with inconsequential human populations
Forests with human populations and agriculture
 • Populated forests: forests with minor human populations
 • Remote forests: forests with inconsequential human populations
 Biotic communities Brown and Lowe’s Biotic Communities of the Southwest (Brown et al. 1979; Source scale 1:1 000 000. Map developed by The Nature Conservancy, Arizona, USA, in 2004.
Conifer Forest/Woodland
 • Petran Subalpine Conifer Forest
 • Great Basin Conifer Woodland
 • Petran Montane Conifer Forest
Evergreen Woodland
 • Madrean Evergreen Woodland
 • Interior Chaparral
 • Arizona Upland Subdivision - Sonoran Desertscrub
 • Sinaloan Thornscrub
 • Chihuahuan Desertscrub
 • Plains of Sonora Subdivision - Sonoran Desertscrub
 • Semidesert Grassland
 • Plains and Great Basin Grassland