Table 2 We quantified fire spatial patterns using metrics representing fire size, shape, and orientation. The table includes a reference for the program used to calculate the metrics, a general description of each, and a summary of their potential ecological significance in terms of pattern−process relationships of interest. Madrean Ecoregion Sky Islands, US and Mexico, 1985 to 2011

Size

Fire size

(VanDerWal et al. 2014)

Area of each fire (ha)

The distribution of fires of varying size defines an important component of a fire regime. Fire size can be influenced by climate (Haire and McGarigal 2009), as well as underlying features of topography that either facilitate or impede fire spread (Mansuy et al. 2014). Greater interior, or core area, reflects a higher proportion of area burned that is isolated from the surrounding unburned matrix and its intact resources. Interior areas are likely to experience very different physical conditions than disturbed areas near intact vegetation (Turner et al. 2001).

Core area index (VanDerWal et al. 2014)

The interior area of the fire, greater than 100 m from the fire perimeter as a percentage of patch area

Shape

Eccentricity

(Bui et al. 2012)

Ratio of the distance between the two farthest extremities of each fire and the distance of the maximum width of its perpendicular axis (i.e., length-to-breadth)

Fire shape can reflect underlying factors such as physiography, hydrography, and surficial deposits (Mansuy et al. 2014), time period, vegetation, climate, and management (Parisien et al. 2006).

Shape interacts with fire size and severity to influence rate and direction of successional trajectories (Turner et al. 1998). Complex fire shapes can include more edge habitats with greater potential interspersion of seed sources.

Shape index

(VanDerWal et al. 2014)

Fire shape complexity; the degree of departure from a circular shape; ranges from 1 (round) to >1 (complex)

Perimeter area ratio (VanDerWal et al. 2014)

Ratio of patch perimeter (m) to area (m²)

Orientation

Bearing

(Hijmans 2016)

Orientation (degrees) of the axis of the two farthest extremities of each fire

The orientation of a burn can reflect predominant winds and/or topographic (e.g., watershed) orientation (Barros et al. 2013). Given this potential interaction with topography, bearing may also influence the traversability of the burned area by wildlife and plants.