## 3 Results and Discussion
### Impacts of Fire  
#### 3.4.1 Impacts on Ecosystem Performance Anomalies  
Species composition, stand age [14, 52], carbon dynamics [43, 53], permafrost condition, and performance [44] are often modulated by fire in the boreal forest. Fires are the dominant disturbance in the boreal forest [43]; 47% of the Yukon River Basin boreal forest burned at least once between 1950 and 2008 and 16% burned at least twice. Estimated fire return intervals in the Alaskan boreal ecoregions ranged from 74 to 326 years from 1950 to 2009 [27]. Fires create a landscape mosaic of large patches in various stages of succession following fire (Figures 2 and 3). Figure 3A depicts severely underperforming EPA within fire scars 1 and 3 years old, while the area within an 8-year-old stand had recovered slightly to include normal and under-performance (Figure 3A). Mid-successful  
Figure 3: **(A)** 2008 Ecosystem Performance Anomaly (EPA) for a region surrounding the confluence of the Yukon and Tanana Rivers. Location indicated by the black circle in the Yukon River Basin (grey). Fire perimeters (1950–2008) are labeled with the year of burn. **(B)** July–September 2008–2011 composite (Bands: 5, 4, 3) of WELD Landsat imagery. Darker green corresponds to coniferous forest, lime green to deciduous forest, and pink-brown to shrub/herbaceous/woody wetlands. stands, 39 and 55 years old, were indistinguishable from the surrounding landscape and contained areas of normal and overperforming EPA, similar to the pattern seen in Figure 4A. Our chromosome demonstrated an average EPA of \(-33.8\) (\(\pm 2.1\) SE), in the year prior to fire (Figure 4A), significantly lower than the overall chromosome average EPA (\(-28.7\pm 0.64\) SE), and the average EPA (\(-4.9\pm 1.1\) SE) in areas that were unburned since 1950. This pattern of underperformance prior to fire may be due to reduced stand performance and photosynthetic rate at old age [91, 92] or insect defoliation/mortality [4, 95] inducing a negative EPA and leaving the forest more  
Figure 4: **(A) Ecosystem Performance Anomalies (EPA) chronosequence for land cover classes following fires in the Yukon River Basin boreal forest. Thick horizontal black lines denote the upper and lower bounds of normal EPA; _i.e._, the area above the top line has an overperforming EPA. The early succession land cover group is not displayed after 20 years due to low sample size. Negative values for years since fire correspond to EPA in areas prior to being burned. (B) Percentage of burned area in each land cover group by year, prior to and following fires. “Deciduous” and “Coniferous” forest groups represent mature stands. **  
susceptible to fire. [PERSON] and [PERSON] [25] found a similar pattern in a study of North American boreal forest, where a declining NDVI trend was found in the 15 years leading up to a fire. The average EPA in a fire scar in the year of burn was \(-158\pm 4.5\) SE, which decreased to \(-208\pm 3.1\) SE in the year immediately after the burn, and recovered to \(-116\pm 2.8\) SE in the second year after the burn (Figure 4A). Fire often reduces performance 30%-80% in the year following a fire [43], and considerably reduces leaf area index [91], since most fires are stand replacing, resulting in severe underperformance. The lower EPA in the year after burn (as compared to the year of burn) was due to the timing of fires: a late summer fire would have smaller impacts on actual GSN in that year. Ecosystem performance, vegetation growth, and CO\({}_{2}\) uptake, return to pre-fire levels 5-9 years after fire [25, 53]. However, average EPA in fire scars remained significantly underperforming for 10-15 years following fire, consistent with [43], and became positive 35 years following fire (Figure 4A). The interannual pattern of EPA recovery became noisier 40 years after fire due to the lower number of sample points available and the lower quality of the oldest fire perimeter data.