## 4 Discussion
### Do Temporal Trends in EVI and NDVI Correlate with Canopy Growth? This section focuses on the second objective, which explores the spatial relationships between changes in CHM and VI trends, particularly in ecotonal areas where increases or decreases in height and foliage are most pronounced, such as those linked to woody vegetation expansion. Pervasive growth across the four study polygons from 2000 to 2016/18 is clear in the \(\Delta\)CHM30 data presented in Figure 6. The range in canopy growth or loss dynamics is captured in five distinct canopy height change classes showing that, overall, \(>\)50% of the study area (55.9 km\({}^{2}\)) had undergone an average canopy height increase of \(>\)1 m, suggesting growth through mechanisms of either (a) in situ vertical stem extension, (b) in-filling of canopy gaps by crown expansion and new growth, (c) succession or lateral expansion of taller canopy cover outwards from woody ecotones or (d) some combination of all. In areas of pre-existing woody cover classes, processes of in situ growth and in-filling of canopy gaps were likely the main mechanisms contributing to increases in overall CHM30. However, lateral expansion of primarily shrub and juvenile tree ecotones, observed using \(\Delta\)P90 CHM (Figure 7), indicated that \(>\)11% of the study area had undergone a change in vegetation cover because of ecotonal expansion. While almost 90% of the study area showed no discernible expansion or contraction of ecotones, the northern Peace Delta sector (areas A, B and C) exhibited higher rates of woody vegetation ecotone expansion between 2.4 and 3.2 m/yr (Table 2). This rapid expansion highlights a dynamic ecological response, driven by local environmental factors [114]. In contrast, the more southerly Athabasca Delta sector (area D) demonstrated a lower ecotone expansion rate closer to half that of the Peace Delta sites at \(\sim\)1.5 m/yr, suggesting some differentiation in the underlying processes driving the expansion of woody ecotones across these two regions. From the limited spatial extent of the survey polygons, it cannot be concluded that the driving mechanisms for ecotone expansion are distinct. However, it is noteworthy that the three northern polygons lie along channels that have historically reversed in flow direction [115], while the southern polygon is along a channel that naturally drains in one direction. Consequently, local channel and floodplain physiography and hydrology play a role in the patterns and rates of vegetation encroachment [8; 65]. For example, surveyed area C, situated at a higher elevation above the main waterways, likely depends on ice-jam floodwater input, which has been less frequent in the Peace Delta compared to the Athabasca Delta between 2000 and 2018. In contrast, surveyed area B is located within a low-lying floodplain that expands with water during periods of elevated Lake Athabasca levels [116]. As expected from the positive correlations of VI with canopy height, and as reported in other northern studies [23; 53; 117] greening (or increasingly positive) trends were observed in both EVI and NDVI across the study sites. Unlike in the direct correlation between VI and canopy height, the spatial correspondence between VI greening and increasing canopy height was most pronounced in the EVI signal. Notably, the most significant areas of EVI greening tend to visually align with and show a similar rate of spatial extent variations in woody ecotones defined using \(\Delta\)P90 CHM (the change in upper canopy height percentile) (Figures 8 and 9, Table 2). Meanwhile, NDVI greening shows less direct correspondence and suggests a higher rate of woody ecotone expansion, while being more sensitive to a wider range of vegetation changes. These observations indicate that NDVI captures a spatially broader range of vegetation response, including shorter stature vegetation, compared to that in the CHM-based analyses.