Spatial language 17 

Since one of the necessities of pattern research is simplifying spatial concepts into meaningful spatial units (O’Neill 1988), the terms and concepts that pertain to post-fire landscape structure are defined. Terms such as wildfire, fire event, fire perimeter, and unburned remnants are intuitively spatial concepts, but their usage as universal language is less obvious (Andison 2012). A brief description of some of the spatial concepts that are used frequently throughout this dissertation along with a hypothetical (pictorial) representation is shown below; the descriptions are in relation the boreal forest landscapes. The spatial language as described in Figure 1.9 is a conceptual representation of a wildfire and its impact on forested landscapes.

Figure 1.9. Spatial language: summary of the spatial terms and features used in this study.

1 Pixel wide internal buffer Peninsular patches Insular patches

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A fire event is a discrete forest disturbance event in time and space that disrupts the physical and biological structure of an ecosystem and the availability of resources. In this study, a fire event is used to describe a fire-disturbed area(s) that occurs during a specific period and is usually caused by a single ignition. A single fire event may include one or more disturbance patches that originated by spot fires (i.e., wind driven embers) and occurred beyond the existing fire boundaries.

Fire footprint – this refers to the spatial boundary that encompasses boreal wildfire processes; the area within the most probable locations of the outer fire boundary. It is defined based on: 1) a binary conceptualization of a classified map where 1 = all burned pixels and 0 = unburned pixels, and 2) a focal window analysis where a 3×3 focal window was passed over the binary layers 0 and 1; a focal sum was computed as a measure of a pixel’s membership in the fire, resulting in focal sum values ranging from 0 (when al pixels are unburned) to 9 (when all pixels are burned). Any pixel with a focal sum value ≥ 1 indicates probability of membership of a fire and is coded as 1 to present a footprint. The footprint was then shrunk inward by 1 pixel to avoid outward bias by the 3×3 focal function

Figure 1.10. A complex mosaic of post-fire landscape structure: a mixture of burned (dark), partially burned (grey), and unburned (green) areas.

Post-fire residual patch – the wildfires do not burn the entire landscape; they rather create a complex mosaic of post-fire landscape structure, with different degree of burn severity (a mosaic of burned, partially burned, and unburned areas (Figure 1.10). The post-fire residuals are broadly defined as remnants of the pre-fire forest ecosystems that have retained their structure and were not entirely reduced to ash or charcoal during the fire. The composition of living and

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dead vegetation patches and their spatial arrangements differ depending on the fire behaviour and other pre-fire forest characteristics. Besides, following large wildfires, the most visible

component of the fire footprint are the burned areas where all living structures are dead. Also, the resulting spatial mosaic encompasses: 1) partially burned areas where the fire passed through but did not kill the entire vegetation and 2) unburned areas that escape burning and retained within the fire footprint (Figure 1.11). The focus of this study was on unburned areas (hereafter described as post-fire residual patch) that entirely escape fire and left completely unaltered within the fire perimeter.

(a)

(b)

Figure 1.11. A mosaic of burned and unburned areas. (a) Completely burned areas within the fire footprint with high abundance of dead trees that are reduced to charcoal. (b) Presence of

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the large island around the wetland escapes burning while the surrounding areas burned during the fire.

A post-fire residual patch is conceptually defined as a mix of live (and dead) vegetation that form a spatial continuum, ranging from undisturbed patches of live trees to a single tree stem (Nikora et al. 1999; Swystun et al. 2001; Perera et al. 2009a). Residual patches can be classified into different categories based on specific study goals or the scale of observations. Based on the spatial scale of their occurrence, for example, residual patch can be classified as 1) live tree patches (patch-level residuals), 2) standing live trees, and 3) snags (standing dead trees) (Perera et al. 2007; Routledge 2007).

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Figure 1.12. Stepwise criteria used to determine insular and peninsular residual patches for fire footprints. Both insular and peninsular patches are extracted from classified Ikonos images based on their size and location in relation to the fire footprint (Source: Perera et al. 2009a).

The conceptual definition of residual patch has been evolved to ensure consistent interpretation and usage of the term. In the Forest Management Guide for Natural Disturbance Pattern and Emulation (NDEP), for example, a residual patch has been used to describe both insular and peninsular patch types (Figure 1.9) (OMNR 2001). The concepts of insular and peninsular patches have also been described respectively as residual islands and residual matrix respectively (Andison 2004; Dragotescu and Kneeshaw 2012). As shown in Figure 1.9 (above), insular patch refers to undisturbed vegetation patches that are entirely contained within a fire

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event, at least 1 pixel dimension inward from the fire perimeter (at the grain of representation) while a peninsular patch is the undisturbed forested patch contained within a fire event but physically connected to the surrounding forested matrix. Specifically, an insular patch has been used to refer to as undisturbed treed or vegetated land cover class greater than or equal to 0.25 ha while a peninsular patch is defined an area that extends into the disturbance and has a base of less than 400 m (for fires ≤ 260 ha) or 1,000 m (for fires > 260 ha, and generally is longer than its base width (OMNR 2001). The method used to define and extract insular and peninsular patches based on their size and location in relation to the disturbance area perimeter is presented in (Figure 1.12).

The use of the term residual has been changed from the one described in NDEP (OMNR 2001) to reflect the patterns considerations, the variation in residual patch size, and suitability of site-specific habitats (OMNR 2010). The live and dead trees that remain standing after a

disturbance in fire-origin forests haven generally referred to as residual trees or snags, or residual structure. In the recent management guide (OMNR 2010), for example, wildlife trees are used in place of ‘residuals’ or ‘residual trees’, which sometimes led to confusion, as mappable stands of trees are usually referred to as ‘stand level residuals’ (OMNR 2010). Wildlife trees are standing individuals trees or stems, or small clumps of trees or stems, within areas of operations; a clump of wildlife tree is < 0.1 ha in size (OMNR 2010). In this study, residual patches are used to describe any (live) undisturbed forest patch that are entirely contained within a fire event but physically not connected to the perimeter of the fire footprint. This is regardless of age and type of forest species that form the patches.